linux-kernel - [tip: sched/urgent] sched/core: Fix use-after-free bug in dup_user_cpus

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Message-ID: <167326115033.4906.12022600051582690009.tip-bot2@tip-bot2>
Date:   Mon, 09 Jan 2023 10:45:50 -0000
From:   "tip-bot2 for Waiman Long" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     wangbiao3@...omi.com, Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, stable@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] sched/core: Fix use-after-free bug in dup_user_cpus_ptr()

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     87ca4f9efbd7cc649ff43b87970888f2812945b8
Gitweb:        https://git.kernel.org/tip/87ca4f9efbd7cc649ff43b87970888f2812945b8
Author:        Waiman Long <longman@...hat.com>
AuthorDate:    Fri, 30 Dec 2022 23:11:19 -05:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Mon, 09 Jan 2023 11:43:07 +01:00

sched/core: Fix use-after-free bug in dup_user_cpus_ptr()

Since commit 07ec77a1d4e8 ("sched: Allow task CPU affinity to be
restricted on asymmetric systems"), the setting and clearing of
user_cpus_ptr are done under pi_lock for arm64 architecture. However,
dup_user_cpus_ptr() accesses user_cpus_ptr without any lock
protection. Since sched_setaffinity() can be invoked from another
process, the process being modified may be undergoing fork() at
the same time.  When racing with the clearing of user_cpus_ptr in
__set_cpus_allowed_ptr_locked(), it can lead to user-after-free and
possibly double-free in arm64 kernel.

Commit 8f9ea86fdf99 ("sched: Always preserve the user requested
cpumask") fixes this problem as user_cpus_ptr, once set, will never
be cleared in a task's lifetime. However, this bug was re-introduced
in commit 851a723e45d1 ("sched: Always clear user_cpus_ptr in
do_set_cpus_allowed()") which allows the clearing of user_cpus_ptr in
do_set_cpus_allowed(). This time, it will affect all arches.

Fix this bug by always clearing the user_cpus_ptr of the newly
cloned/forked task before the copying process starts and check the
user_cpus_ptr state of the source task under pi_lock.

Note to stable, this patch won't be applicable to stable releases.
Just copy the new dup_user_cpus_ptr() function over.

Fixes: 07ec77a1d4e8 ("sched: Allow task CPU affinity to be restricted on asymmetric systems")
Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
Reported-by: David Wang 王标 <wangbiao3@...omi.com>
Signed-off-by: Waiman Long <longman@...hat.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Peter Zijlstra <peterz@...radead.org>
Cc: stable@...r.kernel.org
Link: https://lore.kernel.org/r/20221231041120.440785-2-longman@redhat.com
---
 kernel/sched/core.c | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 965d813..f9f6e54 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2612,19 +2612,43 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
 		      int node)
 {
+	cpumask_t *user_mask;
 	unsigned long flags;

-	if (!src->user_cpus_ptr)
+	/*
+	 * Always clear dst->user_cpus_ptr first as their user_cpus_ptr's
+	 * may differ by now due to racing.
+	 */
+	dst->user_cpus_ptr = NULL;
+
+	/*
+	 * This check is racy and losing the race is a valid situation.
+	 * It is not worth the extra overhead of taking the pi_lock on
+	 * every fork/clone.
+	 */
+	if (data_race(!src->user_cpus_ptr))
 		return 0;

-	dst->user_cpus_ptr = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
-	if (!dst->user_cpus_ptr)
+	user_mask = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
+	if (!user_mask)
 		return -ENOMEM;

-	/* Use pi_lock to protect content of user_cpus_ptr */
+	/*
+	 * Use pi_lock to protect content of user_cpus_ptr
+	 *
+	 * Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent
+	 * do_set_cpus_allowed().
+	 */
 	raw_spin_lock_irqsave(&src->pi_lock, flags);
-	cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+	if (src->user_cpus_ptr) {
+		swap(dst->user_cpus_ptr, user_mask);
+		cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+	}
 	raw_spin_unlock_irqrestore(&src->pi_lock, flags);
+
+	if (unlikely(user_mask))
+		kfree(user_mask);
+
 	return 0;
 }