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Miller" , Jakub Kicinski , Paolo Abeni , Simon Horman , Alexei Starovoitov , Daniel Borkmann , netdev@vger.kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH bpf v4] bpf, sockmap: Fix cork use-after-free in tcp_bpf_sendmsg() In-Reply-To: <20260724103856.3399001-1-nicoyip.dev@gmail.com> (Chengfeng Ye's message of "Fri, 24 Jul 2026 18:38:56 +0800") References: <87fr18lmzo.fsf@cloudflare.com> <20260724103856.3399001-1-nicoyip.dev@gmail.com> User-Agent: mu4e 1.14.1; emacs 30.2 Date: Fri, 24 Jul 2026 12:48:22 +0200 Message-ID: <87a4rgll3d.fsf@cloudflare.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain On Fri, Jul 24, 2026 at 06:38 PM +08, 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. 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") > Reviewed-by: Emil Tsalapatis > Signed-off-by: Chengfeng Ye > --- Thanks! Reviewed-by: Jakub Sitnicki