linux-kernel - [tip: locking/core] futex: Validate waiter correctly in futex_proxy_trylock

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Date:   Tue, 17 Aug 2021 20:14:05 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/core] futex: Validate waiter correctly in
 futex_proxy_trylock_atomic()

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     dc7109aaa233d83b573f75763a9f1ae207042a53
Gitweb:        https://git.kernel.org/tip/dc7109aaa233d83b573f75763a9f1ae207042a53
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Sun, 15 Aug 2021 23:29:04 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 17 Aug 2021 19:05:36 +02:00

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@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20210815211305.193767519@linutronix.de
---
 kernel/futex.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index 21625cb..a1f27fd 100644
--- 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;