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Subject: Re: [PATCH] block: fix NULL pointer dereference in account statistics
 with IDE
To:     Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org
Cc:     Christoph Hellwig <hch@lst.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        kernel test robot <rong.a.chen@intel.com>
References: <20191210184704.24081-1-logang@deltatee.com>
 <81ec258d-3bfe-f55a-ba55-83047743bbbe@kernel.dk>
 <eaa2c3ae-b71b-acca-32a0-494e545d60d9@deltatee.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <bfddce21-0a26-0d3a-6503-189b31c809a5@kernel.dk>
Date:   Tue, 10 Dec 2019 12:08:31 -0700
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On 12/10/19 12:02 PM, Logan Gunthorpe wrote:
> 
> 
> On 2019-12-10 11:59 a.m., Jens Axboe wrote:
>> On 12/10/19 11:47 AM, Logan Gunthorpe wrote:
>>> The IDE driver creates some passthru requests which never get
>>> submitted to the block layer in such a way that blk_account_io_start()
>>> gets called. However, the driver still calls __blk_mq_end_request() in
>>> ide_end_rq() which will call blk_account_io_completion() which tries
>>> to dereferences req->part which is never set. See ide_prep_sense() for
>>> an example of where these requests come from.
>>>
>>> To fix this, blk_account_io_completion() and blk_account_io_done()
>>> should do nothing if req->part is not set.
>>>
>>> The back trace of this bug is:
>>>
>>>     BUG: kernel NULL pointer dereference, address: 000002ac
>>>     #PF: supervisor write access in kernel mode
>>>     #PF: error_code(0x0002) - not-present page
>>>     *pde = 00000000
>>>     Oops: 0002 [#1]
>>>     CPU: 0 PID: 237 Comm: kworker/0:1H Not tainted
>>>     5.4.0-rc2-00011-g48d9b0d43105e #1
>>>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1
>>>     04/01/2014
>>>     Workqueue: kblockd drive_rq_insert_work
>>>     EIP: blk_account_io_completion+0x7a/0xf0
>>>     Code: 89 54 24 08 31 d2 89 4c 24 04 31 c9 c7 04 24 02 00 00 00 c1 ee
>>>     09 e8 f5 21 a6 ff e8 70 5c a7 ff 8b 53 60 8d 04 bd 00 00 00 00 <01> b4
>>>     02 ac 02 00 00 8b 9a 88 02 00 00 85 db 74 11 85 d2 74 51 8b
>>>     EAX: 00000000 EBX: f5b80000 ECX: 00000000 EDX: 00000000
>>>     ESI: 00000000 EDI: 00000000 EBP: f3031e70 ESP: f3031e54
>>>     DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068 EFLAGS: 00010046
>>>     CR0: 80050033 CR2: 000002ac CR3: 03c25000 CR4: 000406d0
>>>     Call Trace:
>>>      <IRQ>
>>>       blk_update_request+0x85/0x420
>>>       ide_end_rq+0x38/0xa0
>>>       ide_complete_rq+0x3d/0x70
>>>       cdrom_newpc_intr+0x258/0xba0
>>>       ide_intr+0x135/0x250
>>>       __handle_irq_event_percpu+0x3e/0x250
>>>       handle_irq_event_percpu+0x1f/0x50
>>>       handle_irq_event+0x32/0x60
>>>       handle_level_irq+0x6c/0x110
>>>       handle_irq+0x72/0xa0
>>>       </IRQ>
>>>       do_IRQ+0x45/0xad
>>>       common_interrupt+0x115/0x11c
>>
>> Why not just:
>>
>> diff --git a/block/blk.h b/block/blk.h
>> index 6842f28c033e..d7407b5d0200 100644
>> --- a/block/blk.h
>> +++ b/block/blk.h
>> @@ -250,7 +250,7 @@ int blk_dev_init(void);
>>   */
>>  static inline bool blk_do_io_stat(struct request *rq)
>>  {
>> -	return rq->rq_disk && (rq->rq_flags & RQF_IO_STAT);
>> +	return rq->part && rq->rq_disk && (rq->rq_flags & RQF_IO_STAT);
>>  }
> 
> Because blk_account_io_start() also checks blk_do_io_stat() and, in that
> case, rq->part will never be set (seeing that's the function that
> typically sets it); thus that solution would disable stats entirely.

Gotcha. I'm fine with the patch you posted in that case.

-- 
Jens Axboe