On Tue, Apr 5, 2022 at 10:04 AM Tadeusz Struk <tadeusz.struk@xxxxxxxxxx> 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 don't preserve the pointer to the link on the cgroup list > in __cgroup_bpf_detach(), but proceed with the cleanup and retry calling > update_effective_progs() again afterwards. I think it's still problematic. BPF link might have been the last one that holds bpf_prog's refcnt, so when link is put, its prog can stay there in effective_progs array(s) and will cause use-after-free anyways. It would be best to make sure that detach never fails. On detach effective prog array can only shrink, so even if update_effective_progs() fails to allocate memory, we should be able to iterate and just replace that prog with NULL, as a fallback strategy. > > > Cc: "Alexei Starovoitov" <ast@xxxxxxxxxx> > Cc: "Daniel Borkmann" <daniel@xxxxxxxxxxxxx> > Cc: "Andrii Nakryiko" <andrii@xxxxxxxxxx> > Cc: "Martin KaFai Lau" <kafai@xxxxxx> > Cc: "Song Liu" <songliubraving@xxxxxx> > Cc: "Yonghong Song" <yhs@xxxxxx> > Cc: "John Fastabend" <john.fastabend@xxxxxxxxx> > Cc: "KP Singh" <kpsingh@xxxxxxxxxx> > Cc: <netdev@xxxxxxxxxxxxxxx> > Cc: <bpf@xxxxxxxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > Cc: <linux-kernel@xxxxxxxxxxxxxxx> > > Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4 > Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment") > Reported-by: <syzbot+f264bffdfbd5614f3bb2@xxxxxxxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx> > --- > kernel/bpf/cgroup.c | 25 ++++++++++++++----------- > 1 file changed, 14 insertions(+), 11 deletions(-) > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c > index 128028efda64..b6307337a3c7 100644 > --- a/kernel/bpf/cgroup.c > +++ b/kernel/bpf/cgroup.c > @@ -723,10 +723,11 @@ 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; > - > - /* now can actually delete it from this cgroup list */ > + /* > + * Proceed regardless of error. The link and/or prog will be freed > + * just after this function returns so just delete it from this > + * cgroup list and retry calling update_effective_progs again later. > + */ > list_del(&pl->node); > kfree(pl); > if (list_empty(progs)) > @@ -735,12 +736,11 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, > if (old_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; > + /* In case of error call update_effective_progs again */ > + if (err) > + err = update_effective_progs(cgrp, atype); there is no guarantee that this will now succeed, right? so it's more like "let's try just in case we are lucky this time"? > + > return err; > } > > @@ -881,6 +881,7 @@ static void bpf_cgroup_link_release(struct bpf_link *link) > struct bpf_cgroup_link *cg_link = > container_of(link, struct bpf_cgroup_link, link); > struct cgroup *cg; > + int err; > > /* link might have been auto-detached by dying cgroup already, > * in that case our work is done here > @@ -896,8 +897,10 @@ static void bpf_cgroup_link_release(struct bpf_link *link) > return; > } > > - WARN_ON(__cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link, > - cg_link->type)); > + err = __cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link, > + cg_link->type); > + if (err) > + pr_warn("cgroup_bpf_detach() failed, err %d\n", err); > > cg = cg_link->cgroup; > cg_link->cgroup = NULL; > -- > 2.35.1
