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

[Weiming Shi <bestswngs@gmail.com>]

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 <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 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