Re: [PATCHv6 13/16] x86/traps: Handle LASS thrown #SS

[Xin Li]

On 6/20/2025 6:53 AM, Kirill A. Shutemov wrote:
> From: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
>
> LASS throws a #GP for any violations except for stack register accesses,
> in which case it throws a #SS instead. Handle this similarly to how other
> LASS violations are handled.
>
> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>   arch/x86/kernel/traps.c | 34 ++++++++++++++++++++++++++++------
>   1 file changed, 28 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 0f558d3369a3..bd8f7e72b238 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -866,6 +860,34 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
>   	cond_local_irq_disable(regs);
>   }
>   
> +DEFINE_IDTENTRY_ERRORCODE(exc_stack_segment)
> +{
> +	if (cpu_feature_enabled(X86_FEATURE_LASS)) {
> +		enum kernel_gp_hint hint = GP_NO_HINT;
> +		unsigned long gp_addr;
> +
> +		if (user_mode(regs)) {
> +			gp_user_force_sig_segv(regs, X86_TRAP_GP, error_code, GPFSTR);
> +			return;
> +		}

Please add the following FRED code:

		if (cpu_feature_enabled(X86_FEATURE_FRED) &&
		    fixup_exception(regs, X86_TRAP_SS, error_code, 0))
			return;

Because the following FRED fixup

 
https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5105e7687ad3dffde77f6e4393b5530e83d672dc

is otherwise skipped, it is interpreted as a LASS #SS before reaching
the FRED fixup (which is indeed a userspace fault but triggered from
a specific kernel IP as described in the commit changelog).

The x86 selftest sigreturn_64 fails w/o the above fix.

> +
> +		hint = get_kernel_gp_address(regs, &gp_addr);
> +		if (hint != GP_NO_HINT) {
> +			printk(GPFSTR ", %s 0x%lx", kernel_gp_hint_help[hint],

GPFSTR?

#define GPFSTR "general protection fault"

It is misleading here, you need something like SSFSTR which tells
exactly which exception caused it.

Thanks!
    Xin