audit: Fix data races of skb_queue_len() readers on audit_queue

Multiple readers access audit_queue.qlen via skb_queue_len() without
holding the queue lock or using READ_ONCE(), while kauditd writes to
this field via the skb_dequeue() → __skb_unlink() path with WRITE_ONCE()
protected by a spinlock. This constitutes data races.

All affected skb_queue_len(&audit_queue) call sites:
  - kauditd_thread() wait_event_freezable() condition
  - audit_receive_msg() AUDIT_GET handler (s.backlog assignment)
  - audit_receive() backlog check
  - audit_log_start() backlog check and pr_warn()

KCSAN reports the following conflicting access pattern (one example):
==================================================================
BUG: KCSAN: data-race in audit_log_start / skb_dequeue

write (marked) to 0xffffffff8512ee20 of 4 bytes by task 661 on cpu 57:
 skb_dequeue+0x70/0xf0
 kauditd_send_queue+0x71/0x220
 kauditd_thread+0x1cb/0x430
 kthread+0x1c2/0x210
 ret_from_fork+0x162/0x1a0
 ret_from_fork_asm+0x1a/0x30

read to 0xffffffff8512ee20 of 4 bytes by task 36586 on cpu 1:
 audit_log_start+0x2a0/0x6b0
 audit_core_dumps+0x64/0xa0
 do_coredump+0x14b/0x1260
 get_signal+0xeb2/0xf70
 arch_do_signal_or_restart+0x41/0x170
 exit_to_user_mode_loop+0xa2/0x1c0
 do_syscall_64+0x1a3/0x1c0
 entry_SYSCALL_64_after_hwframe+0x76/0xe0

value changed: 0x00000001 -> 0x00000000
==================================================================

Resolve the race by switching to lockless helper skb_queue_len_lockless(),
which internally uses READ_ONCE() and properly pairs with the WRITE_ONCE()
write accesses already present on the writer side.

Cc: stable@vger.kernel.org
Fixes: 3197542482df ("audit: rework audit_log_start()")
Signed-off-by: Chi Wang <wangchi@kylinos.cn>
Reviewed-by: Ricardo Robaina <rrobaina@redhat.com>
[PM: line length tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>

diff --git a/kernel/audit.c b/kernel/audit.c
index dcc657d35776d8462760d1cd29e4ed679ae7839a..feaa4e2271d78a0ff9cbe434cd2e722f02fb1aa2 100644 (file)
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -950,7 +950,7 @@ main_queue:
                 *       do the multicast send and rotate records from the
                 *       main queue to the retry/hold queues */
                wait_event_freezable(kauditd_wait,
-                                    (skb_queue_len(&audit_queue) ? 1 : 0));
+                               (skb_queue_len_lockless(&audit_queue) ? 1 : 0));
        }
 
        return 0;
@@ -1283,7 +1283,7 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
                s.rate_limit               = audit_rate_limit;
                s.backlog_limit            = audit_backlog_limit;
                s.lost                     = atomic_read(&audit_lost);
-               s.backlog                  = skb_queue_len(&audit_queue);
+               s.backlog                  = skb_queue_len_lockless(&audit_queue);
                s.feature_bitmap           = AUDIT_FEATURE_BITMAP_ALL;
                s.backlog_wait_time        = audit_backlog_wait_time;
                s.backlog_wait_time_actual = atomic_read(&audit_backlog_wait_time_actual);
@@ -1627,7 +1627,7 @@ static void audit_receive(struct sk_buff *skb)
 
        /* can't block with the ctrl lock, so penalize the sender now */
        if (audit_backlog_limit &&
-           (skb_queue_len(&audit_queue) > audit_backlog_limit)) {
+           (skb_queue_len_lockless(&audit_queue) > audit_backlog_limit)) {
                DECLARE_WAITQUEUE(wait, current);
 
                /* wake kauditd to try and flush the queue */
@@ -1933,7 +1933,7 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
                long stime = audit_backlog_wait_time;
 
                while (audit_backlog_limit &&
-                      (skb_queue_len(&audit_queue) > audit_backlog_limit)) {
+                       (skb_queue_len_lockless(&audit_queue) > audit_backlog_limit)) {
                        /* wake kauditd to try and flush the queue */
                        wake_up_interruptible(&kauditd_wait);
 
@@ -1953,7 +1953,7 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
                        } else {
                                if (audit_rate_check() && printk_ratelimit())
                                        pr_warn("audit_backlog=%d > audit_backlog_limit=%d\n",
-                                               skb_queue_len(&audit_queue),
+                                               skb_queue_len_lockless(&audit_queue),
                                                audit_backlog_limit);
                                audit_log_lost("backlog limit exceeded");
                                return NULL;