linux-kernel - Re: [PATCH v10 08/15] x86/vsyscall: Reorganize the page fault emulation code

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Message-ID: <20251030213700.1e98203e@pumpkin>
Date: Thu, 30 Oct 2025 21:37:00 +0000
From: David Laight <david.laight.linux@...il.com>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: Andy Lutomirski <luto@...nel.org>, Dave Hansen <dave.hansen@...el.com>,
 Rick P Edgecombe <rick.p.edgecombe@...el.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
 <bp@...en8.de>, the arch/x86 maintainers <x86@...nel.org>, Dave Hansen
 <dave.hansen@...ux.intel.com>, Jonathan Corbet <corbet@....net>, Ard
 Biesheuvel <ardb@...nel.org>, "jpoimboe@...nel.org" <jpoimboe@...nel.org>,
 Andrew Cooper <andrew.cooper3@...rix.com>, Tony Luck <tony.luck@...el.com>,
 "Alexander Shishkin" <alexander.shishkin@...ux.intel.com>, "Kirill A .
 Shutemov" <kas@...nel.org>, Sean Christopherson <seanjc@...gle.com>, Randy
 Dunlap <rdunlap@...radead.org>, David Woodhouse <dwmw@...zon.co.uk>, Vegard
 Nossum <vegard.nossum@...cle.com>, Xin Li <xin@...or.com>, Linux Kernel
 Mailing List <linux-kernel@...r.kernel.org>, "linux-doc@...r.kernel.org"
 <linux-doc@...r.kernel.org>, Kees Cook <kees@...nel.org>, "H. Peter Anvin"
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 "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>, Geert Uytterhoeven
 <geert@...ux-m68k.org>
Subject: Re: [PATCH v10 08/15] x86/vsyscall: Reorganize the page fault
 emulation code

On Thu, 30 Oct 2025 12:28:52 -0700
Sohil Mehta <sohil.mehta@...el.com> wrote:

> Thank you for taking a look at these patches.
> 
> On 10/30/2025 9:58 AM, Andy Lutomirski wrote:
> 
> > So I think all that's needed is to update "[PATCH v10 10/15] x86/vsyscall: Add vsyscall emulation for #GP" to check user_64bit_mode(regs) for the vsyscall case.  (As submitted, unless I missed something while composing the patches in my head, it's only checking user_mode(regs), and I think it's worth the single extra line of code to make the result a tiny bit more robust.)  
> 
> I probably don't understand all the nuances here. But, the goal of the
> check seems to ensure a 32-bit process running on a 64-bit kernel
> doesn't ever go through this vsyscall emulation code, right?

Do remember that there is no such thing as a '32-bit process'.
Changing to/from 'long mode' isn't privileged.
OTOH in 32-bit mode you can't generate an address above 4G.
(But I've no idea if the high register bits get cleared before the register
is modified.)

	David

> 
> I guess a user_64bit_mode(regs) check wouldn't harm. I'll add it when
> the vsyscall series is posted.
> 
> 
> 
> 
>