Re: [PATCH] block: fix NULL pointer dereference in account statistics with IDE

[Logan Gunthorpe]

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.

Logan