On Mon, May 23, 2022 at 2:36 PM Tadeusz Struk <tadeusz.struk@xxxxxxxxxx> 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@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> > > --- > > 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
