linux-kernel - Re: [PATCH V4 01/50] x86/entry: Add fence for kernel entry swapgs in paranoid

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Date:   Thu, 28 Oct 2021 08:51:59 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Borislav Petkov <bp@...en8.de>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Chang S . Bae" <chang.seok.bae@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH V4 01/50] x86/entry: Add fence for kernel entry swapgs in
 paranoid_entry()



On 2021/10/27 20:19, Borislav Petkov wrote:
> On Tue, Oct 26, 2021 at 10:13:31PM +0800, Lai Jiangshan wrote:
>> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>>
>> Commit 18ec54fdd6d18 ("x86/speculation: Prepare entry code for Spectre
>> v1 swapgs mitigations") adds FENCE_SWAPGS_{KERNEL|USER}_ENTRY
>> for conditional swapgs.  And in paranoid_entry(), it uses only
>> FENCE_SWAPGS_KERNEL_ENTRY for both branches.  It is because the fence
>> is required for both cases since the CR3 write is conditinal even PTI
>> is enabled.
>>
>> But commit 96b2371413e8f ("x86/entry/64: Switch CR3 before SWAPGS in
>> paranoid entry") switches the code order and changes the branches.
>> And it misses the needed FENCE_SWAPGS_KERNEL_ENTRY for user gsbase case.
>>
>> Add it back.
>>
>> Fixes: Commit 96b2371413e8f ("x86/entry/64: Switch CR3 before SWAPGS in paranoid entry")
>> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
>> Cc: Chang S. Bae <chang.seok.bae@...el.com>
>> Cc: Sasha Levin <sashal@...nel.org>
>> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
>> ---
>>   arch/x86/entry/entry_64.S | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index e38a4cf795d9..1f98188e83ef 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -898,6 +898,7 @@ SYM_CODE_START_LOCAL(paranoid_entry)
>>   	rdmsr
>>   	testl	%edx, %edx
>>   	jns	.Lparanoid_entry_swapgs
>> +	FENCE_SWAPGS_KERNEL_ENTRY
> 
> Wouldn't it be cleaner to simply move the FENCE up from under the swapgs
> to before the jns instruction?

Sure, I will move it just before the testl instruction.

> 
> This way you kill speculation for both cases and have a single fence.
>