| Message ID | 20210303185807.2160264-1-surenb@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v3,1/1] mm/madvise: replace ptrace attach requirement for process_madvise | expand |
On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: > > process_madvise currently requires ptrace attach capability. > PTRACE_MODE_ATTACH gives one process complete control over another > process. It effectively removes the security boundary between the > two processes (in one direction). Granting ptrace attach capability > even to a system process is considered dangerous since it creates an > attack surface. This severely limits the usage of this API. > The operations process_madvise can perform do not affect the correctness > of the operation of the target process; they only affect where the data > is physically located (and therefore, how fast it can be accessed). > What we want is the ability for one process to influence another process > in order to optimize performance across the entire system while leaving > the security boundary intact. > Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ > and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata > and CAP_SYS_NICE for influencing process performance. > > Cc: stable@vger.kernel.org # 5.10+ > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > Reviewed-by: Kees Cook <keescook@chromium.org> > Acked-by: Minchan Kim <minchan@kernel.org> > Acked-by: David Rientjes <rientjes@google.com> > --- > changes in v3 > - Added Reviewed-by: Kees Cook <keescook@chromium.org> > - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 > - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request > - cc'ed linux-security-module@vger.kernel.org per James Morris's request > > mm/madvise.c | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/mm/madvise.c b/mm/madvise.c > index df692d2e35d4..01fef79ac761 100644 > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, > goto release_task; > } > > - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); > + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ > + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); > if (IS_ERR_OR_NULL(mm)) { > ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; > goto release_task; > } > > + /* > + * Require CAP_SYS_NICE for influencing process performance. Note that > + * only non-destructive hints are currently supported. How is non-destructive defined? Is MADV_DONTNEED non-destructive? > + */ > + if (!capable(CAP_SYS_NICE)) { > + ret = -EPERM; > + goto release_mm; > + } > + > total_len = iov_iter_count(&iter); > > while (iov_iter_count(&iter)) { > @@ -1218,6 +1228,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, > if (ret == 0) > ret = total_len - iov_iter_count(&iter); > > +release_mm: > mmput(mm); > release_task: > put_task_struct(task); > -- > 2.30.1.766.gb4fecdf3b7-goog >
On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: > > On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: > > > > process_madvise currently requires ptrace attach capability. > > PTRACE_MODE_ATTACH gives one process complete control over another > > process. It effectively removes the security boundary between the > > two processes (in one direction). Granting ptrace attach capability > > even to a system process is considered dangerous since it creates an > > attack surface. This severely limits the usage of this API. > > The operations process_madvise can perform do not affect the correctness > > of the operation of the target process; they only affect where the data > > is physically located (and therefore, how fast it can be accessed). > > What we want is the ability for one process to influence another process > > in order to optimize performance across the entire system while leaving > > the security boundary intact. > > Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ > > and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata > > and CAP_SYS_NICE for influencing process performance. > > > > Cc: stable@vger.kernel.org # 5.10+ > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > Reviewed-by: Kees Cook <keescook@chromium.org> > > Acked-by: Minchan Kim <minchan@kernel.org> > > Acked-by: David Rientjes <rientjes@google.com> > > --- > > changes in v3 > > - Added Reviewed-by: Kees Cook <keescook@chromium.org> > > - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 > > - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request > > - cc'ed linux-security-module@vger.kernel.org per James Morris's request > > > > mm/madvise.c | 13 ++++++++++++- > > 1 file changed, 12 insertions(+), 1 deletion(-) > > > > diff --git a/mm/madvise.c b/mm/madvise.c > > index df692d2e35d4..01fef79ac761 100644 > > --- a/mm/madvise.c > > +++ b/mm/madvise.c > > @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, > > goto release_task; > > } > > > > - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); > > + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ > > + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); > > if (IS_ERR_OR_NULL(mm)) { > > ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; > > goto release_task; > > } > > > > + /* > > + * Require CAP_SYS_NICE for influencing process performance. Note that > > + * only non-destructive hints are currently supported. > > How is non-destructive defined? Is MADV_DONTNEED non-destructive? Non-destructive in this context means the data is not lost and can be recovered. I follow the logic described in https://lwn.net/Articles/794704/ where Minchan was introducing MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED would be considered destructive hints. Note that process_madvise_behavior_valid() allows only MADV_COLD and MADV_PAGEOUT at the moment, which are both non-destructive. > > > + */ > > + if (!capable(CAP_SYS_NICE)) { > > + ret = -EPERM; > > + goto release_mm; > > + } > > + > > total_len = iov_iter_count(&iter); > > > > while (iov_iter_count(&iter)) { > > @@ -1218,6 +1228,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, > > if (ret == 0) > > ret = total_len - iov_iter_count(&iter); > > > > +release_mm: > > mmput(mm); > > release_task: > > put_task_struct(task); > > -- > > 2.30.1.766.gb4fecdf3b7-goog > >
On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote: > > On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: > > > > On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > process_madvise currently requires ptrace attach capability. > > > PTRACE_MODE_ATTACH gives one process complete control over another > > > process. It effectively removes the security boundary between the > > > two processes (in one direction). Granting ptrace attach capability > > > even to a system process is considered dangerous since it creates an > > > attack surface. This severely limits the usage of this API. > > > The operations process_madvise can perform do not affect the correctness > > > of the operation of the target process; they only affect where the data > > > is physically located (and therefore, how fast it can be accessed). > > > What we want is the ability for one process to influence another process > > > in order to optimize performance across the entire system while leaving > > > the security boundary intact. > > > Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ > > > and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata > > > and CAP_SYS_NICE for influencing process performance. > > > > > > Cc: stable@vger.kernel.org # 5.10+ > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > > Reviewed-by: Kees Cook <keescook@chromium.org> > > > Acked-by: Minchan Kim <minchan@kernel.org> > > > Acked-by: David Rientjes <rientjes@google.com> > > > --- > > > changes in v3 > > > - Added Reviewed-by: Kees Cook <keescook@chromium.org> > > > - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 > > > - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request > > > - cc'ed linux-security-module@vger.kernel.org per James Morris's request > > > > > > mm/madvise.c | 13 ++++++++++++- > > > 1 file changed, 12 insertions(+), 1 deletion(-) > > > > > > diff --git a/mm/madvise.c b/mm/madvise.c > > > index df692d2e35d4..01fef79ac761 100644 > > > --- a/mm/madvise.c > > > +++ b/mm/madvise.c > > > @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, > > > goto release_task; > > > } > > > > > > - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); > > > + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ > > > + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); > > > if (IS_ERR_OR_NULL(mm)) { > > > ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; > > > goto release_task; > > > } > > > > > > + /* > > > + * Require CAP_SYS_NICE for influencing process performance. Note that > > > + * only non-destructive hints are currently supported. > > > > How is non-destructive defined? Is MADV_DONTNEED non-destructive? > > Non-destructive in this context means the data is not lost and can be > recovered. I follow the logic described in > https://lwn.net/Articles/794704/ where Minchan was introducing > MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE > and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED > would be considered destructive hints. > Note that process_madvise_behavior_valid() allows only MADV_COLD and > MADV_PAGEOUT at the moment, which are both non-destructive. > There is a plan to support MADV_DONTNEED for this syscall. Do we need to change these access checks again with that support?
On Wed, Mar 3, 2021 at 4:04 PM Shakeel Butt <shakeelb@google.com> wrote: > > On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote: > > > > On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: > > > > > > On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > > > process_madvise currently requires ptrace attach capability. > > > > PTRACE_MODE_ATTACH gives one process complete control over another > > > > process. It effectively removes the security boundary between the > > > > two processes (in one direction). Granting ptrace attach capability > > > > even to a system process is considered dangerous since it creates an > > > > attack surface. This severely limits the usage of this API. > > > > The operations process_madvise can perform do not affect the correctness > > > > of the operation of the target process; they only affect where the data > > > > is physically located (and therefore, how fast it can be accessed). > > > > What we want is the ability for one process to influence another process > > > > in order to optimize performance across the entire system while leaving > > > > the security boundary intact. > > > > Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ > > > > and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata > > > > and CAP_SYS_NICE for influencing process performance. > > > > > > > > Cc: stable@vger.kernel.org # 5.10+ > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > > > Reviewed-by: Kees Cook <keescook@chromium.org> > > > > Acked-by: Minchan Kim <minchan@kernel.org> > > > > Acked-by: David Rientjes <rientjes@google.com> > > > > --- > > > > changes in v3 > > > > - Added Reviewed-by: Kees Cook <keescook@chromium.org> > > > > - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 > > > > - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request > > > > - cc'ed linux-security-module@vger.kernel.org per James Morris's request > > > > > > > > mm/madvise.c | 13 ++++++++++++- > > > > 1 file changed, 12 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/mm/madvise.c b/mm/madvise.c > > > > index df692d2e35d4..01fef79ac761 100644 > > > > --- a/mm/madvise.c > > > > +++ b/mm/madvise.c > > > > @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, > > > > goto release_task; > > > > } > > > > > > > > - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); > > > > + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ > > > > + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); > > > > if (IS_ERR_OR_NULL(mm)) { > > > > ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; > > > > goto release_task; > > > > } > > > > > > > > + /* > > > > + * Require CAP_SYS_NICE for influencing process performance. Note that > > > > + * only non-destructive hints are currently supported. > > > > > > How is non-destructive defined? Is MADV_DONTNEED non-destructive? > > > > Non-destructive in this context means the data is not lost and can be > > recovered. I follow the logic described in > > https://lwn.net/Articles/794704/ where Minchan was introducing > > MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE > > and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED > > would be considered destructive hints. > > Note that process_madvise_behavior_valid() allows only MADV_COLD and > > MADV_PAGEOUT at the moment, which are both non-destructive. > > > > There is a plan to support MADV_DONTNEED for this syscall. Do we need > to change these access checks again with that support? I think so. Destructive hints affect the data, so we will probably need stricter checks for those hints.
On 04.03.21 01:03, Shakeel Butt wrote: > On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote: >> >> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: >>> >>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: >>>> >>>> process_madvise currently requires ptrace attach capability. >>>> PTRACE_MODE_ATTACH gives one process complete control over another >>>> process. It effectively removes the security boundary between the >>>> two processes (in one direction). Granting ptrace attach capability >>>> even to a system process is considered dangerous since it creates an >>>> attack surface. This severely limits the usage of this API. >>>> The operations process_madvise can perform do not affect the correctness >>>> of the operation of the target process; they only affect where the data >>>> is physically located (and therefore, how fast it can be accessed). >>>> What we want is the ability for one process to influence another process >>>> in order to optimize performance across the entire system while leaving >>>> the security boundary intact. >>>> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ >>>> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata >>>> and CAP_SYS_NICE for influencing process performance. >>>> >>>> Cc: stable@vger.kernel.org # 5.10+ >>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com> >>>> Reviewed-by: Kees Cook <keescook@chromium.org> >>>> Acked-by: Minchan Kim <minchan@kernel.org> >>>> Acked-by: David Rientjes <rientjes@google.com> >>>> --- >>>> changes in v3 >>>> - Added Reviewed-by: Kees Cook <keescook@chromium.org> >>>> - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 >>>> - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request >>>> - cc'ed linux-security-module@vger.kernel.org per James Morris's request >>>> >>>> mm/madvise.c | 13 ++++++++++++- >>>> 1 file changed, 12 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/mm/madvise.c b/mm/madvise.c >>>> index df692d2e35d4..01fef79ac761 100644 >>>> --- a/mm/madvise.c >>>> +++ b/mm/madvise.c >>>> @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, >>>> goto release_task; >>>> } >>>> >>>> - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); >>>> + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ >>>> + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); >>>> if (IS_ERR_OR_NULL(mm)) { >>>> ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; >>>> goto release_task; >>>> } >>>> >>>> + /* >>>> + * Require CAP_SYS_NICE for influencing process performance. Note that >>>> + * only non-destructive hints are currently supported. >>> >>> How is non-destructive defined? Is MADV_DONTNEED non-destructive? >> >> Non-destructive in this context means the data is not lost and can be >> recovered. I follow the logic described in >> https://lwn.net/Articles/794704/ where Minchan was introducing >> MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE >> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED >> would be considered destructive hints. >> Note that process_madvise_behavior_valid() allows only MADV_COLD and >> MADV_PAGEOUT at the moment, which are both non-destructive. >> > > There is a plan to support MADV_DONTNEED for this syscall. Do we need > to change these access checks again with that support? Eh, I absolutely don't think letting another process discard memory in another process' address space is a good idea. The target process can observe that easily and might even run into real issues. What's the use case?
On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david@redhat.com> wrote: > > On 04.03.21 01:03, Shakeel Butt wrote: > > On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote: > >> > >> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: > >>> > >>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: > >>>> > >>>> process_madvise currently requires ptrace attach capability. > >>>> PTRACE_MODE_ATTACH gives one process complete control over another > >>>> process. It effectively removes the security boundary between the > >>>> two processes (in one direction). Granting ptrace attach capability > >>>> even to a system process is considered dangerous since it creates an > >>>> attack surface. This severely limits the usage of this API. > >>>> The operations process_madvise can perform do not affect the correctness > >>>> of the operation of the target process; they only affect where the data > >>>> is physically located (and therefore, how fast it can be accessed). > >>>> What we want is the ability for one process to influence another process > >>>> in order to optimize performance across the entire system while leaving > >>>> the security boundary intact. > >>>> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ > >>>> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata > >>>> and CAP_SYS_NICE for influencing process performance. > >>>> > >>>> Cc: stable@vger.kernel.org # 5.10+ > >>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com> > >>>> Reviewed-by: Kees Cook <keescook@chromium.org> > >>>> Acked-by: Minchan Kim <minchan@kernel.org> > >>>> Acked-by: David Rientjes <rientjes@google.com> > >>>> --- > >>>> changes in v3 > >>>> - Added Reviewed-by: Kees Cook <keescook@chromium.org> > >>>> - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 > >>>> - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request > >>>> - cc'ed linux-security-module@vger.kernel.org per James Morris's request > >>>> > >>>> mm/madvise.c | 13 ++++++++++++- > >>>> 1 file changed, 12 insertions(+), 1 deletion(-) > >>>> > >>>> diff --git a/mm/madvise.c b/mm/madvise.c > >>>> index df692d2e35d4..01fef79ac761 100644 > >>>> --- a/mm/madvise.c > >>>> +++ b/mm/madvise.c > >>>> @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, > >>>> goto release_task; > >>>> } > >>>> > >>>> - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); > >>>> + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ > >>>> + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); > >>>> if (IS_ERR_OR_NULL(mm)) { > >>>> ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; > >>>> goto release_task; > >>>> } > >>>> > >>>> + /* > >>>> + * Require CAP_SYS_NICE for influencing process performance. Note that > >>>> + * only non-destructive hints are currently supported. > >>> > >>> How is non-destructive defined? Is MADV_DONTNEED non-destructive? > >> > >> Non-destructive in this context means the data is not lost and can be > >> recovered. I follow the logic described in > >> https://lwn.net/Articles/794704/ where Minchan was introducing > >> MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE > >> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED > >> would be considered destructive hints. > >> Note that process_madvise_behavior_valid() allows only MADV_COLD and > >> MADV_PAGEOUT at the moment, which are both non-destructive. > >> > > > > There is a plan to support MADV_DONTNEED for this syscall. Do we need > > to change these access checks again with that support? > > Eh, I absolutely don't think letting another process discard memory in > another process' address space is a good idea. The target process can > observe that easily and might even run into real issues. > > What's the use case? > Userspace oom reaper. Please look at https://lore.kernel.org/linux-api/20201014183943.GA1489464@google.com/T/
On 05.03.21 18:45, Shakeel Butt wrote: > On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david@redhat.com> wrote: >> >> On 04.03.21 01:03, Shakeel Butt wrote: >>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote: >>>> >>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: >>>>> >>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: >>>>>> >>>>>> process_madvise currently requires ptrace attach capability. >>>>>> PTRACE_MODE_ATTACH gives one process complete control over another >>>>>> process. It effectively removes the security boundary between the >>>>>> two processes (in one direction). Granting ptrace attach capability >>>>>> even to a system process is considered dangerous since it creates an >>>>>> attack surface. This severely limits the usage of this API. >>>>>> The operations process_madvise can perform do not affect the correctness >>>>>> of the operation of the target process; they only affect where the data >>>>>> is physically located (and therefore, how fast it can be accessed). >>>>>> What we want is the ability for one process to influence another process >>>>>> in order to optimize performance across the entire system while leaving >>>>>> the security boundary intact. >>>>>> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ >>>>>> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata >>>>>> and CAP_SYS_NICE for influencing process performance. >>>>>> >>>>>> Cc: stable@vger.kernel.org # 5.10+ >>>>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com> >>>>>> Reviewed-by: Kees Cook <keescook@chromium.org> >>>>>> Acked-by: Minchan Kim <minchan@kernel.org> >>>>>> Acked-by: David Rientjes <rientjes@google.com> >>>>>> --- >>>>>> changes in v3 >>>>>> - Added Reviewed-by: Kees Cook <keescook@chromium.org> >>>>>> - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 >>>>>> - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request >>>>>> - cc'ed linux-security-module@vger.kernel.org per James Morris's request >>>>>> >>>>>> mm/madvise.c | 13 ++++++++++++- >>>>>> 1 file changed, 12 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/mm/madvise.c b/mm/madvise.c >>>>>> index df692d2e35d4..01fef79ac761 100644 >>>>>> --- a/mm/madvise.c >>>>>> +++ b/mm/madvise.c >>>>>> @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, >>>>>> goto release_task; >>>>>> } >>>>>> >>>>>> - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); >>>>>> + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ >>>>>> + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); >>>>>> if (IS_ERR_OR_NULL(mm)) { >>>>>> ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; >>>>>> goto release_task; >>>>>> } >>>>>> >>>>>> + /* >>>>>> + * Require CAP_SYS_NICE for influencing process performance. Note that >>>>>> + * only non-destructive hints are currently supported. >>>>> >>>>> How is non-destructive defined? Is MADV_DONTNEED non-destructive? >>>> >>>> Non-destructive in this context means the data is not lost and can be >>>> recovered. I follow the logic described in >>>> https://lwn.net/Articles/794704/ where Minchan was introducing >>>> MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE >>>> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED >>>> would be considered destructive hints. >>>> Note that process_madvise_behavior_valid() allows only MADV_COLD and >>>> MADV_PAGEOUT at the moment, which are both non-destructive. >>>> >>> >>> There is a plan to support MADV_DONTNEED for this syscall. Do we need >>> to change these access checks again with that support? >> >> Eh, I absolutely don't think letting another process discard memory in >> another process' address space is a good idea. The target process can >> observe that easily and might even run into real issues. >> >> What's the use case? >> > > Userspace oom reaper. Please look at > https://lore.kernel.org/linux-api/20201014183943.GA1489464@google.com/T/ > Thanks, somehow I missed that (not that it really changed my opinion on the approach while skimming over the discussion :) will have a more detailed look)
On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand <david@redhat.com> wrote: > > On 05.03.21 18:45, Shakeel Butt wrote: > > On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david@redhat.com> wrote: > >> > >> On 04.03.21 01:03, Shakeel Butt wrote: > >>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote: > >>>> > >>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: > >>>>> > >>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: > >>>>>> > >>>>>> process_madvise currently requires ptrace attach capability. > >>>>>> PTRACE_MODE_ATTACH gives one process complete control over another > >>>>>> process. It effectively removes the security boundary between the > >>>>>> two processes (in one direction). Granting ptrace attach capability > >>>>>> even to a system process is considered dangerous since it creates an > >>>>>> attack surface. This severely limits the usage of this API. > >>>>>> The operations process_madvise can perform do not affect the correctness > >>>>>> of the operation of the target process; they only affect where the data > >>>>>> is physically located (and therefore, how fast it can be accessed). > >>>>>> What we want is the ability for one process to influence another process > >>>>>> in order to optimize performance across the entire system while leaving > >>>>>> the security boundary intact. > >>>>>> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ > >>>>>> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata > >>>>>> and CAP_SYS_NICE for influencing process performance. > >>>>>> > >>>>>> Cc: stable@vger.kernel.org # 5.10+ > >>>>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com> > >>>>>> Reviewed-by: Kees Cook <keescook@chromium.org> > >>>>>> Acked-by: Minchan Kim <minchan@kernel.org> > >>>>>> Acked-by: David Rientjes <rientjes@google.com> > >>>>>> --- > >>>>>> changes in v3 > >>>>>> - Added Reviewed-by: Kees Cook <keescook@chromium.org> > >>>>>> - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 > >>>>>> - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request > >>>>>> - cc'ed linux-security-module@vger.kernel.org per James Morris's request > >>>>>> > >>>>>> mm/madvise.c | 13 ++++++++++++- > >>>>>> 1 file changed, 12 insertions(+), 1 deletion(-) > >>>>>> > >>>>>> diff --git a/mm/madvise.c b/mm/madvise.c > >>>>>> index df692d2e35d4..01fef79ac761 100644 > >>>>>> --- a/mm/madvise.c > >>>>>> +++ b/mm/madvise.c > >>>>>> @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, > >>>>>> goto release_task; > >>>>>> } > >>>>>> > >>>>>> - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); > >>>>>> + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ > >>>>>> + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); > >>>>>> if (IS_ERR_OR_NULL(mm)) { > >>>>>> ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; > >>>>>> goto release_task; > >>>>>> } > >>>>>> > >>>>>> + /* > >>>>>> + * Require CAP_SYS_NICE for influencing process performance. Note that > >>>>>> + * only non-destructive hints are currently supported. > >>>>> > >>>>> How is non-destructive defined? Is MADV_DONTNEED non-destructive? > >>>> > >>>> Non-destructive in this context means the data is not lost and can be > >>>> recovered. I follow the logic described in > >>>> https://lwn.net/Articles/794704/ where Minchan was introducing > >>>> MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE > >>>> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED > >>>> would be considered destructive hints. > >>>> Note that process_madvise_behavior_valid() allows only MADV_COLD and > >>>> MADV_PAGEOUT at the moment, which are both non-destructive. > >>>> > >>> > >>> There is a plan to support MADV_DONTNEED for this syscall. Do we need > >>> to change these access checks again with that support? > >> > >> Eh, I absolutely don't think letting another process discard memory in > >> another process' address space is a good idea. The target process can > >> observe that easily and might even run into real issues. > >> > >> What's the use case? > >> > > > > Userspace oom reaper. Please look at > > https://lore.kernel.org/linux-api/20201014183943.GA1489464@google.com/T/ > > > > Thanks, somehow I missed that (not that it really changed my opinion on > the approach while skimming over the discussion :) will have a more > detailed look) The latest version of that patchset is: https://lore.kernel.org/patchwork/patch/1344419/ Yeah, memory reaping is a special case when we are operating on a dying process to speed up the release of its memory. I don't know if for that particular case we need to make the checks stricter. It's a dying process anyway and the data is being destroyed. Allowing to speed up that process probably can still use CAP_SYS_NICE. > > -- > Thanks, > > David / dhildenb >
On 05.03.21 19:08, Suren Baghdasaryan wrote: > On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand <david@redhat.com> wrote: >> >> On 05.03.21 18:45, Shakeel Butt wrote: >>> On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david@redhat.com> wrote: >>>> >>>> On 04.03.21 01:03, Shakeel Butt wrote: >>>>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote: >>>>>> >>>>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: >>>>>>> >>>>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: >>>>>>>> >>>>>>>> process_madvise currently requires ptrace attach capability. >>>>>>>> PTRACE_MODE_ATTACH gives one process complete control over another >>>>>>>> process. It effectively removes the security boundary between the >>>>>>>> two processes (in one direction). Granting ptrace attach capability >>>>>>>> even to a system process is considered dangerous since it creates an >>>>>>>> attack surface. This severely limits the usage of this API. >>>>>>>> The operations process_madvise can perform do not affect the correctness >>>>>>>> of the operation of the target process; they only affect where the data >>>>>>>> is physically located (and therefore, how fast it can be accessed). >>>>>>>> What we want is the ability for one process to influence another process >>>>>>>> in order to optimize performance across the entire system while leaving >>>>>>>> the security boundary intact. >>>>>>>> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ >>>>>>>> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata >>>>>>>> and CAP_SYS_NICE for influencing process performance. >>>>>>>> >>>>>>>> Cc: stable@vger.kernel.org # 5.10+ >>>>>>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com> >>>>>>>> Reviewed-by: Kees Cook <keescook@chromium.org> >>>>>>>> Acked-by: Minchan Kim <minchan@kernel.org> >>>>>>>> Acked-by: David Rientjes <rientjes@google.com> >>>>>>>> --- >>>>>>>> changes in v3 >>>>>>>> - Added Reviewed-by: Kees Cook <keescook@chromium.org> >>>>>>>> - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 >>>>>>>> - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request >>>>>>>> - cc'ed linux-security-module@vger.kernel.org per James Morris's request >>>>>>>> >>>>>>>> mm/madvise.c | 13 ++++++++++++- >>>>>>>> 1 file changed, 12 insertions(+), 1 deletion(-) >>>>>>>> >>>>>>>> diff --git a/mm/madvise.c b/mm/madvise.c >>>>>>>> index df692d2e35d4..01fef79ac761 100644 >>>>>>>> --- a/mm/madvise.c >>>>>>>> +++ b/mm/madvise.c >>>>>>>> @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, >>>>>>>> goto release_task; >>>>>>>> } >>>>>>>> >>>>>>>> - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); >>>>>>>> + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ >>>>>>>> + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); >>>>>>>> if (IS_ERR_OR_NULL(mm)) { >>>>>>>> ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; >>>>>>>> goto release_task; >>>>>>>> } >>>>>>>> >>>>>>>> + /* >>>>>>>> + * Require CAP_SYS_NICE for influencing process performance. Note that >>>>>>>> + * only non-destructive hints are currently supported. >>>>>>> >>>>>>> How is non-destructive defined? Is MADV_DONTNEED non-destructive? >>>>>> >>>>>> Non-destructive in this context means the data is not lost and can be >>>>>> recovered. I follow the logic described in >>>>>> https://lwn.net/Articles/794704/ where Minchan was introducing >>>>>> MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE >>>>>> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED >>>>>> would be considered destructive hints. >>>>>> Note that process_madvise_behavior_valid() allows only MADV_COLD and >>>>>> MADV_PAGEOUT at the moment, which are both non-destructive. >>>>>> >>>>> >>>>> There is a plan to support MADV_DONTNEED for this syscall. Do we need >>>>> to change these access checks again with that support? >>>> >>>> Eh, I absolutely don't think letting another process discard memory in >>>> another process' address space is a good idea. The target process can >>>> observe that easily and might even run into real issues. >>>> >>>> What's the use case? >>>> >>> >>> Userspace oom reaper. Please look at >>> https://lore.kernel.org/linux-api/20201014183943.GA1489464@google.com/T/ >>> >> >> Thanks, somehow I missed that (not that it really changed my opinion on >> the approach while skimming over the discussion :) will have a more >> detailed look) > > The latest version of that patchset is: > https://lore.kernel.org/patchwork/patch/1344419/ > Yeah, memory reaping is a special case when we are operating on a > dying process to speed up the release of its memory. I don't know if > for that particular case we need to make the checks stricter. It's a > dying process anyway and the data is being destroyed. Allowing to > speed up that process probably can still use CAP_SYS_NICE. I know, unrelated discussion (sorry, I don't have above thread in my archive anymore due to automatic cleanups ...) , but introducing MADV_DONTEED on a remote processes, having to tweak range logic because we always want to apply it to the whole MM, just to speed up memory reaping sounds like completely abusing madvise()/process_madvise() to me. You want different semantics than MADV_DONTNEED. You want different semantics than madvise. Simple example: mlock()ed pages in the target process. MADV_DONTNEED would choke on that. For the use case of reaping, you certainly don't care. I am not sure if process_madvise() is the right interface to enforce discarding of all target memory. Not to mention that MADV_FREE doesn't make any sense IMHO for memory reaping ... no to mention exposing this via process_madvise().
On Fri, Mar 5, 2021 at 10:23 AM David Hildenbrand <david@redhat.com> wrote: > > On 05.03.21 19:08, Suren Baghdasaryan wrote: > > On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand <david@redhat.com> wrote: > >> > >> On 05.03.21 18:45, Shakeel Butt wrote: > >>> On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david@redhat.com> wrote: > >>>> > >>>> On 04.03.21 01:03, Shakeel Butt wrote: > >>>>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote: > >>>>>> > >>>>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: > >>>>>>> > >>>>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: > >>>>>>>> > >>>>>>>> process_madvise currently requires ptrace attach capability. > >>>>>>>> PTRACE_MODE_ATTACH gives one process complete control over another > >>>>>>>> process. It effectively removes the security boundary between the > >>>>>>>> two processes (in one direction). Granting ptrace attach capability > >>>>>>>> even to a system process is considered dangerous since it creates an > >>>>>>>> attack surface. This severely limits the usage of this API. > >>>>>>>> The operations process_madvise can perform do not affect the correctness > >>>>>>>> of the operation of the target process; they only affect where the data > >>>>>>>> is physically located (and therefore, how fast it can be accessed). > >>>>>>>> What we want is the ability for one process to influence another process > >>>>>>>> in order to optimize performance across the entire system while leaving > >>>>>>>> the security boundary intact. > >>>>>>>> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ > >>>>>>>> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata > >>>>>>>> and CAP_SYS_NICE for influencing process performance. > >>>>>>>> > >>>>>>>> Cc: stable@vger.kernel.org # 5.10+ > >>>>>>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com> > >>>>>>>> Reviewed-by: Kees Cook <keescook@chromium.org> > >>>>>>>> Acked-by: Minchan Kim <minchan@kernel.org> > >>>>>>>> Acked-by: David Rientjes <rientjes@google.com> > >>>>>>>> --- > >>>>>>>> changes in v3 > >>>>>>>> - Added Reviewed-by: Kees Cook <keescook@chromium.org> > >>>>>>>> - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 > >>>>>>>> - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request > >>>>>>>> - cc'ed linux-security-module@vger.kernel.org per James Morris's request > >>>>>>>> > >>>>>>>> mm/madvise.c | 13 ++++++++++++- > >>>>>>>> 1 file changed, 12 insertions(+), 1 deletion(-) > >>>>>>>> > >>>>>>>> diff --git a/mm/madvise.c b/mm/madvise.c > >>>>>>>> index df692d2e35d4..01fef79ac761 100644 > >>>>>>>> --- a/mm/madvise.c > >>>>>>>> +++ b/mm/madvise.c > >>>>>>>> @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, > >>>>>>>> goto release_task; > >>>>>>>> } > >>>>>>>> > >>>>>>>> - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); > >>>>>>>> + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ > >>>>>>>> + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); > >>>>>>>> if (IS_ERR_OR_NULL(mm)) { > >>>>>>>> ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; > >>>>>>>> goto release_task; > >>>>>>>> } > >>>>>>>> > >>>>>>>> + /* > >>>>>>>> + * Require CAP_SYS_NICE for influencing process performance. Note that > >>>>>>>> + * only non-destructive hints are currently supported. > >>>>>>> > >>>>>>> How is non-destructive defined? Is MADV_DONTNEED non-destructive? > >>>>>> > >>>>>> Non-destructive in this context means the data is not lost and can be > >>>>>> recovered. I follow the logic described in > >>>>>> https://lwn.net/Articles/794704/ where Minchan was introducing > >>>>>> MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE > >>>>>> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED > >>>>>> would be considered destructive hints. > >>>>>> Note that process_madvise_behavior_valid() allows only MADV_COLD and > >>>>>> MADV_PAGEOUT at the moment, which are both non-destructive. > >>>>>> > >>>>> > >>>>> There is a plan to support MADV_DONTNEED for this syscall. Do we need > >>>>> to change these access checks again with that support? > >>>> > >>>> Eh, I absolutely don't think letting another process discard memory in > >>>> another process' address space is a good idea. The target process can > >>>> observe that easily and might even run into real issues. > >>>> > >>>> What's the use case? > >>>> > >>> > >>> Userspace oom reaper. Please look at > >>> https://lore.kernel.org/linux-api/20201014183943.GA1489464@google.com/T/ > >>> > >> > >> Thanks, somehow I missed that (not that it really changed my opinion on > >> the approach while skimming over the discussion :) will have a more > >> detailed look) > > > > The latest version of that patchset is: > > https://lore.kernel.org/patchwork/patch/1344419/ > > Yeah, memory reaping is a special case when we are operating on a > > dying process to speed up the release of its memory. I don't know if > > for that particular case we need to make the checks stricter. It's a > > dying process anyway and the data is being destroyed. Allowing to > > speed up that process probably can still use CAP_SYS_NICE. > > I know, unrelated discussion (sorry, I don't have above thread in my > archive anymore due to automatic cleanups ...) , but introducing > MADV_DONTEED on a remote processes, having to tweak range logic because > we always want to apply it to the whole MM, just to speed up memory > reaping sounds like completely abusing madvise()/process_madvise() to me. > > You want different semantics than MADV_DONTNEED. You want different > semantics than madvise. > > Simple example: mlock()ed pages in the target process. MADV_DONTNEED > would choke on that. For the use case of reaping, you certainly don't care. > > I am not sure if process_madvise() is the right interface to enforce > discarding of all target memory. > > > Not to mention that MADV_FREE doesn't make any sense IMHO for memory > reaping ... no to mention exposing this via process_madvise(). Yeah, that was the last comment from Christoph Hellwig on https://lore.kernel.org/patchwork/patch/1344418/ I'll be rethinking the whole approach. Previously I proposed couple different approaches that would make reaping a part of the kill by adding a new flag for pidfd_send_signal: https://lore.kernel.org/patchwork/patch/1338196/ https://lore.kernel.org/patchwork/patch/1060407/ but maybe a separate syscall for reaping is indeed the right way to go... > > -- > Thanks, > > David / dhildenb >
> Am 05.03.2021 um 19:36 schrieb Suren Baghdasaryan <surenb@google.com>: > > On Fri, Mar 5, 2021 at 10:23 AM David Hildenbrand <david@redhat.com> wrote: >> >>> On 05.03.21 19:08, Suren Baghdasaryan wrote: >>> On Fri, Mar 5, 2021 at 9:52 AM David Hildenbrand <david@redhat.com> wrote: >>>> >>>> On 05.03.21 18:45, Shakeel Butt wrote: >>>>> On Fri, Mar 5, 2021 at 9:37 AM David Hildenbrand <david@redhat.com> wrote: >>>>>> >>>>>> On 04.03.21 01:03, Shakeel Butt wrote: >>>>>>> On Wed, Mar 3, 2021 at 3:34 PM Suren Baghdasaryan <surenb@google.com> wrote: >>>>>>>> >>>>>>>> On Wed, Mar 3, 2021 at 3:17 PM Shakeel Butt <shakeelb@google.com> wrote: >>>>>>>>> >>>>>>>>> On Wed, Mar 3, 2021 at 10:58 AM Suren Baghdasaryan <surenb@google.com> wrote: >>>>>>>>>> >>>>>>>>>> process_madvise currently requires ptrace attach capability. >>>>>>>>>> PTRACE_MODE_ATTACH gives one process complete control over another >>>>>>>>>> process. It effectively removes the security boundary between the >>>>>>>>>> two processes (in one direction). Granting ptrace attach capability >>>>>>>>>> even to a system process is considered dangerous since it creates an >>>>>>>>>> attack surface. This severely limits the usage of this API. >>>>>>>>>> The operations process_madvise can perform do not affect the correctness >>>>>>>>>> of the operation of the target process; they only affect where the data >>>>>>>>>> is physically located (and therefore, how fast it can be accessed). >>>>>>>>>> What we want is the ability for one process to influence another process >>>>>>>>>> in order to optimize performance across the entire system while leaving >>>>>>>>>> the security boundary intact. >>>>>>>>>> Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ >>>>>>>>>> and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata >>>>>>>>>> and CAP_SYS_NICE for influencing process performance. >>>>>>>>>> >>>>>>>>>> Cc: stable@vger.kernel.org # 5.10+ >>>>>>>>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com> >>>>>>>>>> Reviewed-by: Kees Cook <keescook@chromium.org> >>>>>>>>>> Acked-by: Minchan Kim <minchan@kernel.org> >>>>>>>>>> Acked-by: David Rientjes <rientjes@google.com> >>>>>>>>>> --- >>>>>>>>>> changes in v3 >>>>>>>>>> - Added Reviewed-by: Kees Cook <keescook@chromium.org> >>>>>>>>>> - Created man page for process_madvise per Andrew's request: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=a144f458bad476a3358e3a45023789cb7bb9f993 >>>>>>>>>> - cc'ed stable@vger.kernel.org # 5.10+ per Andrew's request >>>>>>>>>> - cc'ed linux-security-module@vger.kernel.org per James Morris's request >>>>>>>>>> >>>>>>>>>> mm/madvise.c | 13 ++++++++++++- >>>>>>>>>> 1 file changed, 12 insertions(+), 1 deletion(-) >>>>>>>>>> >>>>>>>>>> diff --git a/mm/madvise.c b/mm/madvise.c >>>>>>>>>> index df692d2e35d4..01fef79ac761 100644 >>>>>>>>>> --- a/mm/madvise.c >>>>>>>>>> +++ b/mm/madvise.c >>>>>>>>>> @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, >>>>>>>>>> goto release_task; >>>>>>>>>> } >>>>>>>>>> >>>>>>>>>> - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); >>>>>>>>>> + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ >>>>>>>>>> + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); >>>>>>>>>> if (IS_ERR_OR_NULL(mm)) { >>>>>>>>>> ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; >>>>>>>>>> goto release_task; >>>>>>>>>> } >>>>>>>>>> >>>>>>>>>> + /* >>>>>>>>>> + * Require CAP_SYS_NICE for influencing process performance. Note that >>>>>>>>>> + * only non-destructive hints are currently supported. >>>>>>>>> >>>>>>>>> How is non-destructive defined? Is MADV_DONTNEED non-destructive? >>>>>>>> >>>>>>>> Non-destructive in this context means the data is not lost and can be >>>>>>>> recovered. I follow the logic described in >>>>>>>> https://lwn.net/Articles/794704/ where Minchan was introducing >>>>>>>> MADV_COLD and MADV_PAGEOUT as non-destructive versions of MADV_FREE >>>>>>>> and MADV_DONTNEED. Following that logic, MADV_FREE and MADV_DONTNEED >>>>>>>> would be considered destructive hints. >>>>>>>> Note that process_madvise_behavior_valid() allows only MADV_COLD and >>>>>>>> MADV_PAGEOUT at the moment, which are both non-destructive. >>>>>>>> >>>>>>> >>>>>>> There is a plan to support MADV_DONTNEED for this syscall. Do we need >>>>>>> to change these access checks again with that support? >>>>>> >>>>>> Eh, I absolutely don't think letting another process discard memory in >>>>>> another process' address space is a good idea. The target process can >>>>>> observe that easily and might even run into real issues. >>>>>> >>>>>> What's the use case? >>>>>> >>>>> >>>>> Userspace oom reaper. Please look at >>>>> https://lore.kernel.org/linux-api/20201014183943.GA1489464@google.com/T/ >>>>> >>>> >>>> Thanks, somehow I missed that (not that it really changed my opinion on >>>> the approach while skimming over the discussion :) will have a more >>>> detailed look) >>> >>> The latest version of that patchset is: >>> https://lore.kernel.org/patchwork/patch/1344419/ >>> Yeah, memory reaping is a special case when we are operating on a >>> dying process to speed up the release of its memory. I don't know if >>> for that particular case we need to make the checks stricter. It's a >>> dying process anyway and the data is being destroyed. Allowing to >>> speed up that process probably can still use CAP_SYS_NICE. >> >> I know, unrelated discussion (sorry, I don't have above thread in my >> archive anymore due to automatic cleanups ...) , but introducing >> MADV_DONTEED on a remote processes, having to tweak range logic because >> we always want to apply it to the whole MM, just to speed up memory >> reaping sounds like completely abusing madvise()/process_madvise() to me. >> >> You want different semantics than MADV_DONTNEED. You want different >> semantics than madvise. >> >> Simple example: mlock()ed pages in the target process. MADV_DONTNEED >> would choke on that. For the use case of reaping, you certainly don't care. >> >> I am not sure if process_madvise() is the right interface to enforce >> discarding of all target memory. >> >> >> Not to mention that MADV_FREE doesn't make any sense IMHO for memory >> reaping ... no to mention exposing this via process_madvise(). > > Yeah, that was the last comment from Christoph Hellwig on > https://lore.kernel.org/patchwork/patch/1344418/ > I'll be rethinking the whole approach. Previously I proposed couple > different approaches that would make reaping a part of the kill by > adding a new flag for pidfd_send_signal: > https://lore.kernel.org/patchwork/patch/1338196/ > https://lore.kernel.org/patchwork/patch/1060407/ > but maybe a separate syscall for reaping is indeed the right way to go... Yeah, most likely! >
diff --git a/mm/madvise.c b/mm/madvise.c index df692d2e35d4..01fef79ac761 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -1198,12 +1198,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, goto release_task; } - mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); + /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */ + mm = mm_access(task, PTRACE_MODE_READ_FSCREDS); if (IS_ERR_OR_NULL(mm)) { ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; goto release_task; } + /* + * Require CAP_SYS_NICE for influencing process performance. Note that + * only non-destructive hints are currently supported. + */ + if (!capable(CAP_SYS_NICE)) { + ret = -EPERM; + goto release_mm; + } + total_len = iov_iter_count(&iter); while (iov_iter_count(&iter)) { @@ -1218,6 +1228,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec, if (ret == 0) ret = total_len - iov_iter_count(&iter); +release_mm: mmput(mm); release_task: put_task_struct(task);