[tip:sched/core] sched/core: Fix a potential double-fetch bug in sched_copy_attr()

[tip-bot for Kangjie Lu <tipbot@zytor.com>]

Commit-ID:  120e4e76857ddbc9268e1aa3f9de61a498e84618
Gitweb:     https://git.kernel.org/tip/120e4e76857ddbc9268e1aa3f9de61a498e84618
Author:     Kangjie Lu <kjlu@umn.edu>
AuthorDate: Wed, 9 Jan 2019 01:45:24 -0600
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 21 Jan 2019 11:26:17 +0100

sched/core: Fix a potential double-fetch bug in sched_copy_attr()

"uattr->size" is copied in from user space and checked. However, it is
copied in again after the security check. A malicious user may race to
change it. The fix sets uattr->size to be the checked size.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: pakki001@umn.edu
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20190109074524.10176-1-kjlu@umn.edu
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a674c7db2f29..d4d3514c4fe9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4499,6 +4499,9 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
 	if (ret)
 		return -EFAULT;
 
+	/* In case attr->size was changed by user-space: */
+	attr->size = size;
+
 	/*
 	 * XXX: Do we want to be lenient like existing syscalls; or do we want
 	 * to be strict and return an error on out-of-bounds values?