Re: [PATCH 3/5] x86/vsyscall: Add vsyscall emulation for #GP

[H. Peter Anvin]

On 2026-03-03 13:20, Sohil Mehta wrote:
> 
> Sure, combining things from the cover letter and what you suggested
> here. How about?
>

[...]

> 
> "The vsyscall page is located in the high/kernel part of the address
> space. LASS prevents access to this page from userspace. The current
> kernel only enables LASS when all vsyscall modes are disabled.

Suggest making an introductory paragraph here with the background information,
instead of mixing it into the rest of the text in a somewhat incoherent manner:

"vsyscall emulation can be execute-only (XONLY) or read-execute (EMULATE),
specified by the vsyscall= kernel command line option. XONLY mode is the
default. The EMULATE mode has been deprecated since 2022 and is considered
insecure.

This patch adds support for LASS with XONLY vsyscall emulation.

> With LASS, vsyscall page accesses trigger a #GP instead of a #PF. In
> XONLY (execute-only) mode, directly reading the vsyscall page is
> disallowed. So, the faulting RIP can be easily used to determine if the
> #GP was triggered due to a vsyscall access.

How about:

"With LASS, vsyscall page accesses trigger a #GP instead of a #PF. For XONLY
mode, all that is needed is the faulting RIP, which is trivially available
regardless of the type of fault."

> Reuse the #PF emulation code during a #GP and emulate the vsyscall
> access in XONLY mode. As multiple fault handlers are now using the
> emulation code, add a sanity check to ensure that the fault truly
> happened in 64-bit user mode.
> 
> In contrast, when vsyscall=emulate (deprecated) is configured, it maps
> the vsyscall page as readable. Supporting EMULATE mode with LASS is much
> harder because the #GP doesn't provide enough error information (such as
> PFEC and CR2 in case of #PF). So, complex instruction decoding would be
> required in the #GP handler which isn't worth the effort.

"... as remaining users of EMULATE mode can be reasonably assumed to be niche
users, who are already trading off security for compatibility."

Use "EMULATE mode" consistently here. Captializing it makes it clear that it
is a term and not just a prose word.

> LASS and
> vsyscall=emulate will be kept mutually exclusive for simplicity."

	-hpa