Re: [PATCH] bpf, sockmap: Fix cork use-after-free in tcp_bpf_sendmsg()
From: "Emil Tsalapatis" <emil@etsalapatis.com>
Date: 2026-07-20 20:12:20
Also in:
bpf, lkml, stable
On Sun Jul 19, 2026 at 12:16 PM EDT, Chengfeng Ye wrote:
tcp_bpf_sendmsg() keeps msg_tx across sk_stream_wait_memory(), which
drops and reacquires the socket lock. Its error path tries to decide
whether msg_tx names the local temporary message by comparing it with
the current value of psock->cork.
This comparison is unsafe when two threads send on the same socket:
Thread A Thread B
msg_tx = psock->cork
sk_msg_alloc() fails
sk_stream_wait_memory()
releases the socket lock acquires the socket lock
completes the cork
psock->cork = NULL
frees the cork
reacquires the socket lock
msg_tx != psock->cork
sk_msg_free(msg_tx)
The stale cork is therefore mistaken for the local temporary message
and freed again. KASAN reported:
BUG: KASAN: slab-use-after-free in sk_msg_free+0x49/0x50
Read of size 4 at addr ffff88810c908800 by task poc/90
Call Trace:
sk_msg_free+0x49/0x50
tcp_bpf_sendmsg+0x14f5/0x1cc0
__sys_sendto+0x32c/0x3a0
__x64_sys_sendto+0xdb/0x1b0
Allocated by task 89:
__kasan_kmalloc+0x8f/0xa0
tcp_bpf_sendmsg+0x16b3/0x1cc0
Freed by task 91:
__kasan_slab_free+0x43/0x70
kfree+0x131/0x3c0
tcp_bpf_sendmsg+0xec3/0x1cc0
msg_tx can only name the stack-local tmp or the shared cork. Test for
tmp directly so a changed psock->cork cannot turn a shared message into
an apparent local one.
Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <redacted>
---Hi Chengfeng, The patch looks good: Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com> There is one caveat: Normally we ignore pre-existing issues Sashiko finds while reviewing the patch that are unrelated to the change itself. For this function, however, I think we should make an exception because it has multiple glaring issues we can fix more cleanly if we do it all at once. E.g., tmp never gets cleaned up even if there are allocations hanging off of it. Would you be willing to expand the patch that addresses the Sashiko comments, even if unrelated to your fix? That would save us the time to review the inevitable followups and provide more coherent refactoring.
quoted hunk ↗ jump to hunk
net/ipv4/tcp_bpf.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c index 8e905b50dead..a30475afb6f8 100644 --- a/net/ipv4/tcp_bpf.c +++ b/net/ipv4/tcp_bpf.c@@ -604,7 +604,7 @@ static int tcp_bpf_sendmsg(struct sock *sk, struct msghdr *msg, size_t size) wait_for_memory: err = sk_stream_wait_memory(sk, &timeo); if (err) { - if (msg_tx && msg_tx != psock->cork) + if (msg_tx == &tmp) sk_msg_free(sk, msg_tx); goto out_err; }