[PATCH net] net: Fix TOCTOU issue in sk_is_readable()

[Michal Luczaj]

sk->sk_prot->sock_is_readable is a valid function pointer when sk resides
in a sockmap. After the last sk_psock_put() (which usually happens when
socket is removed from sockmap), sk->sk_prot gets restored and
sk->sk_prot->sock_is_readable becomes NULL.

This makes sk_is_readable() racy, if the value of sk->sk_prot is reloaded
after the initial check. Which in turn may lead to a null pointer
dereference.

Ensure the function pointer does not turn NULL after the check.

Fixes: 8934ce2fd081 ("bpf: sockmap redirect ingress support")
Suggested-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
Signed-off-by: Michal Luczaj <mhal@xxxxxxx>
---
 include/net/sock.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 92e7c1aae3ccafe3a806dcee07ec77a469c0f43d..4c37015b7cf71e4902bdf411cfa57528a5d16ab3 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -3010,8 +3010,11 @@ int sock_ioctl_inout(struct sock *sk, unsigned int cmd,
 int sk_ioctl(struct sock *sk, unsigned int cmd, void __user *arg);
 static inline bool sk_is_readable(struct sock *sk)
 {
-	if (sk->sk_prot->sock_is_readable)
-		return sk->sk_prot->sock_is_readable(sk);
+	const struct proto *prot = READ_ONCE(sk->sk_prot);
+
+	if (prot->sock_is_readable)
+		return prot->sock_is_readable(sk);
+
 	return false;
 }
 #endif	/* _SOCK_H */

---
base-commit: 82cbd06f327f3c2ccdee990bd356c9303ae168f9
change-id: 20250525-skisreadable-toctou-382c42ffb685

Best regards,
-- 
Michal Luczaj <mhal@xxxxxxx>