Overview

In the Linux kernel, the following vulnerability has been resolved:

net: Fix null-ptr-deref by sock_lock_init_class_and_name() and rmmod.

When I ran the repro 0 and waited a few seconds, I observed two LOCKDEP splats: a warning immediately followed by a null-ptr-deref. 1

Reproduction Steps:

1. Mount CIFS
2. Add an iptables rule to drop incoming FIN packets for CIFS
3. Unmount CIFS
4. Unload the CIFS module
5. Remove the iptables rule

At step 3), the CIFS module calls sock_release() for the underlying TCP socket, and it returns quickly. However, the socket remains in FIN_WAIT_1 because incoming FIN packets are dropped.

At this point, the module's refcnt is 0 while the socket is still alive, so the following rmmod command succeeds.

ss -tan

State Recv-Q Send-Q Local Address:Port Peer Address:Port FIN-WAIT-1 0 477 10.0.2.15:51062 10.0.0.137:445

lsmod | grep cifs

cifs 1159168 0

This highlights a discrepancy between the lifetime of the CIFS module and the underlying TCP socket. Even after CIFS calls sock_release() and it returns, the TCP socket does not die immediately in order to close the connection gracefully.

While this is generally fine, it causes an issue with LOCKDEP because CIFS assigns a different lock class to the TCP socket's sk->sk_lock using sock_lock_init_class_and_name().

Once an incoming packet is processed for the socket or a timer fires, sk->sk_lock is acquired.

Then, LOCKDEP checks the lock context in check_wait_context(), where hlock_class() is called to retrieve the lock class. However, since the module has already been unloaded, hlock_class() logs a warning and returns NULL, triggering the null-ptr-deref.

If LOCKDEP is enabled, we must ensure that a module calling sock_lock_init_class_and_name() (CIFS, NFS, etc) cannot be unloaded while such a socket is still alive to prevent this issue.

Let's hold the module reference in sock_lock_init_class_and_name() and release it when the socket is freed in sk_prot_free().

Note that sock_lock_init() clears sk->sk_owner for svc_create_socket() that calls sock_lock_init_class_and_name() for a listening socket, which clones a socket by sk_clone_lock() without GFP_ZERO.

CIFS_PATH="//${CIFS_SERVER}/Users/Administrator/Desktop/CIFS_TEST" DEV="enp0s3" CRED="/root/WindowsCredential.txt"

MNT=$(mktemp -d /tmp/XXXXXX) mount -t cifs ${CIFS_PATH} ${MNT} -o vers=3.0,credentials=${CRED},cache=none,echo_interval=1

iptables -A INPUT -s ${CIFS_SERVER} -j DROP

for i in $(seq 10); do umount ${MNT} rmmod cifs sleep 1 done

rm -r ${MNT}

iptables -D INPUT -s ${CIFS_SERVER} -j DROP

WARNING: CPU: 10 PID: 0 at kernel/locking/lockdep.c:234 hlock_class (kernel/locking/lockdep.c:234 kernel/locking/lockdep.c:223) Modules linked in: cifs_arc4 nls_ucs2_utils cifs_md4 [last unloaded: cifs] CPU: 10 UID: 0 PID: 0 Comm: swapper/10 Not tainted 6.14.0 #36 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014 RIP: 0010:hlock_class (kernel/locking/lockdep.c:234 kernel/locking/lockdep.c:223) ... Call Trace: <IRQ> __lock_acquire (kernel/locking/lockdep.c:4853 kernel/locking/lockdep.c:5178) lock_acquire (kernel/locking/lockdep.c:469 kernel/locking/lockdep.c:5853 kernel/locking/lockdep.c:5816) _raw_spin_lock_nested (kernel/locking/spinlock.c:379) tcp_v4_rcv (./include/linux/skbuff.h:1678 ./include/net/tcp.h:2547 net/ipv4/tcp_ipv4.c:2350) ...

BUG: kernel NULL pointer dereference, address: 00000000000000c4 PF: supervisor read access in kernel mode PF: error_code(0x0000) - not-present page PGD 0 Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI CPU: 10 UID: 0 PID: 0 Comm: swapper/10 Tainted: G W 6.14.0 #36 Tainted: [W]=WARN Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014 RIP: 0010:__lock_acquire (kernel/ ---truncated---

Affected

• Linux >= ed07536ed6731775219c1df7fa26a7588753e693 < 83083c5fc7cf9b0f136a42f26aba60da380f3601
• Linux >= ed07536ed6731775219c1df7fa26a7588753e693 < b7489b753667bc9245958a4896c9419743083c27
• Linux >= ed07536ed6731775219c1df7fa26a7588753e693 < d51e47e2ab6ef10a317d576075cf625cdbf96426
• Linux >= ed07536ed6731775219c1df7fa26a7588753e693 < feda73ad44a5cc80f6bf796bb1099a3fe71576d4
• Linux >= ed07536ed6731775219c1df7fa26a7588753e693 < 905d43b8ad2436c240f844acb3ebcc7a99b8ebf1
• Linux >= ed07536ed6731775219c1df7fa26a7588753e693 < 5f7f6abd92b6c8dc8f19625ef93c3a18549ede04
• Linux >= ed07536ed6731775219c1df7fa26a7588753e693 < c11247a21aab4b50a23c8b696727d7483de2f1e1
• Linux >= ed07536ed6731775219c1df7fa26a7588753e693 < 2155802d3313d7b8365935c6b8d6edc0ddd7eb94
• Linux >= ed07536ed6731775219c1df7fa26a7588753e693 < 0bb2f7a1ad1f11d861f58e5ee5051c8974ff9569
• Linux 2.6.20

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.