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

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Message-ID: <5ed7d3c8-63c3-48f3-aaeb-a19514f4ef5e@oracle.com>
Date: Thu, 23 May 2024 19:03:21 +0200
From: Alexandre Chartre <alexandre.chartre@...cle.com>
To: 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,
        andrew.cooper3@...rix.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 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

On the console:

[ 1546.656252] WARNING: CPU: 124 PID: 8413 at arch/x86/kernel/traps.c:1009 exc_debug_kernel+0xd2/0x160
..
[ 1546.656352] RIP: 0010:clear_bhb_loop+0x0/0xb0


alex.