RE: [PATCH v2] drm/dp: Fix aux->transfer NULL pointer dereference on drm_dp_dpcd_access

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 02 Nov 2021, "Yuan, Perry" <Perry.Yuan@amd.com> wrote:
> [AMD Official Use Only]
>
> Hi Jani:
> Thanks for your comments.
>
>> -----Original Message-----
>> From: Jani Nikula <jani.nikula@linux.intel.com>
>> Sent: Monday, November 1, 2021 9:07 PM
>> To: Yuan, Perry <Perry.Yuan@amd.com>; Maarten Lankhorst
>> <maarten.lankhorst@linux.intel.com>; Maxime Ripard <mripard@kernel.org>;
>> Thomas Zimmermann <tzimmermann@suse.de>; David Airlie <airlied@linux.ie>;
>> Daniel Vetter <daniel@ffwll.ch>
>> Cc: Yuan, Perry <Perry.Yuan@amd.com>; dri-devel@lists.freedesktop.org; linux-
>> kernel@vger.kernel.org; Huang, Shimmer <Xinmei.Huang@amd.com>; Huang,
>> Ray <Ray.Huang@amd.com>
>> Subject: Re: [PATCH v2] drm/dp: Fix aux->transfer NULL pointer dereference on
>> drm_dp_dpcd_access
>> 
>> [CAUTION: External Email]
>> 
>> On Mon, 01 Nov 2021, Perry Yuan <Perry.Yuan@amd.com> wrote:
>> > Fix below crash by adding a check in the drm_dp_dpcd_access which
>> > ensures that aux->transfer was actually initialized earlier.
>> 
>> Gut feeling says this is papering over a real usage issue somewhere else. Why is
>> the aux being used for transfers before ->transfer has been set? Why should the
>> dp helper be defensive against all kinds of misprogramming?
>> 
>> 
>> BR,
>> Jani.
>> 
>
> The issue was found by Intel IGT test suite, graphic by pass test case.
> https://gitlab.freedesktop.org/drm/igt-gpu-tools
> normally use case will not see the issue. 
> To avoid this issue happy again when we run the test case , it will be nice to add a check before the transfer is called.
> And we can see that it really needs to have a check here to make ITG &kernel happy.

You're missing my point. What is the root cause? Why do you have the aux
device or connector registered before ->transfer function is
initialized. I don't think you should do that.

BR,
Jani.


>
> Perry.
>
>> 
>> >
>> > BUG: kernel NULL pointer dereference, address: 0000000000000000 PGD 0
>> > P4D 0
>> > Oops: 0010 [#1] SMP NOPTI
>> > RIP: 0010:0x0
>> > Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
>> > RSP: 0018:ffffa8d64225bab8 EFLAGS: 00010246
>> > RAX: 0000000000000000 RBX: 0000000000000020 RCX: ffffa8d64225bb5e
>> > RDX: ffff93151d921880 RSI: ffffa8d64225bac8 RDI: ffff931511a1a9d8
>> > RBP: ffffa8d64225bb10 R08: 0000000000000001 R09: ffffa8d64225ba60
>> > R10: 0000000000000002 R11: 000000000000000d R12: 0000000000000001
>> > R13: 0000000000000000 R14: ffffa8d64225bb5e R15: ffff931511a1a9d8
>> > FS: 00007ff8ea7fa9c0(0000) GS:ffff9317fe6c0000(0000)
>> > knlGS:0000000000000000
>> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> > CR2: ffffffffffffffd6 CR3: 000000010d5a4000 CR4: 0000000000750ee0
>> > PKRU: 55555554
>> > Call Trace:
>> > drm_dp_dpcd_access+0x72/0x110 [drm_kms_helper]
>> > drm_dp_dpcd_read+0xb7/0xf0 [drm_kms_helper]
>> > drm_dp_start_crc+0x38/0xb0 [drm_kms_helper]
>> > amdgpu_dm_crtc_set_crc_source+0x1ae/0x3e0 [amdgpu]
>> > crtc_crc_open+0x174/0x220 [drm]
>> > full_proxy_open+0x168/0x1f0
>> > ? open_proxy_open+0x100/0x100
>> > do_dentry_open+0x156/0x370
>> > vfs_open+0x2d/0x30
>> >
>> > v2: fix some typo
>> >
>> > Signed-off-by: Perry Yuan <Perry.Yuan@amd.com>
>> > ---
>> >  drivers/gpu/drm/drm_dp_helper.c | 4 ++++
>> >  1 file changed, 4 insertions(+)
>> >
>> > diff --git a/drivers/gpu/drm/drm_dp_helper.c
>> > b/drivers/gpu/drm/drm_dp_helper.c index 6d0f2c447f3b..76b28396001a
>> > 100644
>> > --- a/drivers/gpu/drm/drm_dp_helper.c
>> > +++ b/drivers/gpu/drm/drm_dp_helper.c
>> > @@ -260,6 +260,10 @@ static int drm_dp_dpcd_access(struct drm_dp_aux
>> *aux, u8 request,
>> >       msg.buffer = buffer;
>> >       msg.size = size;
>> >
>> > +     /* No transfer function is set, so not an available DP connector */
>> > +     if (!aux->transfer)
>> > +             return -EINVAL;
>> > +
>> >       mutex_lock(&aux->hw_mutex);
>> >
>> >       /*
>> 
>> --
>> Jani Nikula, Intel Open Source Graphics Center

-- 
Jani Nikula, Intel Open Source Graphics Center