| Message ID | 20220517180420.87954-1-tadeusz.struk@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Delegated to: | BPF |
| Headers | show |
| Series | [v4] bpf: Fix KASAN use-after-free Read in compute_effective_progs | expand |
On 5/17/22 11:04, Tadeusz Struk wrote: > Syzbot found a Use After Free bug in compute_effective_progs(). > The reproducer creates a number of BPF links, and causes a fault > injected alloc to fail, while calling bpf_link_detach on them. > Link detach triggers the link to be freed by bpf_link_free(), > which calls __cgroup_bpf_detach() and update_effective_progs(). > If the memory allocation in this function fails, the function restores > the pointer to the bpf_cgroup_link on the cgroup list, but the memory > gets freed just after it returns. After this, every subsequent call to > update_effective_progs() causes this already deallocated pointer to be > dereferenced in prog_list_length(), and triggers KASAN UAF error. > > To fix this issue don't preserve the pointer to the prog or link in the > list, but remove it and replace it with a dummy prog without shrinking > the table. The subsequent call to __cgroup_bpf_detach() or > __cgroup_bpf_detach() will correct it. > > Cc: "Alexei Starovoitov" <ast@kernel.org> > Cc: "Daniel Borkmann" <daniel@iogearbox.net> > Cc: "Andrii Nakryiko" <andrii@kernel.org> > Cc: "Martin KaFai Lau" <kafai@fb.com> > Cc: "Song Liu" <songliubraving@fb.com> > Cc: "Yonghong Song" <yhs@fb.com> > Cc: "John Fastabend" <john.fastabend@gmail.com> > Cc: "KP Singh" <kpsingh@kernel.org> > Cc: <netdev@vger.kernel.org> > Cc: <bpf@vger.kernel.org> > Cc: <stable@vger.kernel.org> > Cc: <linux-kernel@vger.kernel.org> > > Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4 > Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment") > Reported-by: <syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com> > Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org> > --- > v2: Add a fall back path that removes a prog from the effective progs > table in case detach fails to allocate memory in compute_effective_progs(). > > v3: Implement the fallback in a separate function purge_effective_progs > > v4: Changed purge_effective_progs() to manipulate the array in a similar way > how replace_effective_prog() does it. > --- > kernel/bpf/cgroup.c | 68 +++++++++++++++++++++++++++++++++++++++------ > 1 file changed, 60 insertions(+), 8 deletions(-) > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c > index 128028efda64..6f1a6160c99e 100644 > --- a/kernel/bpf/cgroup.c > +++ b/kernel/bpf/cgroup.c > @@ -681,6 +681,60 @@ static struct bpf_prog_list *find_detach_entry(struct list_head *progs, > return ERR_PTR(-ENOENT); > } > > +/** > + * purge_effective_progs() - After compute_effective_progs fails to alloc new > + * cgrp->bpf.inactive table we can recover by > + * recomputing the array in place. > + * > + * @cgrp: The cgroup which descendants to travers > + * @prog: A program to detach or NULL > + * @link: A link to detach or NULL > + * @atype: Type of detach operation > + */ > +static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog, > + struct bpf_cgroup_link *link, > + enum cgroup_bpf_attach_type atype) > +{ > + struct cgroup_subsys_state *css; > + struct bpf_prog_array *progs; > + struct bpf_prog_list *pl; > + struct list_head *head; > + struct cgroup *cg; > + int pos; > + > + /* recompute effective prog array in place */ > + css_for_each_descendant_pre(css, &cgrp->self) { > + struct cgroup *desc = container_of(css, struct cgroup, self); > + > + if (percpu_ref_is_zero(&desc->bpf.refcnt)) > + continue; > + > + /* find position of link or prog in effective progs array */ > + for (pos = 0, cg = desc; cg; cg = cgroup_parent(cg)) { > + if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI)) > + continue; > + > + head = &cg->bpf.progs[atype]; > + list_for_each_entry(pl, head, node) { > + if (!prog_list_prog(pl)) > + continue; > + if (pl->prog == prog && pl->link == link) > + goto found; > + pos++; > + } > + } > +found: > + BUG_ON(!cg); > + progs = rcu_dereference_protected( > + desc->bpf.effective[atype], > + lockdep_is_held(&cgroup_mutex)); > + > + /* Remove the program from the array */ > + WARN_ONCE(bpf_prog_array_delete_safe_at(progs, pos), > + "Failed to purge a prog from array at index %d", pos); > + } > +} > + > /** > * __cgroup_bpf_detach() - Detach the program or link from a cgroup, and > * propagate the change to descendants > @@ -723,8 +777,12 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, > pl->link = NULL; > > err = update_effective_progs(cgrp, atype); > - if (err) > - goto cleanup; > + if (err) { > + /* If update affective array failed replace the prog with a dummy prog*/ > + pl->prog = old_prog; > + pl->link = link; > + purge_effective_progs(cgrp, old_prog, link, atype); > + } > > /* now can actually delete it from this cgroup list */ > list_del(&pl->node); > @@ -736,12 +794,6 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, > bpf_prog_put(old_prog); > static_branch_dec(&cgroup_bpf_enabled_key[atype]); > return 0; > - > -cleanup: > - /* restore back prog or link */ > - pl->prog = old_prog; > - pl->link = link; > - return err; > } > > static int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, Hi Andrii, Do you have any more feedback? Does it look better to you now?
On Mon, May 23, 2022 at 2:36 PM Tadeusz Struk <tadeusz.struk@linaro.org> wrote: > > On 5/17/22 11:04, Tadeusz Struk wrote: > > Syzbot found a Use After Free bug in compute_effective_progs(). > > The reproducer creates a number of BPF links, and causes a fault > > injected alloc to fail, while calling bpf_link_detach on them. > > Link detach triggers the link to be freed by bpf_link_free(), > > which calls __cgroup_bpf_detach() and update_effective_progs(). > > If the memory allocation in this function fails, the function restores > > the pointer to the bpf_cgroup_link on the cgroup list, but the memory > > gets freed just after it returns. After this, every subsequent call to > > update_effective_progs() causes this already deallocated pointer to be > > dereferenced in prog_list_length(), and triggers KASAN UAF error. > > > > To fix this issue don't preserve the pointer to the prog or link in the > > list, but remove it and replace it with a dummy prog without shrinking > > the table. The subsequent call to __cgroup_bpf_detach() or > > __cgroup_bpf_detach() will correct it. > > > > Cc: "Alexei Starovoitov" <ast@kernel.org> > > Cc: "Daniel Borkmann" <daniel@iogearbox.net> > > Cc: "Andrii Nakryiko" <andrii@kernel.org> > > Cc: "Martin KaFai Lau" <kafai@fb.com> > > Cc: "Song Liu" <songliubraving@fb.com> > > Cc: "Yonghong Song" <yhs@fb.com> > > Cc: "John Fastabend" <john.fastabend@gmail.com> > > Cc: "KP Singh" <kpsingh@kernel.org> > > Cc: <netdev@vger.kernel.org> > > Cc: <bpf@vger.kernel.org> > > Cc: <stable@vger.kernel.org> > > Cc: <linux-kernel@vger.kernel.org> > > > > Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4 > > Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment") > > Reported-by: <syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com> > > Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org> > > --- > > v2: Add a fall back path that removes a prog from the effective progs > > table in case detach fails to allocate memory in compute_effective_progs(). > > > > v3: Implement the fallback in a separate function purge_effective_progs > > > > v4: Changed purge_effective_progs() to manipulate the array in a similar way > > how replace_effective_prog() does it. > > --- > > kernel/bpf/cgroup.c | 68 +++++++++++++++++++++++++++++++++++++++------ > > 1 file changed, 60 insertions(+), 8 deletions(-) > > > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c > > index 128028efda64..6f1a6160c99e 100644 > > --- a/kernel/bpf/cgroup.c > > +++ b/kernel/bpf/cgroup.c > > @@ -681,6 +681,60 @@ static struct bpf_prog_list *find_detach_entry(struct list_head *progs, > > return ERR_PTR(-ENOENT); > > } > > > > +/** > > + * purge_effective_progs() - After compute_effective_progs fails to alloc new > > + * cgrp->bpf.inactive table we can recover by > > + * recomputing the array in place. > > + * > > + * @cgrp: The cgroup which descendants to travers > > + * @prog: A program to detach or NULL > > + * @link: A link to detach or NULL > > + * @atype: Type of detach operation > > + */ > > +static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog, > > + struct bpf_cgroup_link *link, > > + enum cgroup_bpf_attach_type atype) > > +{ > > + struct cgroup_subsys_state *css; > > + struct bpf_prog_array *progs; > > + struct bpf_prog_list *pl; > > + struct list_head *head; > > + struct cgroup *cg; > > + int pos; > > + > > + /* recompute effective prog array in place */ > > + css_for_each_descendant_pre(css, &cgrp->self) { > > + struct cgroup *desc = container_of(css, struct cgroup, self); > > + > > + if (percpu_ref_is_zero(&desc->bpf.refcnt)) > > + continue; > > + > > + /* find position of link or prog in effective progs array */ > > + for (pos = 0, cg = desc; cg; cg = cgroup_parent(cg)) { > > + if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI)) > > + continue; > > + > > + head = &cg->bpf.progs[atype]; > > + list_for_each_entry(pl, head, node) { > > + if (!prog_list_prog(pl)) > > + continue; > > + if (pl->prog == prog && pl->link == link) > > + goto found; > > + pos++; > > + } > > + } > > +found: > > + BUG_ON(!cg); > > + progs = rcu_dereference_protected( > > + desc->bpf.effective[atype], > > + lockdep_is_held(&cgroup_mutex)); > > + > > + /* Remove the program from the array */ > > + WARN_ONCE(bpf_prog_array_delete_safe_at(progs, pos), > > + "Failed to purge a prog from array at index %d", pos); > > + } > > +} > > + > > /** > > * __cgroup_bpf_detach() - Detach the program or link from a cgroup, and > > * propagate the change to descendants > > @@ -723,8 +777,12 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, > > pl->link = NULL; > > > > err = update_effective_progs(cgrp, atype); > > - if (err) > > - goto cleanup; > > + if (err) { > > + /* If update affective array failed replace the prog with a dummy prog*/ > > + pl->prog = old_prog; > > + pl->link = link; > > + purge_effective_progs(cgrp, old_prog, link, atype); > > + } > > > > /* now can actually delete it from this cgroup list */ > > list_del(&pl->node); > > @@ -736,12 +794,6 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, > > bpf_prog_put(old_prog); > > static_branch_dec(&cgroup_bpf_enabled_key[atype]); > > return 0; > > - > > -cleanup: > > - /* restore back prog or link */ > > - pl->prog = old_prog; > > - pl->link = link; > > - return err; > > } > > > > static int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, > > Hi Andrii, > Do you have any more feedback? Does it look better to you now? Hi, this is on my TODO list, but I need a bit more focused time to think all this through and I haven't managed to get it in last week. I'm worried about the percpu_ref_is_zero(&desc->bpf.refcnt) portion and whether it can cause some skew in the calculated array index, I need to look at this a bit more in depth. Sorry for the delay. > -- > Thanks, > Tadeusz
On 5/23/22 15:47, Andrii Nakryiko wrote: >> Hi Andrii, >> Do you have any more feedback? Does it look better to you now? > Hi, this is on my TODO list, but I need a bit more focused time to > think all this through and I haven't managed to get it in last week. > I'm worried about the percpu_ref_is_zero(&desc->bpf.refcnt) portion > and whether it can cause some skew in the calculated array index, I > need to look at this a bit more in depth. Sorry for the delay. That's fine. take your time and let me know if there is anything else to change/improve. FWIW I tested it extensively with the syzbot repro and the issue doesn't trigger anymore.
Hi Andrii, On 5/23/22 15:47, Andrii Nakryiko wrote: >> Hi Andrii, >> Do you have any more feedback? Does it look better to you now? > Hi, this is on my TODO list, but I need a bit more focused time to > think all this through and I haven't managed to get it in last week. > I'm worried about the percpu_ref_is_zero(&desc->bpf.refcnt) portion > and whether it can cause some skew in the calculated array index, I > need to look at this a bit more in depth. Sorry for the delay. Did you get a chance to look at this yet?
On Thu, Jun 2, 2022 at 7:37 AM Tadeusz Struk <tadeusz.struk@linaro.org> wrote: > > Hi Andrii, > On 5/23/22 15:47, Andrii Nakryiko wrote: > >> Hi Andrii, > >> Do you have any more feedback? Does it look better to you now? > > Hi, this is on my TODO list, but I need a bit more focused time to > > think all this through and I haven't managed to get it in last week. > > I'm worried about the percpu_ref_is_zero(&desc->bpf.refcnt) portion > > and whether it can cause some skew in the calculated array index, I > > need to look at this a bit more in depth. Sorry for the delay. > > Did you get a chance to look at this yet? > Hm.. I've applied it two days ago, but for some reason there was no notification from the bot. It's now c89c79fda9b6 ("bpf: Fix KASAN use-after-free Read in compute_effective_progs"). > -- > Thanks, > Tadeusz
On 6/2/22 09:11, Andrii Nakryiko wrote: >> Did you get a chance to look at this yet? >> > Hm.. I've applied it two days ago, but for some reason there was no > notification from the bot. It's now c89c79fda9b6 ("bpf: Fix KASAN > use-after-free Read in compute_effective_progs"). Great! Thank you.
On 6/2/22 09:25, Tadeusz Struk wrote: > On 6/2/22 09:11, Andrii Nakryiko wrote: >>> Did you get a chance to look at this yet? >>> >> Hm.. I've applied it two days ago, but for some reason there was no >> notification from the bot. It's now c89c79fda9b6 ("bpf: Fix KASAN >> use-after-free Read in compute_effective_progs"). FYI. Just requested a test on bpf-next and it passed fine. https://groups.google.com/g/syzkaller-android-bugs/c/nr6mD4vhRA4
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c index 128028efda64..6f1a6160c99e 100644 --- a/kernel/bpf/cgroup.c +++ b/kernel/bpf/cgroup.c @@ -681,6 +681,60 @@ static struct bpf_prog_list *find_detach_entry(struct list_head *progs, return ERR_PTR(-ENOENT); } +/** + * purge_effective_progs() - After compute_effective_progs fails to alloc new + * cgrp->bpf.inactive table we can recover by + * recomputing the array in place. + * + * @cgrp: The cgroup which descendants to travers + * @prog: A program to detach or NULL + * @link: A link to detach or NULL + * @atype: Type of detach operation + */ +static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog, + struct bpf_cgroup_link *link, + enum cgroup_bpf_attach_type atype) +{ + struct cgroup_subsys_state *css; + struct bpf_prog_array *progs; + struct bpf_prog_list *pl; + struct list_head *head; + struct cgroup *cg; + int pos; + + /* recompute effective prog array in place */ + css_for_each_descendant_pre(css, &cgrp->self) { + struct cgroup *desc = container_of(css, struct cgroup, self); + + if (percpu_ref_is_zero(&desc->bpf.refcnt)) + continue; + + /* find position of link or prog in effective progs array */ + for (pos = 0, cg = desc; cg; cg = cgroup_parent(cg)) { + if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI)) + continue; + + head = &cg->bpf.progs[atype]; + list_for_each_entry(pl, head, node) { + if (!prog_list_prog(pl)) + continue; + if (pl->prog == prog && pl->link == link) + goto found; + pos++; + } + } +found: + BUG_ON(!cg); + progs = rcu_dereference_protected( + desc->bpf.effective[atype], + lockdep_is_held(&cgroup_mutex)); + + /* Remove the program from the array */ + WARN_ONCE(bpf_prog_array_delete_safe_at(progs, pos), + "Failed to purge a prog from array at index %d", pos); + } +} + /** * __cgroup_bpf_detach() - Detach the program or link from a cgroup, and * propagate the change to descendants @@ -723,8 +777,12 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, pl->link = NULL; err = update_effective_progs(cgrp, atype); - if (err) - goto cleanup; + if (err) { + /* If update affective array failed replace the prog with a dummy prog*/ + pl->prog = old_prog; + pl->link = link; + purge_effective_progs(cgrp, old_prog, link, atype); + } /* now can actually delete it from this cgroup list */ list_del(&pl->node); @@ -736,12 +794,6 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, bpf_prog_put(old_prog); static_branch_dec(&cgroup_bpf_enabled_key[atype]); return 0; - -cleanup: - /* restore back prog or link */ - pl->prog = old_prog; - pl->link = link; - return err; } static int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
Syzbot found a Use After Free bug in compute_effective_progs(). The reproducer creates a number of BPF links, and causes a fault injected alloc to fail, while calling bpf_link_detach on them. Link detach triggers the link to be freed by bpf_link_free(), which calls __cgroup_bpf_detach() and update_effective_progs(). If the memory allocation in this function fails, the function restores the pointer to the bpf_cgroup_link on the cgroup list, but the memory gets freed just after it returns. After this, every subsequent call to update_effective_progs() causes this already deallocated pointer to be dereferenced in prog_list_length(), and triggers KASAN UAF error. To fix this issue don't preserve the pointer to the prog or link in the list, but remove it and replace it with a dummy prog without shrinking the table. The subsequent call to __cgroup_bpf_detach() or __cgroup_bpf_detach() will correct it. Cc: "Alexei Starovoitov" <ast@kernel.org> Cc: "Daniel Borkmann" <daniel@iogearbox.net> Cc: "Andrii Nakryiko" <andrii@kernel.org> Cc: "Martin KaFai Lau" <kafai@fb.com> Cc: "Song Liu" <songliubraving@fb.com> Cc: "Yonghong Song" <yhs@fb.com> Cc: "John Fastabend" <john.fastabend@gmail.com> Cc: "KP Singh" <kpsingh@kernel.org> Cc: <netdev@vger.kernel.org> Cc: <bpf@vger.kernel.org> Cc: <stable@vger.kernel.org> Cc: <linux-kernel@vger.kernel.org> Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4 Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment") Reported-by: <syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org> --- v2: Add a fall back path that removes a prog from the effective progs table in case detach fails to allocate memory in compute_effective_progs(). v3: Implement the fallback in a separate function purge_effective_progs v4: Changed purge_effective_progs() to manipulate the array in a similar way how replace_effective_prog() does it. --- kernel/bpf/cgroup.c | 68 +++++++++++++++++++++++++++++++++++++++------ 1 file changed, 60 insertions(+), 8 deletions(-)