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SubjectRe: [PATCH] drm: assure aux_dev is nonzero before using it
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On 5/24/19 4:36 AM, Jani Nikula wrote:
> On Thu, 23 May 2019, tcamuso <tcamuso@redhat.com> wrote:
>> From Daniel Kwon <dkwon@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 --

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