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Subject: Re: [PATCH V2 01/41] x86/entry: Fix swapgs fence
To:     Thomas Gleixner <tglx@linutronix.de>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        "Chang S . Bae" <chang.seok.bae@intel.com>,
        Sasha Levin <sashal@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
References: <20210926150838.197719-1-jiangshanlai@gmail.com>
 <20210926150838.197719-2-jiangshanlai@gmail.com> <87r1dbawzq.ffs@tglx>
From:   Lai Jiangshan <laijs@linux.alibaba.com>
Message-ID: <9312a767-f1d3-d283-80a9-e6b3854252e1@linux.alibaba.com>
Date:   Mon, 27 Sep 2021 09:10:49 +0800
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On 2021/9/27 04:43, Thomas Gleixner wrote:
> Lai,
> 
> On Sun, Sep 26 2021 at 23:07, Lai Jiangshan wrote:
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -898,17 +898,12 @@ SYM_CODE_START_LOCAL(paranoid_entry)
>>   	rdmsr
>>   	testl	%edx, %edx
>>   	jns	.Lparanoid_entry_swapgs
>> +	FENCE_SWAPGS_KERNEL_ENTRY
> 
> Good catch.
> 
>>   	ret
>>   
>>   .Lparanoid_entry_swapgs:
>>   	swapgs
>> -
>> -	/*
>> -	 * The above SAVE_AND_SWITCH_TO_KERNEL_CR3 macro doesn't do an
>> -	 * unconditional CR3 write, even in the PTI case.  So do an lfence
>> -	 * to prevent GS speculation, regardless of whether PTI is enabled.
>> -	 */
>> -	FENCE_SWAPGS_KERNEL_ENTRY
>> +	FENCE_SWAPGS_USER_ENTRY
> 
> This change is wrong.
> 
> In the paranoid entry path even if user GS base is set then the entry
> does not necessarily come from user space so there is no guarantee that
> there was a CR3 write on PTI enabled systems before the SWAPGS.
> 
> FENCE_SWAPGS_USER_ENTRY does not emit a LFENCE when PTI is enabled, so
> both the comment and FENCE_SWAPGS_KERNEL_ENTRY which emits LFENCE on
> affected CPUs unconditionaly are correct. Though the comment could do
> with some polishing to make this entirely clear.


I didn't notice FENCE_SWAPGS_USER_ENTRY depends on PTI.

I will add FENCE_SWAPGS_KERNEL_ENTRY only on the kernel path.

Thanks
Lai