| Message ID | 1562005928-1929-1-git-send-email-akaher@vmware.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v5,1/3,v4.9.y] coredump: fix race condition between mmget_not_zero()/get_task_mm() and core dumping | expand |
On Tue, Jul 02, 2019 at 12:02:05AM +0530, Ajay Kaher wrote: > From: Andrea Arcangeli <aarcange@redhat.com> > > commit 04f5866e41fb70690e28397487d8bd8eea7d712a upstream. > > The core dumping code has always run without holding the mmap_sem for > writing, despite that is the only way to ensure that the entire vma > layout will not change from under it. Only using some signal > serialization on the processes belonging to the mm is not nearly enough. > This was pointed out earlier. For example in Hugh's post from Jul 2017: > > https://lkml.kernel.org/r/alpine.LSU.2.11.1707191716030.2055@eggly.anvils > > "Not strictly relevant here, but a related note: I was very surprised > to discover, only quite recently, how handle_mm_fault() may be called > without down_read(mmap_sem) - when core dumping. That seems a > misguided optimization to me, which would also be nice to correct" > > In particular because the growsdown and growsup can move the > vm_start/vm_end the various loops the core dump does around the vma will > not be consistent if page faults can happen concurrently. > > Pretty much all users calling mmget_not_zero()/get_task_mm() and then > taking the mmap_sem had the potential to introduce unexpected side > effects in the core dumping code. > > Adding mmap_sem for writing around the ->core_dump invocation is a > viable long term fix, but it requires removing all copy user and page > faults and to replace them with get_dump_page() for all binary formats > which is not suitable as a short term fix. > > For the time being this solution manually covers the places that can > confuse the core dump either by altering the vma layout or the vma flags > while it runs. Once ->core_dump runs under mmap_sem for writing the > function mmget_still_valid() can be dropped. > > Allowing mmap_sem protected sections to run in parallel with the > coredump provides some minor parallelism advantage to the swapoff code > (which seems to be safe enough by never mangling any vma field and can > keep doing swapins in parallel to the core dumping) and to some other > corner case. > > In order to facilitate the backporting I added "Fixes: 86039bd3b4e6" > however the side effect of this same race condition in /proc/pid/mem > should be reproducible since before 2.6.12-rc2 so I couldn't add any > other "Fixes:" because there's no hash beyond the git genesis commit. > > Because find_extend_vma() is the only location outside of the process > context that could modify the "mm" structures under mmap_sem for > reading, by adding the mmget_still_valid() check to it, all other cases > that take the mmap_sem for reading don't need the new check after > mmget_not_zero()/get_task_mm(). The expand_stack() in page fault > context also doesn't need the new check, because all tasks under core > dumping are frozen. > > Link: http://lkml.kernel.org/r/20190325224949.11068-1-aarcange@redhat.com > Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization") > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> > Reported-by: Jann Horn <jannh@google.com> > Suggested-by: Oleg Nesterov <oleg@redhat.com> > Acked-by: Peter Xu <peterx@redhat.com> > Reviewed-by: Mike Rapoport <rppt@linux.ibm.com> > Reviewed-by: Oleg Nesterov <oleg@redhat.com> > Reviewed-by: Jann Horn <jannh@google.com> > Acked-by: Jason Gunthorpe <jgg@mellanox.com> > Acked-by: Michal Hocko <mhocko@suse.com> > Cc: <stable@vger.kernel.org> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org> > Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > [akaher@vmware.com: stable 4.9 backport > - handle binder_update_page_range - mhocko@suse.com] > Signed-off-by: Ajay Kaher <akaher@vmware.com> > --- > drivers/android/binder.c | 6 ++++++ > fs/proc/task_mmu.c | 18 ++++++++++++++++++ > fs/userfaultfd.c | 9 +++++++++ > include/linux/mm.h | 21 +++++++++++++++++++++ > mm/mmap.c | 6 +++++- > 5 files changed, 59 insertions(+), 1 deletion(-) > > diff --git a/drivers/android/binder.c b/drivers/android/binder.c > index 80499f4..f05ab8f 100644 > --- a/drivers/android/binder.c > +++ b/drivers/android/binder.c > @@ -581,6 +581,12 @@ static int binder_update_page_range(struct binder_proc *proc, int allocate, > if (mm) { > down_write(&mm->mmap_sem); > + if (!mmget_still_valid(mm)) { > + if (allocate == 0) > + goto free_range; > + goto err_no_vma; > + } > + > vma = proc->vma; > if (vma && mm != proc->vma_vm_mm) { > pr_err("%d: vma mm and task mm mismatch\n", > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index 5138e78..4b207b1 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1057,6 +1057,24 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf, This patch is oddly corrupted, and I can't figure out how to fix it up. When applying it, I get following error: patching file drivers/android/binder.c patch: **** malformed patch at line 102: diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c So something is odd here. Can you please fix this up, and resend the series so that they can be applied? thanks, greg k-h
diff --git a/drivers/android/binder.c b/drivers/android/binder.c index 80499f4..f05ab8f 100644 --- a/drivers/android/binder.c +++ b/drivers/android/binder.c @@ -581,6 +581,12 @@ static int binder_update_page_range(struct binder_proc *proc, int allocate, if (mm) { down_write(&mm->mmap_sem); + if (!mmget_still_valid(mm)) { + if (allocate == 0) + goto free_range; + goto err_no_vma; + } + vma = proc->vma; if (vma && mm != proc->vma_vm_mm) { pr_err("%d: vma mm and task mm mismatch\n", diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 5138e78..4b207b1 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1057,6 +1057,24 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf, count = -EINTR; goto out_mm; } + /* + * Avoid to modify vma->vm_flags + * without locked ops while the + * coredump reads the vm_flags. + */ + if (!mmget_still_valid(mm)) { + /* + * Silently return "count" + * like if get_task_mm() + * failed. FIXME: should this + * function have returned + * -ESRCH if get_task_mm() + * failed like if + * get_proc_task() fails? + */ + up_write(&mm->mmap_sem); + goto out_mm; + } for (vma = mm->mmap; vma; vma = vma->vm_next) { vma->vm_flags &= ~VM_SOFTDIRTY; vma_set_page_prot(vma); diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 784d667..8bf425a 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -479,6 +479,8 @@ static int userfaultfd_release(struct inode *inode, struct file *file) * taking the mmap_sem for writing. */ down_write(&mm->mmap_sem); + if (!mmget_still_valid(mm)) + goto skip_mm; prev = NULL; for (vma = mm->mmap; vma; vma = vma->vm_next) { cond_resched(); @@ -501,6 +503,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file) vma->vm_flags = new_flags; vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; } +skip_mm: up_write(&mm->mmap_sem); mmput(mm); wakeup: @@ -802,6 +805,9 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx, goto out; down_write(&mm->mmap_sem); + if (!mmget_still_valid(mm)) + goto out_unlock; + vma = find_vma_prev(mm, start, &prev); if (!vma) goto out_unlock; @@ -947,6 +953,9 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, goto out; down_write(&mm->mmap_sem); + if (!mmget_still_valid(mm)) + goto out_unlock; + vma = find_vma_prev(mm, start, &prev); if (!vma) goto out_unlock; diff --git a/include/linux/mm.h b/include/linux/mm.h index e3c8d40..c239984 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1189,6 +1189,27 @@ void zap_page_range(struct vm_area_struct *vma, unsigned long address, void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma, unsigned long start, unsigned long end); +/* + * This has to be called after a get_task_mm()/mmget_not_zero() + * followed by taking the mmap_sem for writing before modifying the + * vmas or anything the coredump pretends not to change from under it. + * + * NOTE: find_extend_vma() called from GUP context is the only place + * that can modify the "mm" (notably the vm_start/end) under mmap_sem + * for reading and outside the context of the process, so it is also + * the only case that holds the mmap_sem for reading that must call + * this function. Generally if the mmap_sem is hold for reading + * there's no need of this check after get_task_mm()/mmget_not_zero(). + * + * This function can be obsoleted and the check can be removed, after + * the coredump code will hold the mmap_sem for writing before + * invoking the ->core_dump methods. + */ +static inline bool mmget_still_valid(struct mm_struct *mm) +{ + return likely(!mm->core_state); +} + /** * mm_walk - callbacks for walk_page_range * @pmd_entry: if set, called for each non-empty PMD (3rd-level) entry diff --git a/mm/mmap.c b/mm/mmap.c index 3f2314a..19368fb 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2448,7 +2448,8 @@ find_extend_vma(struct mm_struct *mm, unsigned long addr) vma = find_vma_prev(mm, addr, &prev); if (vma && (vma->vm_start <= addr)) return vma; - if (!prev || expand_stack(prev, addr)) + /* don't alter vm_end if the coredump is running */ + if (!prev || !mmget_still_valid(mm) || expand_stack(prev, addr)) return NULL; if (prev->vm_flags & VM_LOCKED) populate_vma_page_range(prev, addr, prev->vm_end, NULL); @@ -2474,6 +2475,9 @@ find_extend_vma(struct mm_struct *mm, unsigned long addr) return vma; if (!(vma->vm_flags & VM_GROWSDOWN)) return NULL; + /* don't alter vm_start if the coredump is running */ + if (!mmget_still_valid(mm)) + return NULL; start = vma->vm_start; if (expand_stack(vma, addr)) return NULL;