linux-kernel - Re: [PATCHv6 07/16] x86/vsyscall: Reorganize the #PF emulation code

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Message-ID: <f6d73a46-7ca0-4898-bb23-9095be403ea9@intel.com>
Date: Fri, 20 Jun 2025 16:18:28 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>, <dave.hansen@...el.com>
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Subject: Re: [PATCHv6 07/16] x86/vsyscall: Reorganize the #PF emulation code

On 6/20/2025 4:08 PM, Andrew Cooper wrote:
>> But, the resulting code is wonky. It needs to do something more like this:
>>
>> 	if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
>> 		return false;
>>
>> 	if (error_code & X86_PF_INSTR))
>> 		return __emulate_vsyscall(regs, address);
> 
> To do this, LASS needs a proper interlink against NX || SMEP.
> 
> If neither NX nor SMEP are active, the CPU does not report X86_PF_INSTR,
> meaning that fetches are reported as plain reads.
> 
> This leads to some fun corner cases in SMAP and now LASS too for virt.


Maybe I am missing something, but LASS works pre-paging so it wouldn't
generate a PF, right?

We have a new vsyscall emulation for #GP (patch 9) that wouldn't follow
this path.