Re: [PATCH] block, bfq: NULL out the bic when it's no longer valid

[Guenter Roeck <linux@roeck-us.net>]

On Thu, Jun 27, 2019 at 09:44:09PM -0700, Douglas Anderson wrote:
> In reboot tests on several devices we were seeing a "use after free"
> when slub_debug or KASAN was enabled.  The kernel complained about:
> 
>   Unable to handle kernel paging request at virtual address 6b6b6c2b
> 
> ...which is a classic sign of use after free under slub_debug.  The
> stack crawl in kgdb looked like:
> 
>  0  test_bit (addr=<optimized out>, nr=<optimized out>)
>  1  bfq_bfqq_busy (bfqq=<optimized out>)
>  2  bfq_select_queue (bfqd=<optimized out>)
>  3  __bfq_dispatch_request (hctx=<optimized out>)
>  4  bfq_dispatch_request (hctx=<optimized out>)
>  5  0xc056ef00 in blk_mq_do_dispatch_sched (hctx=0xed249440)
>  6  0xc056f728 in blk_mq_sched_dispatch_requests (hctx=0xed249440)
>  7  0xc0568d24 in __blk_mq_run_hw_queue (hctx=0xed249440)
>  8  0xc0568d94 in blk_mq_run_work_fn (work=<optimized out>)
>  9  0xc024c5c4 in process_one_work (worker=0xec6d4640, work=0xed249480)
>  10 0xc024cff4 in worker_thread (__worker=0xec6d4640)
> 
> Digging in kgdb, it could be found that, though bfqq looked fine,
> bfqq->bic had been freed.
> 
> Through further digging, I postulated that perhaps it is illegal to
> access a "bic" (AKA an "icq") after bfq_exit_icq() had been called
> because the "bic" can be freed at some point in time after this call
> is made.  I confirmed that there certainly were cases where the exact
> crashing code path would access the "bic" after bfq_exit_icq() had
> been called.  Sspecifically I set the "bfqq->bic" to (void *)0x7 and
                ^^^

> saw that the bic was 0x7 at the time of the crash.
> 
> To understand a bit more about why this crash was fairly uncommon (I
> saw it only once in a few hundred reboots), you can see that much of
> the time bfq_exit_icq_fbqq() fully frees the bfqq and thus it can't
> access the ->bic anymore.  The only case it doesn't is if
> bfq_put_queue() sees a reference still held.
> 
> However, even in the case when bfqq isn't freed, the crash is still
> rare.  Why?  I tracked what happened to the "bic" after the exit
> routine.  It doesn't get freed right away.  Rather,
> put_io_context_active() eventually called put_io_context() which
> queued up freeing on a workqueue.  The freeing then actually happened
> later than that through call_rcu().  Despite all these delays, some
> extra debugging showed that all the hoops could be jumped through in
> time and the memory could be freed causing the original crash.  Phew!
> 
> To make a long story short, assuming it truly is illegal to access an
> icq after the "exit_icq" callback is finished, this patch is needed.
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>

Nicely done ... thanks!

Reviewed-by: Guenter Roeck <groeck@chromium.org>

> ---
> Most of the testing of this was done on the Chrome OS 4.19 kernel with
> BFQ backported (thanks to Paolo's help).  I did manage to reproduce a
> crash on mainline Linux (v5.2-rc6) though.
> 
> To see some of the techniques used to debug this, see
> <https://crrev.com/c/1679134> and <https://crrev.com/c/1681258/1>.
> 
> I'll also note that on linuxnext (next-20190627) I saw some other
> use-after-frees that seemed related to BFQ but haven't had time to
> debug.  They seemed unrelated.
> 
>  block/bfq-iosched.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index f8d430f88d25..6c0cff03f8f6 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -4584,6 +4584,7 @@ static void bfq_exit_icq_bfqq(struct bfq_io_cq *bic, bool is_sync)
>  		unsigned long flags;
>  
>  		spin_lock_irqsave(&bfqd->lock, flags);
> +		bfqq->bic = NULL;
>  		bfq_exit_bfqq(bfqd, bfqq);
>  		bic_set_bfqq(bic, NULL, is_sync);
>  		spin_unlock_irqrestore(&bfqd->lock, flags);
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
>