Messages in this thread | | | Date | Mon, 23 May 2022 14:36:15 -0700 | Subject | Re: [PATCH v4] bpf: Fix KASAN use-after-free Read in compute_effective_progs | From | Tadeusz Struk <> |
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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? -- Thanks, Tadeusz
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