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Date:   Wed,  9 Jan 2019 01:45:24 -0600
From:   Kangjie Lu <kjlu@....edu>
To:     kjlu@....edu
Cc:     pakki001@....edu, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2] sched: 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@....edu>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6fedf3a98581..e868cc25ac2a 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 in the 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?
-- 
2.17.1

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