linux-kernel - [PATCH] sched/core: Fix a potential double fetch bug on attr->size

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Date:   Mon, 25 Sep 2017 08:53:16 -0400
From:   Meng Xu <meng.xu@...ech.edu>
To:     mingo@...hat.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org
Cc:     meng.xu@...ech.edu, sanidhya@...ech.edu, taesoo@...ech.edu,
        Meng Xu <mengxu.gatech@...il.com>
Subject: [PATCH] sched/core: Fix a potential double fetch bug on attr->size

From: Meng Xu <mengxu.gatech@...il.com>

`attr->size` after the second fetch `copy_from_user(attr, uattr, size)`,
can be different from what is initially fetched in and checked
`get_user(size, &uattr->size)` by racing condition in the userspace.

The issue and the patch are both similar to commit f12f42a
(in kernel/events/core.c).

Signed-off-by: Meng Xu <mengxu.gatech@...il.com>
---
 kernel/sched/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0869b20..c22d2b4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4349,6 +4349,8 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
 	if (ret)
 		return -EFAULT;
 
+	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?
-- 
2.7.4