Subject: Re: [PATCH bpf] bpf, sockmap: Fix wrong rsge offset in bpf_msg_push_data()

From: Jiayuan Chen <[email-masked]>
Date: Sat, 25 Apr 2026 16:25:15 +0800

On 4/23/26 11:58 PM, Weiming Shi wrote:
> When bpf_msg_push_data() splits a scatterlist element into head and
> tail, the tail's page offset is advanced by `start` (absolute message
> byte offset) instead of `start - offset` (byte position within the
> element). This makes rsge.offset overshoot by `offset` bytes, pointing
> to the wrong location within the page or beyond its boundary. Consumers
> of the corrupted entry either silently read wrong data or trigger an
> out-of-bounds access.
>
>   BUG: KASAN: slab-use-after-free in bpf_msg_pull_data (net/core/filter.c:2728)
>   Read of size 32752 at addr ffff8881042f0010 by task poc/130
>   Call Trace:
>    __asan_memcpy (mm/kasan/shadow.c:105)
>    bpf_msg_pull_data (net/core/filter.c:2728)
>    bpf_prog_run_pin_on_cpu (include/linux/bpf.h:1402)
>    sk_psock_msg_verdict (net/core/skmsg.c:934)
>    tcp_bpf_send_verdict (net/ipv4/tcp_bpf.c:421)
>    sock_sendmsg_nosec (net/socket.c:727)
>
> Fixes: 6fff607e2f14 ("bpf: sk_msg program helper bpf_msg_push_data")
> Reported-by: Xiang Mei <[email-masked]>
> Signed-off-by: Weiming Shi <[email-masked]>

Reviewed-by: Jiayuan Chen <[email-masked]>

Fix looks correct.
The bug only surfaces when the split branch is taken and start lands
inside a non-first SG element (offset > 0).

It took me 2 hours to reproduce it. Repos or self-tests are welcome next 
time you reported a bug...

> ---
>   net/core/filter.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 3e56b567bd18..f12fbc49bc03 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2865,7 +2865,7 @@ BPF_CALL_4(bpf_msg_push_data, struct sk_msg *, msg, u32, start,
>
>   		psge->length = start - offset;
>   		rsge.length -= psge->length;
> -		rsge.offset += start;
> +		rsge.offset += start - offset;
>
>   		sk_msg_iter_var_next(i);
>   		sg_unmark_end(psge);
> --
> 2.43.0