netdev - Re: [PATCH net] bpf: test_run: fix WARNING in format

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Date: Mon, 27 Nov 2023 15:10:52 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Yonghong Song <yonghong.song@...ux.dev>,
 Edward Adam Davis <eadavis@...com>,
 syzbot+e2c932aec5c8a6e1d31c@...kaller.appspotmail.com
Cc: andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
 davem@...emloft.net, edumazet@...gle.com, haoluo@...gle.com,
 hawk@...nel.org, john.fastabend@...il.com, jolsa@...nel.org,
 kpsingh@...nel.org, kuba@...nel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, martin.lau@...nel.org,
 martin.lau@...ux.dev, mhiramat@...nel.org, netdev@...r.kernel.org,
 pabeni@...hat.com, rostedt@...dmis.org, sdf@...gle.com, song@...nel.org,
 syzkaller-bugs@...glegroups.com, yhs@...com
Subject: Re: [PATCH net] bpf: test_run: fix WARNING in format_decode

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@...kaller.appspotmail.com
>> Signed-off-by: Edward Adam Davis <eadavis@...com>
> 
> 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;