Re: [PATCH net] bpf: test_run: fix WARNING in format_decode

[Daniel Borkmann]

On 11/22/23 6:28 AM, Yonghong Song wrote:
> On 11/21/23 7:50 PM, Edward Adam Davis wrote:
> > Confirm that skb->len is not 0 to ensure that skb length is valid.
> >
> > Fixes: 114039b34201 ("bpf: Move skb->len == 0 checks into __bpf_redirect")
> > Reported-by: syzbot+e2c932aec5c8a6e1d31c@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
>
> Stan, Could you take a look at this patch?

I think this only papers over the bug.. also BPF selftests seem to break
with this change.

Looking again at the syzkaller trace :

  [...]
  Please remove unsupported %\0 in format string
  WARNING: CPU: 0 PID: 5068 at lib/vsprintf.c:2675 format_decode+0xa03/0xba0 lib/vsprintf.c:2675
  [...]

We need to fix bpf_bprintf_prepare() instead to reject invalid fmts such
as %0 and similar.

> >   net/bpf/test_run.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> > index c9fdcc5cdce1..78258a822a5c 100644
> > --- a/net/bpf/test_run.c
> > +++ b/net/bpf/test_run.c
> > @@ -845,6 +845,9 @@ static int convert___skb_to_skb(struct sk_buff *skb, struct __sk_buff *__skb)
> >   {
> >       struct qdisc_skb_cb *cb = (struct qdisc_skb_cb *)skb->cb;
> > +    if (!skb->len)
> > +        return -EINVAL;
> > +
> >       if (!__skb)
> >           return 0;