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

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Message-ID: <bd81a98b-f8d4-4304-ac55-d4151a1a77ab@intel.com>
Date: Mon, 23 Jun 2025 08:32:53 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
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Subject: Re: [PATCHv6 07/16] x86/vsyscall: Reorganize the #PF emulation code

On 6/23/25 05:41, Kirill A. Shutemov wrote:
> So, IIUC, that's dependency of vsyscall PF on NX. Do we want to disable
> vsyscall on boot if NX is not available?

Well, vsyscall=none can break old userspace, so forcing it on old
hardware doesn't seem like a great idea.

But, either way, this doesn't really appear to be a LASS issue. This code:

>         if (!(error_code & X86_PF_INSTR)) {
>                 /* Failed vsyscall read */
>                 if (vsyscall_mode == EMULATE)
>                         return false;

Is really asking the question:

	Is this #PF from an instruction fetch in the vsyscall page?

That _should_ be able to be done by comparing CR2 and regs->rip. In
fact, that's done just below anyway:

	WARN_ON_ONCE(address != regs->ip);

So I think we can fix this up with something like the attached patch
which just drives the if() from regs->rip and make the warning NX-only.

But this code has been like this a long time and I'm 99% sure the x86
selftests poke at all these cases. I'm curious what they do on those old
P4's (or a 64-bit VM with NX turned off), but it's not super important
either way.
View attachment "vsyscall-nx.patch" of type "text/x-patch" (1152 bytes)