Re: [PATCH net v2] net/smc: validate peer CDC cursor against RMBE size before accepting it

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 stays
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 and a single
advance can be at most one bufferful; the wire cursor is peer-controlled
and was never checked against either.

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 the advance without clamping against size. A peer can inflate it
two ways: an out-of-range prod.count (e.g. 0x7fffffff), or -- since on a
wrap increment smc_curs_diff() returns (size - old.count) + new.count --
a cursor with a bumped wrap and new.count above old.count. Either 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.

Reject the bad cursor at ingress, mirroring the cursor-sanity idiom two
lines above: drop any incoming cursor whose count is outside the buffer
(>= size), and any whose advance from the last accepted cursor exceeds
one bufferful (smc_curs_diff() > size), which catches the wrap case.
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, bounding 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 <redacted>
Assisted-by: AuditCode-AI:2026.07
---
v2: also bound the cursor *advance* (smc_curs_diff() <= size), not just
    the count. A wrap increment with new.count above old.count passes the
    count check yet still inflates bytes_to_rcv past the buffer -- thanks
    to sashiko-bot for spotting the count-only gap. Resent standalone.

 net/smc/smc_cdc.h | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/net/smc/smc_cdc.h b/net/smc/smc_cdc.h
index 696cc11f2303..b0ae4d43b017 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 +236,21 @@ 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;
+	/* Bound the advance too: a wrap increment with temp.count above
+	 * old.count makes smc_curs_diff() report more than one bufferful,
+	 * which would inflate bytes_to_rcv / peer_rmbe_space past the buffer
+	 * in smc_cdc_msg_recv_action(). Reject such a cursor.
+	 */
+	if (smc_curs_diff(size, &old, &temp) > size)
+		return;

 	smc_curs_copy(local, &temp, conn);
 }
 

@@ -246,8 +262,10 @@ 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)