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

From: Ville Syrjälä
Date: Mon Sep 23 2019 - 11:22:13 EST


On Mon, Sep 23, 2019 at 11:03:35AM -0400, Tony Camuso wrote:
> On 7/12/19 1:06 PM, Ville Syrjälä wrote:
> > On Fri, Jul 12, 2019 at 12:07:46PM -0400, Tony Camuso wrote:
> >> On 7/10/19 9:56 AM, Ville Syrjälä wrote:
> >>> On Wed, Jul 10, 2019 at 09:47:11AM -0400, Tony Camuso wrote:
> >>>> 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.
> >>>>>
> >>>>> 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.
> >>>>>
> >>>>>
> >>>>> BR,
> >>>>> Jani.
> >>>>
> >>>> I have not been able to reproduce this problem.
> >>>
> >>> mknod /dev/foo c <drm_dp_aux major> 255
> >>> cat /dev/foo
> >>>
> >>> should do it.
> >>
> >> How do I determine <drm_dp_aux major>?
> >
> > ls,file,stat. Take your pick.
> >
>
> Problem here is I can't ls,file,stat /dev/foo until after it's created,
> but I need to know the drm_dp_aux major number befroe I can use mknod.
>
> What am I missing here?

udev/whatever should create a bunch of these for you so you can check
from them. If not, then dig around in /sys/class/drm_dp_aux_dev.

--
Ville Syrjälä
Intel