[PATCH 5.15] net/smc: reject CHID-0 ACCEPT that matches an empty ism_dev slot

[Andrey Troshin]

From: Xiang Mei <xmei5@xxxxxxx>

[ Upstream commit 277740023def559a4a2ddc3e8e784ee37a0f16a9 ]

On the SMC-D client, slot 0 of ini->ism_dev[]/ini->ism_chid[] is
reserved for an SMC-Dv1 device. smc_find_ism_v2_device_clnt()
populates V2 entries starting at index 1, so when no V1 device is
selected slot 0 is left in its kzalloc()'ed state with ism_dev[0] ==
NULL and ism_chid[0] == 0.

smc_v2_determine_accepted_chid() then matches the peer's CHID against
the array starting from index 0 using the CHID alone. A malicious
peer replying to a SMC-Dv2-only proposal with d1.chid == 0 matches
the empty slot, ini->ism_selected becomes 0, and the subsequent
ism_dev[0]->lgr_lock dereference in smc_conn_create() faults at
offsetof(struct smcd_dev, lgr_lock) == 0x68:

  BUG: KASAN: null-ptr-deref in _raw_spin_lock_bh+0x79/0xe0
  Write of size 4 at addr 0000000000000068 by task exploit/144
  Call Trace:
   _raw_spin_lock_bh
   smc_conn_create (net/smc/smc_core.c:1997)
   __smc_connect (net/smc/af_smc.c:1447)
   smc_connect (net/smc/af_smc.c:1720)
   __sys_connect
   __x64_sys_connect
   do_syscall_64

Require ism_dev[i] to be non-NULL before accepting a CHID match.

Fixes: a7c9c5f4af7f ("net/smc: CLC accept / confirm V2")
Reported-by: Weiming Shi <bestswngs@xxxxxxxxx>
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
Link: https://patch.msgid.link/20260511062138.2839584-1-xmei5@xxxxxxx
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Signed-off-by: Andrey Troshin <drtrosh@xxxxxxxxxxxxxx>
---
Backport fix for CVE-2026-64048
Link: https://nvd.nist.gov/vuln/detail/CVE-2026-64048
---
 net/smc/af_smc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index 6b60a5dd240d..c283f0272a77 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -1107,7 +1107,8 @@ smc_v2_determine_accepted_chid(struct smc_clc_msg_accept_confirm_v2 *aclc,
 	int i;
 
 	for (i = 0; i < ini->ism_offered_cnt + 1; i++) {
-		if (ini->ism_chid[i] == ntohs(aclc->chid)) {
+		if (ini->ism_dev[i] &&
+			ini->ism_chid[i] == ntohs(aclc->chid)) {
 			ini->ism_selected = i;
 			return 0;
 		}
-- 
2.34.1