Subject: Re: [PATCH net] net/smc: fix sleep-inside-lock in __smc_setsockopt() causing local DoS

From: Dust Li <[email-masked]>
Date: Sat, 9 May 2026 09:26:46 +0800

On 2026-05-08 21:33:10, Nicolò Coccia wrote:

Hi Nicolò,

This patch doesn't apply on net/main

>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.
>

You should add a Fixes tag here.
>Signed-off-by: Nicolò Coccia [email-masked]>
>---
> net/smc/af_smc.c | 16 ++++++++--------
> 1 file changed, 8 insertions(+), 8 deletions(-)
>
>diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
>--- a/net/smc/af_smc.c
>+++ b/net/smc/af_smc.c
>@@ -XXXX,X +XXXX,X @@ 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;
>- }
>-

The indenting is all messed up

Best regards,
Dust