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Subject: Re: [PATCH V2 01/41] x86/entry: Fix swapgs fence
From:   Lai Jiangshan <laijs@linux.alibaba.com>
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>
 <9312a767-f1d3-d283-80a9-e6b3854252e1@linux.alibaba.com>
Message-ID: <445de475-c223-be11-325f-fa6679e45cb0@linux.alibaba.com>
Date:   Mon, 27 Sep 2021 11:27:58 +0800
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On 2021/9/27 09:10, Lai Jiangshan wrote:

>>
>> 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.

The commit c75890700455 ("x86/entry/64: Remove unneeded kernel CR3 switching")
( https://lore.kernel.org/all/20200419144049.1906-2-laijs@linux.alibaba.com/ )
also made it wrong.

When the SWITCH_TO_KERNEL_CR3 in the path is removed, FENCE_SWAPGS_USER_ENTRY
should also be changed to FENCE_SWAPGS_KERNEL_ENTRY. (Or just jmp to
.Lerror_entry_done_lfence which has FENCE_SWAPGS_KERNEL_ENTRY already.)

And FENCE_SWAPGS_USER_ENTRY could be documented with "it should be followed with
serializing operations such as SWITCH_TO_KERNEL_CR3".  Or we can add a
SWAPGS_AND_SWITCH_TO_KERNEL_CR3 to combine them.

I will fix it in v3. (Or should I do it separately before v3?)

Sorry for my fault.
Lai

> 
> I will add FENCE_SWAPGS_KERNEL_ENTRY only on the kernel path.
> 
> Thanks
> Lai