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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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