[PATCH net] net/smc: validate peer CDC cursor against RMBE size before accepting it

[Ibrahim Hashimov <security@auditcode.ai>]

smc_cdc_cursor_to_host() converts the wire-format producer/consumer
cursor of an incoming CDC message into a host smc_host_cursor. It rejects
a cursor that goes backwards, but never checks that the cursor's count is
inside the buffer it indexes. Per smc_host_cursor ("an offset in an
RMBE") and the invariant smc_curs_add() enforces for every local advance
(0 <= count < size), a valid cursor count must be < size; the wire cursor
is peer-controlled and was never checked against that.

smcr_cdc_msg_to_host() accepts the producer and consumer cursors, and the
unbounded value feeds smc_curs_diff() in smc_cdc_msg_recv_action(), which
computes new->count - old->count without clamping against size. A peer
that puts an out-of-range prod.count on the wire (e.g. 0x7fffffff) drives
bytes_to_rcv far past rmb_desc->len -- the very invariant the comment
above the atomic_add() claims but does not enforce.

smc_rx_recvmsg() then trusts bytes_to_rcv as the amount of valid RMB
data. Its first copy chunk is safely bounded by rmb_desc->len -
cons.count, but the second chunk copies (copylen - chunk_len) bytes from
offset 0, and copylen came from the inflated readable -- an out-of-bounds
read of the RMB's backing (v)malloc allocation, copied straight to the
receiving process via _copy_to_iter(). This is a remote kernel-memory
disclosure driven by a malicious SMC-R peer, needing no local privilege
on the victim. The same unbounded count also reaches
smc_cdc_handle_urg_data_arrival() (base + urg_curs.count - 1) -- a
second, narrower OOB read.

Fix it where the wire cursor is accepted, mirroring the cursor-sanity
idiom two lines above: reject any incoming cursor whose count is >= the
size of the buffer it will index. smc_cdc_cursor_to_host() gains a size
parameter; smcr_cdc_msg_to_host() passes conn->rmb_desc->len for the
producer cursor and conn->peer_rmbe_size for the consumer cursor. This
bounds it at its single origin instead of patching every downstream
consumer. SMC-D (smcd_cdc_msg_to_host()) does not use this helper and is
a separate, out-of-scope gap.

Reproduced on a v6.19 KASAN kernel (SMC-Rv2 over soft-RoCE): a peer
emitting prod.count=0x7fffffff triggers a vmalloc-out-of-bounds read in
_copy_to_iter() via smc_rx_recvmsg(), returning ~960 KB of RMB-adjacent
kernel heap to userspace; an in-range cursor transfers cleanly. With the
fix the out-of-range cursor is dropped before it reaches
conn->local_rx_ctrl.prod, and the reproducer returns rc=0 with no report.

Fixes: 5f08318f617b ("smc: connection data control (CDC)")
Cc: stable@vger.kernel.org
Signed-off-by: Ibrahim Hashimov <security@auditcode.ai>
Assisted-by: AuditCode-AI:2026.07
---
 net/smc/smc_cdc.h | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/net/smc/smc_cdc.h b/net/smc/smc_cdc.h
index 696cc11f2303..f07bbef47073 100644
--- a/net/smc/smc_cdc.h
+++ b/net/smc/smc_cdc.h
@@ -221,6 +221,7 @@ static inline void smc_host_msg_to_cdc(struct smc_cdc_msg *peer,
 
 static inline void smc_cdc_cursor_to_host(union smc_host_cursor *local,
 					  union smc_cdc_cursor *peer,
+					  unsigned int size,
 					  struct smc_connection *conn)
 {
 	union smc_host_cursor temp, old;
@@ -235,6 +236,14 @@ static inline void smc_cdc_cursor_to_host(union smc_host_cursor *local,
 	if ((old.wrap == temp.wrap) &&
 	    (old.count > temp.count))
 		return;
+	/* count is an offset into the RMBE and must always stay inside
+	 * it (see smc_curs_add()); the peer is untrusted, so reject an
+	 * out-of-range wire cursor the same way an out-of-order one is
+	 * already rejected above, instead of letting it drive
+	 * bytes_to_rcv / the urgent-byte offset past the buffer end
+	 */
+	if (temp.count >= size)
+		return;
 	smc_curs_copy(local, &temp, conn);
 }
 
@@ -246,8 +255,10 @@ static inline void smcr_cdc_msg_to_host(struct smc_host_cdc_msg *local,
 	local->len = peer->len;
 	local->seqno = ntohs(peer->seqno);
 	local->token = ntohl(peer->token);
-	smc_cdc_cursor_to_host(&local->prod, &peer->prod, conn);
-	smc_cdc_cursor_to_host(&local->cons, &peer->cons, conn);
+	smc_cdc_cursor_to_host(&local->prod, &peer->prod,
+			       conn->rmb_desc->len, conn);
+	smc_cdc_cursor_to_host(&local->cons, &peer->cons,
+			       conn->peer_rmbe_size, conn);
 	local->prod_flags = peer->prod_flags;
 	local->conn_state_flags = peer->conn_state_flags;
 }
-- 
2.50.1 (Apple Git-155)