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    Subject[PATCH 3.18 005/185] aio: fix io_destroy(2) vs. lookup_ioctx() race
    Date
    3.18-stable review patch.  If anyone has any objections, please let me know.

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

    From: Al Viro <viro@zeniv.linux.org.uk>

    commit baf10564fbb66ea222cae66fbff11c444590ffd9 upstream.

    kill_ioctx() used to have an explicit RCU delay between removing the
    reference from ->ioctx_table and percpu_ref_kill() dropping the refcount.
    At some point that delay had been removed, on the theory that
    percpu_ref_kill() itself contained an RCU delay. Unfortunately, that was
    the wrong kind of RCU delay and it didn't care about rcu_read_lock() used
    by lookup_ioctx(). As the result, we could get ctx freed right under
    lookup_ioctx(). Tejun has fixed that in a6d7cff472e ("fs/aio: Add explicit
    RCU grace period when freeing kioctx"); however, that fix is not enough.

    Suppose io_destroy() from one thread races with e.g. io_setup() from another;
    CPU1 removes the reference from current->mm->ioctx_table[...] just as CPU2
    has picked it (under rcu_read_lock()). Then CPU1 proceeds to drop the
    refcount, getting it to 0 and triggering a call of free_ioctx_users(),
    which proceeds to drop the secondary refcount and once that reaches zero
    calls free_ioctx_reqs(). That does
    INIT_RCU_WORK(&ctx->free_rwork, free_ioctx);
    queue_rcu_work(system_wq, &ctx->free_rwork);
    and schedules freeing the whole thing after RCU delay.

    In the meanwhile CPU2 has gotten around to percpu_ref_get(), bumping the
    refcount from 0 to 1 and returned the reference to io_setup().

    Tejun's fix (that queue_rcu_work() in there) guarantees that ctx won't get
    freed until after percpu_ref_get(). Sure, we'd increment the counter before
    ctx can be freed. Now we are out of rcu_read_lock() and there's nothing to
    stop freeing of the whole thing. Unfortunately, CPU2 assumes that since it
    has grabbed the reference, ctx is *NOT* going away until it gets around to
    dropping that reference.

    The fix is obvious - use percpu_ref_tryget_live() and treat failure as miss.
    It's not costlier than what we currently do in normal case, it's safe to
    call since freeing *is* delayed and it closes the race window - either
    lookup_ioctx() comes before percpu_ref_kill() (in which case ctx->users
    won't reach 0 until the caller of lookup_ioctx() drops it) or lookup_ioctx()
    fails, ctx->users is unaffected and caller of lookup_ioctx() doesn't see
    the object in question at all.

    Cc: stable@kernel.org
    Fixes: a6d7cff472e "fs/aio: Add explicit RCU grace period when freeing kioctx"
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/aio.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/fs/aio.c
    +++ b/fs/aio.c
    @@ -1006,8 +1006,8 @@ static struct kioctx *lookup_ioctx(unsig

    ctx = rcu_dereference(table->table[id]);
    if (ctx && ctx->user_id == ctx_id) {
    - percpu_ref_get(&ctx->users);
    - ret = ctx;
    + if (percpu_ref_tryget_live(&ctx->users))
    + ret = ctx;
    }
    out:
    rcu_read_unlock();

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