futex: Validate waiter correctly in futex_proxy_trylock_atomic()

The loop in futex_requeue() has a sanity check for the waiter, which is
missing in futex_proxy_trylock_atomic(). In theory the key2 check is
sufficient, but futexes are cursed so add it for completeness and paranoia
sake.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211305.193767519@linutronix.de

diff --git a/kernel/futex.c b/kernel/futex.c
index 21625cb..a1f27fd 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1879,6 +1879,13 @@ futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1,
        if (!top_waiter)
                return 0;
 
+       /*
+        * Ensure that this is a waiter sitting in futex_wait_requeue_pi()
+        * and waiting on the 'waitqueue' futex which is always !PI.
+        */
+       if (!top_waiter->rt_waiter || top_waiter->pi_state)
+               ret = -EINVAL;
+
        /* Ensure we requeue to the expected futex. */
        if (!match_futex(top_waiter->requeue_pi_key, key2))
                return -EINVAL;