linux-kernel - Re: [PATCH] x86/bhi: BHI mitigation can trigger warning in #DB handler

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Date: Thu, 23 May 2024 20:30:44 +0200
From: Alexandre Chartre <alexandre.chartre@...cle.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>,
        Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org
Cc: alexandre.chartre@...cle.com, linux-kernel@...r.kernel.org,
        daniel.sneddon@...ux.intel.com, pawan.kumar.gupta@...ux.intel.com,
        tglx@...utronix.de, konrad.wilk@...cle.com, peterz@...radead.org,
        gregkh@...uxfoundation.org, seanjc@...gle.com,
        dave.hansen@...ux.intel.com, nik.borisov@...e.com, kpsingh@...nel.org,
        longman@...hat.com, bp@...en8.de, pbonzini@...hat.com
Subject: Re: [PATCH] x86/bhi: BHI mitigation can trigger warning in #DB
 handler



On 5/23/24 20:27, Alexandre Chartre wrote:
> 
> 
> On 5/23/24 19:53, Andrew Cooper wrote:
>> On 23/05/2024 6:03 pm, Alexandre Chartre wrote:
>>>
>>> On 5/23/24 17:36, Dave Hansen wrote:
>>>> On 5/23/24 07:52, Alexandre Chartre wrote:
>>>>>> Should we wrap up this gem and put it with the other entry selftests?
>>>>>
>>>>> It looks like tools/testing/selftests/x86/single_step_syscall.c tests
>>>>> sysenter with TF set but it doesn't check if the kernel issues any
>>>>> warning.
>>>>
>>>> Does it actually trip the warning though? I'm a bit surprised that
>>>> nobody reported it if so.
>>>
>>> single_step_syscall does trigger the warning:
>>>
>>> $ ./single_step_syscall
>>> [RUN]    Set TF and check nop
>>> [OK]    Survived with TF set and 26 traps
>>> [RUN]    Set TF and check syscall-less opportunistic sysret
>>> [OK]    Survived with TF set and 30 traps
>>> [RUN]    Set TF and check a fast syscall
>>> [OK]    Survived with TF set and 40 traps
>>> [RUN]    Fast syscall with TF cleared
>>> [OK]    Nothing unexpected happened
>>> [RUN]    Set TF and check SYSENTER
>>>      Got SIGSEGV with RIP=ed7fe579, TF=256
>>> [RUN]    Fast syscall with TF cleared
>>> [OK]    Nothing unexpected happened
>>
>> :-/
>>
>> What about the exit code?
>>
>> I find the absence of a [FAIL] concerning...
>>
> 
> $ ./single_step_syscall
> [RUN]    Set TF and check nop
> [OK]    Survived with TF set and 26 traps
> [RUN]    Set TF and check syscall-less opportunistic sysret
> [OK]    Survived with TF set and 30 traps
> [RUN]    Set TF and check a fast syscall
> [OK]    Survived with TF set and 40 traps
> [RUN]    Fast syscall with TF cleared
> [OK]    Nothing unexpected happened
> [RUN]    Set TF and check SYSENTER
>      Got SIGSEGV with RIP=bb44b579, TF=256
> [RUN]    Fast syscall with TF cleared
> [OK]    Nothing unexpected happened
> 
> $ echo $?
> 0
> 
> The program runs as expected (but it doesn't expect much than a SIGSEGV).
> It triggers a warning from the kernel but it doesn't check if a warning
> was produced.
> 

Actually it checks that TF isn't cleared after the sysenter, but that's all.

alex.