Re: [PATCH net v4 3/3] net/smc: bound the send length to the send buffer in smc_tx_sendmsg()

[Dust Li]

On 2026-07-05 02:54:07, Bryam Vargas via B4 Relay wrote:
>From: Bryam Vargas <hexlabsecurity@xxxxxxxxx>
>
>On the SMC-D DMB-merge (nocopy) path, smc_cdc_msg_recv_action()
>advances conn->sndbuf_space from the peer's wire-controlled consumer
>cursor via smc_curs_diff(), which can return more than sndbuf_desc->len;
>a forged cursor drives sndbuf_space past the send buffer, and over many
>CDC messages overflows the signed counter negative. smc_tx_sendmsg()
>reads it as the write space and does a wrap-around copy whose second
>chunk is not re-bounded to sndbuf_desc->len, spilling the local
>sender's outbound data past the send buffer at a peer-controlled
>length: a heap out-of-bounds write. The nearby len > sndbuf_desc->len
>test only feeds SMC_STAT_RMB_TX_SIZE_SMALL on the user length; it does
>not bound the copy.
>
>Bound the write space to sndbuf_desc->len at the consumer, treating a
>negative (sign-overflowed) value as out of range too, so the copy can
>never exceed the ring. This enforces the documented
>0 <= sndbuf_space <= sndbuf_desc->len invariant where it is race-free
>against the CDC tasklet; conforming peers are unaffected.
>
>Fixes: cc0ab806fc52 ("net/smc: adapt cursor update when sndbuf and peer DMB are merged")
>Cc: stable@xxxxxxxxxxxxxxx
>Signed-off-by: Bryam Vargas <hexlabsecurity@xxxxxxxxx>

Reviewed-by: Dust Li <dust.li@xxxxxxxxxxxxxxxxx>

Best regards,
Dust


>---
> net/smc/smc_tx.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
>diff --git a/net/smc/smc_tx.c b/net/smc/smc_tx.c
>index 3144b4b1fe29..5916f02060fb 100644
>--- a/net/smc/smc_tx.c
>+++ b/net/smc/smc_tx.c
>@@ -233,6 +233,19 @@ int smc_tx_sendmsg(struct smc_sock *smc, struct msghdr *msg, size_t len)
> 		/* initialize variables for 1st iteration of subsequent loop */
> 		/* could be just 1 byte, even after smc_tx_wait above */
> 		writespace = atomic_read(&conn->sndbuf_space);
>+		/* sndbuf_space is advanced from the peer's wire-controlled
>+		 * consumer cursor on the SMC-D DMB-merge path; a forged cursor
>+		 * can inflate it past the send buffer, or overflow the signed
>+		 * accumulator to a negative value across many CDC messages
>+		 * (which a plain "> len" check would miss before the size_t
>+		 * cast below turns it huge).  Bound it to the send buffer in
>+		 * either case so the wrap-around write cannot run past
>+		 * sndbuf_desc->len.  This enforces the documented
>+		 * 0 <= sndbuf_space <= sndbuf_desc->len invariant at the
>+		 * producer, race-free against the CDC tasklet.
>+		 */
>+		if (writespace < 0 || writespace > conn->sndbuf_desc->len)
>+			writespace = conn->sndbuf_desc->len;
> 		/* not more than what user space asked for */
> 		copylen = min_t(size_t, send_remaining, writespace);
> 		/* determine start of sndbuf */
>
>-- 
>2.43.0
>