From a3fdd924d88c30b9f488636ce0e4696012cf5511 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Nicol=C3=B2=20Coccia?= Date: Sun, 10 May 2026 12:34:13 -0400 Subject: [PATCH] net/smc: fix sleep-inside-lock in __smc_setsockopt() causing local DoS MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit A logic flaw in __smc_setsockopt() allows a local unprivileged user to cause a Denial of Service (DoS) by holding the socket lock indefinitely. The function __smc_setsockopt() calls copy_from_sockptr() while holding lock_sock(sk). By passing a userfaultfd-monitored memory page (or FUSE-backed memory on systems where unprivileged userfaultfd is disabled) as the optval, an attacker can halt execution during the copy operation, keeping the lock held. Combined with asynchronous tear-down operations like shutdown(), this exhausts the kernel wq (kworkers) and triggers the hung task watchdog. [ 240.123456] INFO: task kworker/u8:2 blocked for more than 120 seconds. [ 240.123489] Call Trace: [ 240.123501] smc_shutdown+... [ 240.123512] lock_sock_nested+... This patch moves the user-space copy outside the lock_sock() critical section to prevent the issue. Fixes: a6a6fe27bab4 ("net/smc: Dynamic control handshake limitation by socket options") Signed-off-by: Nicolò Coccia Reviewed-by: Dust Li Tested-by: Dust Li Signed-off-by: Jakub Kicinski --- net/smc/af_smc.c | 17 ++++++++--------- 1 file changed, 8 insertions(+), 9 deletions(-) diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c index 185dbed7de5d..da28652f6810 100644 --- a/net/smc/af_smc.c +++ b/net/smc/af_smc.c @@ -3054,18 +3054,17 @@ static int __smc_setsockopt(struct socket *sock, int level, int optname, smc = smc_sk(sk); + /* pre-fetch user data outside the lock */ + if (optname == SMC_LIMIT_HS) { + if (optlen < sizeof(int)) + return -EINVAL; + if (copy_from_sockptr(&val, optval, sizeof(int))) + return -EFAULT; + } + lock_sock(sk); switch (optname) { case SMC_LIMIT_HS: - if (optlen < sizeof(int)) { - rc = -EINVAL; - break; - } - if (copy_from_sockptr(&val, optval, sizeof(int))) { - rc = -EFAULT; - break; - } - smc->limit_smc_hs = !!val; rc = 0; break;