Re: [PATCH] drm/nouveau: avoid null deref on bad arguments to nouveau_vma_getmap

[Maarten Lankhorst <maarten.lankhorst@canonical.com>]

Op 03-09-13 16:20, Pasi Kärkkäinen schreef:
> On Wed, Aug 28, 2013 at 09:44:15AM +0200, Maarten Lankhorst wrote:
>> Op 28-08-13 08:29, Pasi Kärkkäinen schreef:
>>> On Fri, Aug 23, 2013 at 11:20:42PM +0300, Pasi Kärkkäinen wrote:
>>>> On Thu, Aug 22, 2013 at 09:12:40AM +0200, Maarten Lankhorst wrote:
>>>>> Op 22-08-13 02:10, Ilia Mirkin schreef:
>>>>>> The code expects non-VRAM mem nodes to have a pages list. If that's not
>>>>>> set, it will do a null deref down the line. Warn on that condition and
>>>>>> return an error.
>>>>>>
>>>>>> See https://bugs.freedesktop.org/show_bug.cgi?id=64774
>>>>>>
>>>>>> Reported-by: Pasi Kärkkäinen <pasik@iki.fi>
>>>>>> Tested-by: Pasi Kärkkäinen <pasik@iki.fi>
>>>>>> Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
>>>>>> Cc: <stable@vger.kernel.org> # 3.8+
>>>>>> ---
>>>>>>
>>>>>> I don't exactly understand what's going on, but this is just a
>>>>>> straightforward way to avoid a null deref that you see happens in the
>>>>>> bug. I haven't figured out the root cause of this, but it's getting
>>>>>> well into the "I have no idea how TTM works" space. However this seems
>>>>>> like a bit of defensive programming -- nouveau_vm_map_sg will pass
>>>>>> node->pages as a list down, which will be dereferenced by
>>>>>> nvc0_vm_map_sg. Perhaps the other arguments should make that
>>>>>> dereferencing not happen, but it definitely was happening here, as you
>>>>>> can see in the bug.
>>>>>>
>>>>>> Ben/Maarten, I'll let you judge whether this check is appropriate,
>>>>>> since like I hope I was able to convey above, I'm just not really sure :)
>>>>> Not it really isn't appropriate..
>>>>>
>>>>> You'd have to call call nouveau_vm_map_sg_table instead, the only place that doesn't handle that correctly
>>>>> is where it's not expected to be called.
>>>>>
>>>>> Here, have a completely untested patch to fix things...
>>>>>
>>>> Maarten: I've been testing this stuff with Linux 3.10.x, so I had to modify the patch a bit to make it apply there.. 
>>>> I've attached the modified copy that applies to 3.10.9, hopefully I did the backport correctly.
>>>>
>>>> With Linux 3.10.9 and the patch applied the kernel doesn't crash anymore, and I get this error in dmesg:
>>>>
>>>> [   76.105643] nouveau W[     DRM] Trying to create a fb in vram with valid_domains=00000004
>>>>
>>>> Does that help? 
>>>>
>>> Any comments? 
>>>
>>> Maarten's patch works for me, I get that warning instead of a kernel crash,
>>> but it's also a bigger change that doesn't apply to older kernels as-is. 
>>>
>>> Ilia's original patch in this thread can be applied to older kernels as-is,
>>> and it also prevents the kernel crash from happening.
>>>
>>> Should we get both applied, so Ilia's patch can be CC'd to stable ? 
>>>
>> You haven't understood the root cause then. You're trying to move an IMPORTED dma-buf into VRAM.
>> Doing so would seem to work, but at that point it's no longer a dma-buf so changes done by the exporter
>> would not show up in nouveau because they no longer refer to the same piece of memory.
>>
> Yes, that's true, I don't understand the root cause. 
> That's mostly because I'm not familiar with the nouveau code/internals. 
>
>
>> Failing is the only right option here.
>>
> Sorry but I'm not sure I understand that correctly.. what exactly do you suggest? What should fail?
>
>
> Because I'm not familiar with the code (yet) understanding and finding the root cause
> will take time for me.. that's why I was suggesting to meanwhile apply Ilia's very simple patch
> from this thread which actually prevents the hard kernel crash from happening, because it seems 
> like an unharmful fix to have to protect against this kind of bugs (the root cause) ?
> Or is that unacceptable? 
>
> (To recap: The kernel crash happens very often when trying to use the nouveau adapter in Optimus mode, with all kernels at least 3.8+, and it's very annoying that your laptop crashes when trying to enable a nouveau output. So Ilia's patch doesn't make the driver working properly, but at least it prevents the hard kernel crash from happening. The crash bug is in many kernel versions, including the long-term v3.10 tree. I've had the crash happening with 3.8.x, 3.9.x and 3.10.x)
The fix probably requires commit fdfb8332651db7a280851dfccfc4f0cff4bcd052 to apply cleanly "drm/nouveau: fix some error-path leaks in fbcon handling code"

> All comments welcome.
>
> Thanks,
>
> -- Pasi
>