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    Subject[PATCH 4.19 009/109] gfs2: Fix use-after-free in gfs2_glock_shrink_scan
    Date
    From: Hillf Danton <hdanton@sina.com>

    [ Upstream commit 1ab19c5de4c537ec0d9b21020395a5b5a6c059b2 ]

    The GLF_LRU flag is checked under lru_lock in gfs2_glock_remove_from_lru() to
    remove the glock from the lru list in __gfs2_glock_put().

    On the shrink scan path, the same flag is cleared under lru_lock but because
    of cond_resched_lock(&lru_lock) in gfs2_dispose_glock_lru(), progress on the
    put side can be made without deleting the glock from the lru list.

    Keep GLF_LRU across the race window opened by cond_resched_lock(&lru_lock) to
    ensure correct behavior on both sides - clear GLF_LRU after list_del under
    lru_lock.

    Reported-by: syzbot <syzbot+34ba7ddbf3021981a228@syzkaller.appspotmail.com>
    Signed-off-by: Hillf Danton <hdanton@sina.com>
    Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/gfs2/glock.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
    index c20d71d86812..14d11ccda868 100644
    --- a/fs/gfs2/glock.c
    +++ b/fs/gfs2/glock.c
    @@ -1457,6 +1457,7 @@ __acquires(&lru_lock)
    while(!list_empty(list)) {
    gl = list_entry(list->next, struct gfs2_glock, gl_lru);
    list_del_init(&gl->gl_lru);
    + clear_bit(GLF_LRU, &gl->gl_flags);
    if (!spin_trylock(&gl->gl_lockref.lock)) {
    add_back_to_lru:
    list_add(&gl->gl_lru, &lru_list);
    @@ -1502,7 +1503,6 @@ static long gfs2_scan_glock_lru(int nr)
    if (!test_bit(GLF_LOCK, &gl->gl_flags)) {
    list_move(&gl->gl_lru, &dispose);
    atomic_dec(&lru_count);
    - clear_bit(GLF_LRU, &gl->gl_flags);
    freed++;
    continue;
    }
    --
    2.30.2
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