Re: [PATCH] bpf: cgroup: remove WARN_ON at bpf_cgroup_link_release

[Dongliang Mu]

On Wed, Mar 2, 2022 at 12:28 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 2/27/22 2:40 PM, Dongliang Mu wrote:
> > From: Dongliang Mu <mudongliangabcd@xxxxxxxxx>
> >
> > When syzkaller injects fault into memory allocation at
> > bpf_prog_array_alloc, the kernel encounters a memory failure and
> > returns non-zero, thus leading to one WARN_ON at
> > bpf_cgroup_link_release. The stack trace is as follows:
> >
> >   __kmalloc+0x7e/0x3d0
> >   bpf_prog_array_alloc+0x4f/0x60
> >   compute_effective_progs+0x132/0x580
> >   ? __sanitizer_cov_trace_pc+0x1a/0x40
> >   update_effective_progs+0x5e/0x260
> >   __cgroup_bpf_detach+0x293/0x760
> >   bpf_cgroup_link_release+0xad/0x400
> >   bpf_link_free+0xca/0x190
> >   bpf_link_put+0x161/0x1b0
> >   bpf_link_release+0x33/0x40
> >   __fput+0x286/0x9f0
> >
> > Fix this by removing the WARN_ON for __cgroup_bpf_detach.
> >
> > Reported-by: syzkaller <syzkaller@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Dongliang Mu <mudongliangabcd@xxxxxxxxx>
> > ---
> >   kernel/bpf/cgroup.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > index 514b4681a90a..fdbdcee6c9fa 100644
> > --- a/kernel/bpf/cgroup.c
> > +++ b/kernel/bpf/cgroup.c
> > @@ -896,8 +896,8 @@ 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));
> > +     __cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link,
> > +                                 cg_link->type);
>
> "Fixing" by removing WARN_ON is just papering over the issue which in this case as
> mentioned is allocation failure on detach/teardown when allocating and recomputing
> effective prog arrays..

Hi Daniel,

you're right. This is not a good fix, any idea to fix the underlying
bug perfectly?

>
> >       cg = cg_link->cgroup;
> >       cg_link->cgroup = NULL;
> >
>