linux-kernel - Re: [PATCH V2 01/41] x86/entry: Fix swapgs fence

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Message-ID: <445de475-c223-be11-325f-fa6679e45cb0@linux.alibaba.com>
Date:   Mon, 27 Sep 2021 11:27:58 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        "Chang S . Bae" <chang.seok.bae@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH V2 01/41] x86/entry: Fix swapgs fence

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