Re: [PATCHv7 13/16] x86/traps: Handle LASS thrown #SS

[Xin Li]

On 6/25/2025 5:51 AM, Kirill A. Shutemov wrote:
> 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.
>
> In case of FRED, before handling #SS as LASS violation, kernel has to
> check if there's a fixup for the exception. It can address #SS due to
> invalid user context on ERETU[1]. See 5105e7687ad3 ("x86/fred: Fixup

Forgot to put the link to [1]?  Maybe just remove "[1]"?

> fault on ERETU by jumping to fred_entrypoint_user") for more details.
>
> Co-developed-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>   arch/x86/kernel/traps.c | 39 +++++++++++++++++++++++++++++++++------
>   1 file changed, 33 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index e2ad760b17ea..f1f92e1ba524 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -418,12 +418,6 @@ DEFINE_IDTENTRY_ERRORCODE(exc_segment_not_present)
>   		      SIGBUS, 0, NULL);
>   }
>   
> -DEFINE_IDTENTRY_ERRORCODE(exc_stack_segment)
> -{
> -	do_error_trap(regs, error_code, "stack segment", X86_TRAP_SS, SIGBUS,
> -		      0, NULL);
> -}
> -
>   DEFINE_IDTENTRY_ERRORCODE(exc_alignment_check)
>   {
>   	char *str = "alignment check";
> @@ -866,6 +860,39 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
>   	cond_local_irq_disable(regs);
>   }
>   
> +#define SSFSTR "stack segment fault"
> +
> +DEFINE_IDTENTRY_ERRORCODE(exc_stack_segment)
> +{
> +	if (user_mode(regs))
> +		goto error_trap;
> +
> +	if (cpu_feature_enabled(X86_FEATURE_FRED) &&
> +	    fixup_exception(regs, X86_TRAP_SS, error_code, 0))
> +		return;
> +

Thanks for making the change for FRED.

> +	if (cpu_feature_enabled(X86_FEATURE_LASS)) {
> +		enum kernel_gp_hint hint;
> +		unsigned long gp_addr;
> +
> +		hint = get_kernel_gp_address(regs, &gp_addr);
> +		if (hint != GP_NO_HINT) {
> +			printk(SSFSTR ", %s 0x%lx", kernel_gp_hint_help[hint],
> +			       gp_addr);
> +		}
> +
> +		if (hint != GP_NON_CANONICAL)
> +			gp_addr = 0;

Nit: GP/gp don't seem fit here, maybe we need a more generic name?

Sorry I don't have a recommendation.

> +
> +		die_addr(SSFSTR, regs, error_code, gp_addr);
> +		return;
> +	}
> +
> +error_trap:
> +	do_error_trap(regs, error_code, "stack segment", X86_TRAP_SS, SIGBUS,
> +		0, NULL);

The indentation has changed; I believe the original formatting is
preferable.

> +}
> +
>   static bool do_int3(struct pt_regs *regs)
>   {
>   	int res;

Just minor comments, so

Reviewed-by: Xin Li (Intel) <xin@xxxxxxxxx>