Re: [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@xxxxxxxxxx> wrote:
> >  From Daniel Kwon <dkwon@xxxxxxxxxx>
> >
> > 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 --