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Date:   Wed, 10 Nov 2021 10:32:21 -0500
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Subject: Re: [PATCH v2] drm/dp: Fix aux->transfer NULL pointer dereference on
 drm_dp_dpcd_access
Content-Language: en-US
To:     "Yuan, Perry" <Perry.Yuan@amd.com>,
        Jani Nikula <jani.nikula@linux.intel.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:     "Huang, Shimmer" <Xinmei.Huang@amd.com>,
        "Huang, Ray" <Ray.Huang@amd.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "Limonciello, Mario" <Mario.Limonciello@amd.com>
References: <20211101061053.38173-1-Perry.Yuan@amd.com>
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On 2021-11-05 03:35, Yuan, Perry wrote:
> [AMD Official Use Only]
> 
> Hi Jani:
> 
> 
>> -----Original Message-----
>> From: Jani Nikula <jani.nikula@linux.intel.com>
>> Sent: Wednesday, November 3, 2021 7:31 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: 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 Wed, 03 Nov 2021, "Yuan, Perry" <Perry.Yuan@amd.com> wrote:
>>> [AMD Official Use Only]
>>>
>>> Hi Jani:
>>>
>>>> -----Original Message-----
>>>> From: Jani Nikula <jani.nikula@linux.intel.com>
>>>> Sent: Tuesday, November 2, 2021 4:40 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: 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 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://g
 itl
>>>>> ab.freedesktop.org%2Fdrm%2Figt-gpu-
>>>> tools&amp;data=04%7C01%7CPerry.Yuan
>>>>> %40amd.com%7C83d011acfe65437c0fa808d99ddc65b0%7C3dd8961fe4
>> 884e6
>>>> 08e11a8
>>>>>
>>>>
>> 2d994e183d%7C0%7C0%7C637714392203200313%7CUnknown%7CTWFpbG
>> Zsb
>>>> 3d8eyJWIj
>>>>>
>>>>
>> oiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1
>> 00
>>>> 0&am
>>>>>
>>>>
>> p;sdata=snPpRYLGeJtTpNGle1YHZAvevcABbgLkgOsffiNzQPw%3D&amp;reser
>> ved
>>>> =0
>>>>> 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.
>>>>
>>>
>>> One potential IGT fix patch to resolve the test case failure is:
>>>
>>> tests/amdgpu/amd_bypass.c
>>>       data->pipe_crc = igt_pipe_crc_new(data->drm_fd, data->pipe_id,
>>>                                        - AMDGPU_PIPE_CRC_SOURCE_DPRX);
>>>                                        + INTEL_PIPE_CRC_SOURCE_AUTO);
>>> The kernel panic error gone after change  "dprx" to "auto" in the IGT test.
>>>
>>> In my view ,the IGT amdgpu bypass test will do some common setup work
>> including crc piple, source.
>>> When the IGT sets up a new CRC pipe capture source for amdgpu bypass
>> test,  the SOURCE was set as "dprx" instead of "auto"
>>> It makes "amdgpu_dm_crtc_set_crc_source()"  failed to set correct  AUX
>> and it's  transfer function invalid .
>>> The system I tested use HDMI port connected to monitor .
>>>
>>> amdgpu_dm_crtc_set_crc_source->    (aux = (aconn->port) ? &aconn-
>>> port->aux : &aconn->dm_dp_aux.aux;)
>>>        drm_dp_start_crc ->
>>>               drm_dp_dpcd_readb->   aux->transfer is NULL, issue here.
>>> The fix will  use the "auto" keyword, which will let the driver select a
>> default source of frame CRCs for this CRTC.
>>>
>>> Correct me if have some wrong points.
>>
>> Apparently I'm completely failing to communicate my POV to you.
>>
>> If you have a kernel oops, the fix needs to be in the kernel, not IGT.
>>
>> The question is, why is it possible for IGT (or any userspace) to trigger AUX
>> communication when the ->transfer function is not set? In my opinion, the
>> kernel driver should not have exposed the interface at all if the ->transfer
>> function is not set. The interface is useless without the ->transfer function.
>> IMO, that's the bug.
>>
> 
> Yes , you are correct , the transfer shouldn't be called before it is ready !
> 
> Let me explain more details in my view .
> Maybe the root cause is not why the aux->transfer is not called before it is registered in this case.
> I suppose the issue was triggered by wrong CRC pipe source .
> 
> Actually the aux->transfer has been registered when amdgpu DM registered at kernel boot.
> IGT test was run when system login to Gnome desktop.
> 
> amdgpu_dm_initialize_dp_connector->
> aconnector->dm_dp_aux.aux.transfer = dm_dp_aux_transfer;
> 
> The test case failed when the IGT  set an  "DPRX"  CRC pipe source while the HDMI connected to monitor only.
> At this time, the aux->transfer is NULL,  and dp helper did not check the transfer pointer NULL or not.
> It calls the transfers to DPCD read, then you see the kernel panic log.
>  
> amdgpu_dm_crtc_funcs->  amdgpu_dm_crtc_set_crc_source-> drm_dp_start_crc 
> 
> * And if the DP cable connected only, the issue will not happen.  Test will pass.
> * If I change the CRC source to "auto", kernel will not see the panic as well.
> Maybe the failed test case need to run on the DP  instead of HDMI, I am not sure at now.
> 

Two things need to happen:
1) IGT should skip tests requiring DPRX CRC source if not on a
   DP connector.
2) Driver should return EINVAL (or another appropriate error) if
   DPRX CRC source is requested when the CRTC is not connected to
   a DP display. Alternatively we could make sure that DPRX is
   not advertised as a CRC source in this case but I'm not sure
   how difficult that would be.

Like Jani said, I don't think the current patch is the correct one
as it doesn't get to the root cause. The root cause fix should be
in the CRC debugfs handling code.

Harry

> 
> Hopping this info can help. 
> 
> Perry.
>  
> 
>>
>> BR,
>> Jani.
>>
>>>
>>> Thank you!
>>> Perry.
>>>
>>>>
>>>>>
>>>>> 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
>>
>> --
>> Jani Nikula, Intel Open Source Graphics Center