linux-kernel - [PATCH] futex: Check for uaddr alignment as early as possible

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Date:   Fri,  8 Dec 2017 17:06:29 -0800
From:   Darren Hart <dvhart@...radead.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     "Darren Hart (VMware)" <dvhart@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH] futex: Check for uaddr alignment as early as possible

From: "Darren Hart (VMware)" <dvhart@...radead.org>

uaddr alignment is currently tested by get_futex_key(). We can catch
misalignment earlier in sys_futex and return -EINVAL sooner. This
simplifies get_futex_key() a little, but more importantly exits the
kernel as soon as an invalid parameter is detected.

Passes all selftests/futex testcases on a dual socket Xeon E5-2670, 16
physical cores total, 32 threads total.

Signed-off-by: Darren Hart (VMware) <dvhart@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Darren Hart <dvhart@...radead.org>
---
 kernel/futex.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 76ed592..c3ee6c4 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -509,8 +509,6 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
 	 * The futex address must be "naturally" aligned.
 	 */
 	key->both.offset = address % PAGE_SIZE;
-	if (unlikely((address % sizeof(u32)) != 0))
-		return -EINVAL;
 	address -= key->both.offset;
 
 	if (unlikely(!access_ok(rw, uaddr, sizeof(u32))))
@@ -3525,6 +3523,11 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
 	u32 val2 = 0;
 	int cmd = op & FUTEX_CMD_MASK;
 
+	/* Only allow for aligned uaddr variables */
+	if (unlikely((unsigned long)uaddr % sizeof(u32) != 0 ||
+		     (unsigned long)uaddr2 % sizeof(u32) != 0))
+		return -EINVAL;
+
 	if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
 		      cmd == FUTEX_WAIT_BITSET ||
 		      cmd == FUTEX_WAIT_REQUEUE_PI)) {
-- 
2.9.4