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

[Nicolò Coccia]

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.

Signed-off-by: Nicolò Coccia nicolo.coccia@xxxxxxxxxxxx>
---
 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;
- }
-
  smc->limit_smc_hs = !!val;
  rc = 0;
  break;