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

[David Laight]

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