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Subject: Re: KASAN: null-ptr-deref Write in blk_mq_map_swqueue
To:     Bart Van Assche <bvanassche@acm.org>
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Cc:     syzbot <syzbot+313d95e8a7a49263f88d@syzkaller.appspotmail.com>,
        axboe@kernel.dk, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
From:   Dongli Zhang <dongli.zhang@oracle.com>
Message-ID: <4a703c7f-3192-6303-87cf-b0db0ebb821a@oracle.com>
Date:   Fri, 27 Mar 2020 06:30:46 -0700
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On 3/26/20 7:52 PM, Bart Van Assche wrote:
> On 2020-03-26 17:19, Dongli Zhang wrote:
>> I think the issue is because of line 2827, that is, the q->nr_hw_queues is
>> updated too earlier. It is still possible the init would fail later.
>>
>> 2809 static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
>> 2810                                                 struct request_queue *q)
>> 2811 {
>> 2812         int i, j, end;
>> 2813         struct blk_mq_hw_ctx **hctxs = q->queue_hw_ctx;
>> 2814
>> 2815         if (q->nr_hw_queues < set->nr_hw_queues) {
>> 2816                 struct blk_mq_hw_ctx **new_hctxs;
>> 2817
>> 2818                 new_hctxs = kcalloc_node(set->nr_hw_queues,
>> 2819                                        sizeof(*new_hctxs), GFP_KERNEL,
>> 2820                                        set->numa_node);
>> 2821                 if (!new_hctxs)
>> 2822                         return;
>> 2823                 if (hctxs)
>> 2824                         memcpy(new_hctxs, hctxs, q->nr_hw_queues *
>> 2825                                sizeof(*hctxs));
>> 2826                 q->queue_hw_ctx = new_hctxs;
>> 2827                 q->nr_hw_queues = set->nr_hw_queues;
>> 2828                 kfree(hctxs);
>> 2829                 hctxs = new_hctxs;
>> 2830         }
> 
> Which kernel tree does this syzbot report refer to? Commit
> d0930bb8f46b ("blk-mq: Fix a recently introduced regression in
> blk_mq_realloc_hw_ctxs()") in Jens' tree removed line 2827 shown above.
> 

Thank you very much for sharing this. The below is in Jens' tree for 5.7.

commit d0930bb8f46b8fb4a7d429c0bf1c91b3ed00a7cf
Author: Bart Van Assche <bvanassche@acm.org>
Date:   Mon Mar 9 21:26:18 2020 -0700

    blk-mq: Fix a recently introduced regression in blk_mq_realloc_hw_ctxs()

    q->nr_hw_queues must only be updated once it is known that
    blk_mq_realloc_hw_ctxs() has succeeded. Otherwise it can happen that
    reallocation fails and that q->nr_hw_queues is larger than the number of
    allocated hardware queues. This patch fixes the following crash if
    increasing the number of hardware queues fails:

    BUG: KASAN: null-ptr-deref in blk_mq_map_swqueue+0x775/0x810
    Write of size 8 at addr 0000000000000118 by task check/977

    CPU: 3 PID: 977 Comm: check Not tainted 5.6.0-rc1-dbg+ #8
    Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
    Call Trace:
     dump_stack+0xa5/0xe6
     __kasan_report.cold+0x65/0x99
     kasan_report+0x16/0x20
     check_memory_region+0x140/0x1b0
     memset+0x28/0x40
     blk_mq_map_swqueue+0x775/0x810
     blk_mq_update_nr_hw_queues+0x468/0x710
     nullb_device_submit_queues_store+0xf7/0x1a0 [null_blk]
     configfs_write_file+0x1c4/0x250 [configfs]
     __vfs_write+0x4c/0x90
     vfs_write+0x145/0x2c0
     ksys_write+0xd7/0x180
     __x64_sys_write+0x47/0x50
     do_syscall_64+0x6f/0x2f0
     entry_SYSCALL_64_after_hwframe+0x49/0xbe

    Fixes: ac0d6b926e74 ("block: Reduce the amount of memory required per
request queue")
    Signed-off-by: Bart Van Assche <bvanassche@acm.org>
    Reviewed-by: Ming Lei <ming.lei@redhat.com>
    Cc: Keith Busch <kbusch@kernel.org>
    Cc: Johannes Thumshirn <jth@kernel.org>
    Cc: Hannes Reinecke <hare@suse.com>
    Cc: Christoph Hellwig <hch@infradead.org>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index d4bd9b961726..37ff8dfb8ab9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2824,7 +2824,6 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
                        memcpy(new_hctxs, hctxs, q->nr_hw_queues *
                               sizeof(*hctxs));
                q->queue_hw_ctx = new_hctxs;
-               q->nr_hw_queues = set->nr_hw_queues;
                kfree(hctxs);
                hctxs = new_hctxs;
        }


That should be the reason why "init_hctx() fault injection" was introduced.

Thank you very much!

Dongli Zhang