linux-kernel - Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

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Message-ID: <373674e4-cbd0-44d7-98c7-304b0ab4f34b@kernel.org>
Date: Tue, 27 Feb 2024 10:43:53 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: Nikolay Borisov <nik.borisov@...e.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
 Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Cc: Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

On 27. 02. 24, 9:47, Nikolay Borisov wrote:
> 
> 
> On 27.02.24 г. 1:52 ч., Pawan Gupta wrote:
>> Macro used for MDS mitigation executes VERW with relative addressing for
>> the operand. This is unnecessary and creates a problem for backports on
>> older kernels that don't support relocations in alternatives. Relocation
>> support was added by commit 270a69c4485d ("x86/alternative: Support
>> relocations in alternatives"). Also asm for fixed addressing is much
>> more cleaner than relative RIP addressing.
>>
>> Simplify the asm by using fixed addressing for VERW operand.
>>
>> Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
>> Reported-by: Nikolay Borisov <nik.borisov@...e.com>
>> Closes: 
>> https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
>> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
>> ---
>>   arch/x86/include/asm/nospec-branch.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/nospec-branch.h 
>> b/arch/x86/include/asm/nospec-branch.h
>> index 2aa52cab1e46..ab19c7f1167b 100644
>> --- a/arch/x86/include/asm/nospec-branch.h
>> +++ b/arch/x86/include/asm/nospec-branch.h
>> @@ -323,7 +323,7 @@
>>    * Note: Only the memory operand variant of VERW clears the CPU 
>> buffers.
>>    */
>>   .macro CLEAR_CPU_BUFFERS
>> -    ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), 
>> X86_FEATURE_CLEAR_CPU_BUF
>> +    ALTERNATIVE "", __stringify(verw mds_verw_sel), 
>> X86_FEATURE_CLEAR_CPU_BUF
> 
> Actually thinking about it more and discussing with Jiri (cc'ed), will 
> this work with KASLR enabled?

I might of course be wrong. We appear to rely on the asm+linker here. 
Please see also:
https://lore.kernel.org/r/fd8f2df0-563e-4f5c-aca4-bc92a14e9426@kernel.org

thanks,
-- 
js
suse labs