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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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