linux-kernel - Re: commit 7ffb791423c7 breaks steam game

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <2cafd3e1-9265-403a-9854-7200d84ca397@nvidia.com>
Date: Fri, 21 Mar 2025 10:55:27 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Bert Karwatzki <spasswolf@....de>, Alex Deucher <alexdeucher@...il.com>,
 Kees Cook <kees@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Andy Lutomirski <luto@...nel.org>,
 linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org
Subject: Re: commit 7ffb791423c7 breaks steam game

On 3/20/25 20:01, Ingo Molnar wrote:
> 
> * Balbir Singh <balbirs@...dia.com> wrote:
> 
>> On 3/17/25 00:09, Bert Karwatzki wrote:
>>> This is related to the admgpu.gttsize. My laptop has the maximum amount 
>>> of memory (64G) and usually gttsize is half of main memory size. I just 
>>> tested with cmdline="nokaslr amdgpi.gttsize=2048" and the problem does 
>>> not occur. So I did some more testing with varying gttsize and got this
>>> for the built-in GPU
>>>
>>> 08:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI]
>>> Cezanne [Radeon Vega Series / Radeon Vega Mobile Series] (rev c5)
>>>
>>> (nokaslr is always enabeld)
>>> gttssize   input behaviour
>>>  2048		GOOD
>>>  2064		GOOD
>>>  2080		SEMIBAD (i.e. noticeable input lag but not as bad as below)
>>>  3072		BAD
>>>  4096		BAD
>>>  8192		BAD
>>> 16384		BAD
>>>
>>> As the build-in GPU has ~512 VRAM there seems to be problems when gttsize >
>>> 4*VRAM so I tested for the discrete GPU with 8G of VRAM
>>> gttsize   input behaviour
>>> 49152		GOOD
>>> 64000		GOOD
>>>
>>> So for the discrete GPU increasing gttsize does no reproduce the bug.
>>>
>>
>> Very interesting, I am not a GTT expert, but with these experiments do you
>> find anything interesting in
>>
>> /sys/kernel/debug/x86/pat_memtype_list?
>>
>> It's weird that you don't see any issues in Xorg (Xfce), just the games.
>> May be we should get help from the amd-gfx experts to further diagnose/debug
>> the interaction of nokaslr with the game.
> 
> So basically your commit:
> 
>   7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
> 
> inflicts part of the effects of a 'nokaslr' boot command line option, 
> and triggers the regression due to that?
> 
> Or is there some other cause?
> 

You are right in your assessment of the root cause. Just to reiterate
- nokaslr does not work with the iGPU, specifically for the games mentioned
- There is a workaround for the problem, which involves reducing the amdgpu.gttsize
- The patch exposes the system to nokaslr situation (effect) when PCI_P2PDMA is enabled

Balbir Singh