[PATCH] drm: assure aux_dev is nonzero before using it

[tony camuso]

On 5/24/19 4:36 AM, Jani Nikula wrote:
> On Thu, 23 May 2019, tcamuso <tcamuso at redhat.com> wrote:
>>  From Daniel Kwon <dkwon at redhat.com>
>>
>> The system was crashed due to invalid memory access while trying to access
>> auxiliary device.
>>
>> crash> bt
>> PID: 9863   TASK: ffff89d1bdf11040  CPU: 1   COMMAND: "ipmitool"
>>   #0 [ffff89cedd7f3868] machine_kexec at ffffffffb0663674
>>   #1 [ffff89cedd7f38c8] __crash_kexec at ffffffffb071cf62
>>   #2 [ffff89cedd7f3998] crash_kexec at ffffffffb071d050
>>   #3 [ffff89cedd7f39b0] oops_end at ffffffffb0d6d758
>>   #4 [ffff89cedd7f39d8] no_context at ffffffffb0d5bcde
>>   #5 [ffff89cedd7f3a28] __bad_area_nosemaphore at ffffffffb0d5bd75
>>   #6 [ffff89cedd7f3a78] bad_area at ffffffffb0d5c085
>>   #7 [ffff89cedd7f3aa0] __do_page_fault at ffffffffb0d7080c
>>   #8 [ffff89cedd7f3b10] do_page_fault at ffffffffb0d70905
>>   #9 [ffff89cedd7f3b40] page_fault at ffffffffb0d6c758
>>      [exception RIP: drm_dp_aux_dev_get_by_minor+0x3d]
>>      RIP: ffffffffc0a589bd  RSP: ffff89cedd7f3bf0  RFLAGS: 00010246
>>      RAX: 0000000000000000  RBX: 0000000000000000  RCX: ffff89cedd7f3fd8
>>      RDX: 0000000000000000  RSI: 0000000000000000  RDI: ffffffffc0a613e0
>>      RBP: ffff89cedd7f3bf8   R8: ffff89f1bcbabbd0   R9: 0000000000000000
>>      R10: ffff89f1be7a1cc0  R11: 0000000000000000  R12: 0000000000000000
>>      R13: ffff89f1b32a2830  R14: ffff89d18fadfa00  R15: 0000000000000000
>>      ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
>>      RIP: 00002b45f0d80d30  RSP: 00007ffc416066a0  RFLAGS: 00010246
>>      RAX: 0000000000000002  RBX: 000056062e212d80  RCX: 00007ffc41606810
>>      RDX: 0000000000000000  RSI: 0000000000000002  RDI: 00007ffc41606ec0
>>      RBP: 0000000000000000   R8: 000056062dfed229   R9: 00002b45f0cdf14d
>>      R10: 0000000000000002  R11: 0000000000000246  R12: 00007ffc41606ec0
>>      R13: 00007ffc41606ed0  R14: 00007ffc41606ee0  R15: 0000000000000000
>>      ORIG_RAX: 0000000000000002  CS: 0033  SS: 002b
>>
>> ----------------------------------------------------------------------------
>>
>> It was trying to open '/dev/ipmi0', but as no entry in aux_dir, it returned
>> NULL from 'idr_find()'. This drm_dp_aux_dev_get_by_minor() should have done a
>> check on this, but had failed to do it.
> 
> I think the better question is, *why* does the idr_find() return NULL? I
> don't think it should, under any circumstances. I fear adding the check
> here papers over some other problem, taking us further away from the
> root cause.

That's a very good question.

> Also, can you reproduce this on a recent upstream kernel? The aux device
> nodes were introduced in kernel v4.6. Whatever you reproduced on v3.10
> is pretty much irrelevant for upstream.

I will look into this deeper, using the upstream kernel.

> 
> 
> BR,
> Jani.

-- snip --