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

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Message-ID: <83e4ffb4-26e9-4fc9-90ad-256a1613ba7d@citrix.com>
Date: Sat, 21 Jun 2025 00:29:40 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Sohil Mehta <sohil.mehta@...el.com>, dave.hansen@...el.com
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Subject: Re: [PATCHv6 07/16] x86/vsyscall: Reorganize the #PF emulation code

On 21/06/2025 12:18 am, Sohil Mehta wrote:
> 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?

Oh right, yes.  This is a preexisting bug in vsyscall #PF handling.

It simply became obvious with Dave's suggested rearrangement.

~Andrew