bpf, sockmap: Fix cork use-after-free in tcp_bpf_sendmsg()

bpf, sockmap: Fix cork use-after-free in tcp_bpf_sendmsg()

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. Check 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")
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/87fr18lmzo.fsf%40cloudflare.com/
Link: https://lore.kernel.org/netdev/20260719161630.2901208-1-nicoyip.dev%40gmail.com/
Link: https://patch.msgid.link/20260724103856.3399001-1-nicoyip.dev@gmail.com
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>