Re: [PATCHv8 14/17] x86/traps: Handle LASS thrown #SS

[Sohil Mehta]

On 7/1/2025 2:58 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.
> 

Maybe I've misunderstood something:

Is the underlying assumption here that #SS were previously only
generated by userspace, but now they can also be generated by the
kernel? And we want the kernel generated #SS to behave the same as the #GP?

> 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. See 5105e7687ad3 ("x86/fred: Fixup
> 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 ceb091f17a5b..f9ca5b911141 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;
> +
> +	if (cpu_feature_enabled(X86_FEATURE_LASS)) {
> +		enum kernel_exc_hint hint;
> +		unsigned long exc_addr;
> +
> +		hint = get_kernel_exc_address(regs, &exc_addr);
> +		if (hint != EXC_NO_HINT) {

The brackets are not needed for singular statements. Also the max line
length is longer now. You can fit this all in a single line.

> +			printk(SSFSTR ", %s 0x%lx", kernel_exc_hint_help[hint],
> +			       exc_addr);
> +		}
> +

> +		if (hint != EXC_NON_CANONICAL)
> +			exc_addr = 0;
> +
> +		die_addr(SSFSTR, regs, error_code, exc_addr);

The variable names in die_addr() should be generalized as well. They
seem to assume the caller to be a #GP handler.

> +		return;
> +	}
> +
> +error_trap:
> +	do_error_trap(regs, error_code, "stack segment", X86_TRAP_SS, SIGBUS,
> +		      0, NULL);
> +}
> +
>  static bool do_int3(struct pt_regs *regs)
>  {
>  	int res;