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

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>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://lore.kernel.org/r/20260615021959.140010-3-jiayuan.chen@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/net/core/filter.c b/net/core/filter.c
index 4b159045881ddc71878b9f9c20037ec197438439..978e740792beb3b2b6f951fb1d8c507f6543ac09 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2872,7 +2872,7 @@ 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);