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Subject[PATCH 4.14 311/496] mm/kmemleak.c: wait for scan completion before disabling free
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Vinayak Menon <vinmenon@codeaurora.org>

[ Upstream commit 914b6dfff790544d9b77dfd1723adb3745ec9700 ]

A crash is observed when kmemleak_scan accesses the object->pointer,
likely due to the following race.

TASK A TASK B TASK C
kmemleak_write
(with "scan" and
NOT "scan=on")
kmemleak_scan()
create_object
kmem_cache_alloc fails
kmemleak_disable
kmemleak_do_cleanup
kmemleak_free_enabled = 0
kfree
kmemleak_free bails out
(kmemleak_free_enabled is 0)
slub frees object->pointer
update_checksum
crash - object->pointer
freed (DEBUG_PAGEALLOC)

kmemleak_do_cleanup waits for the scan thread to complete, but not for
direct call to kmemleak_scan via kmemleak_write. So add a wait for
kmemleak_scan completion before disabling kmemleak_free, and while at it
fix the comment on stop_scan_thread.

[vinmenon@codeaurora.org: fix stop_scan_thread comment]
Link: http://lkml.kernel.org/r/1522219972-22809-1-git-send-email-vinmenon@codeaurora.org
Link: http://lkml.kernel.org/r/1522063429-18992-1-git-send-email-vinmenon@codeaurora.org
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/kmemleak.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)

--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -1658,8 +1658,7 @@ static void start_scan_thread(void)
}

/*
- * Stop the automatic memory scanning thread. This function must be called
- * with the scan_mutex held.
+ * Stop the automatic memory scanning thread.
*/
static void stop_scan_thread(void)
{
@@ -1922,12 +1921,15 @@ static void kmemleak_do_cleanup(struct w
{
stop_scan_thread();

+ mutex_lock(&scan_mutex);
/*
- * Once the scan thread has stopped, it is safe to no longer track
- * object freeing. Ordering of the scan thread stopping and the memory
- * accesses below is guaranteed by the kthread_stop() function.
+ * Once it is made sure that kmemleak_scan has stopped, it is safe to no
+ * longer track object freeing. Ordering of the scan thread stopping and
+ * the memory accesses below is guaranteed by the kthread_stop()
+ * function.
*/
kmemleak_free_enabled = 0;
+ mutex_unlock(&scan_mutex);

if (!kmemleak_found_leaks)
__kmemleak_do_cleanup();

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