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    Subject[PATCH 3.4 044/172] dm snapshot: fix a possible invalid memory access on unload
    Date
    From: Mikulas Patocka <mpatocka@redhat.com>

    3.4.108-rc1 review patch. If anyone has any objections, please let me know.

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


    commit 22aa66a3ee5b61e0f4a0bfeabcaa567861109ec3 upstream.

    When the snapshot target is unloaded, snapshot_dtr() waits until
    pending_exceptions_count drops to zero. Then, it destroys the snapshot.
    Therefore, the function that decrements pending_exceptions_count
    should not touch the snapshot structure after the decrement.

    pending_complete() calls free_pending_exception(), which decrements
    pending_exceptions_count, and then it performs up_write(&s->lock) and it
    calls retry_origin_bios() which dereferences s->origin. These two
    memory accesses to the fields of the snapshot may touch the dm_snapshot
    struture after it is freed.

    This patch moves the call to free_pending_exception() to the end of
    pending_complete(), so that the snapshot will not be destroyed while
    pending_complete() is in progress.

    Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    Signed-off-by: Zefan Li <lizefan@huawei.com>
    ---
    drivers/md/dm-snap.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
    index b092338..696a16b 100644
    --- a/drivers/md/dm-snap.c
    +++ b/drivers/md/dm-snap.c
    @@ -1444,8 +1444,6 @@ out:
    full_bio->bi_end_io = pe->full_bio_end_io;
    full_bio->bi_private = pe->full_bio_private;
    }
    - free_pending_exception(pe);
    -
    increment_pending_exceptions_done_count();

    up_write(&s->lock);
    @@ -1462,6 +1460,8 @@ out:
    }

    retry_origin_bios(s, origin_bios);
    +
    + free_pending_exception(pe);
    }

    static void commit_callback(void *context, int success)
    --
    1.9.1


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