From: Alexander Shishkin <alexander.shishkin@linux.intel.com>

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@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 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
@@ -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 +860,34 @@ 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;
+		}
+
+		hint = get_kernel_gp_address(regs, &gp_addr);
+		if (hint != GP_NO_HINT) {
+			printk(GPFSTR ", %s 0x%lx", kernel_gp_hint_help[hint],
+			       gp_addr);
+		}
+
+		if (hint != GP_NON_CANONICAL)
+			gp_addr = 0;
+
+		die_addr(GPFSTR, regs, error_code, gp_addr);
+		return;
+	}
+
+	do_error_trap(regs, error_code, "stack segment", X86_TRAP_SS, SIGBUS,
+		0, NULL);
+}
+
 static bool do_int3(struct pt_regs *regs)
 {
 	int res;
-- 
2.47.2

On 6/20/2025 6:53 AM, Kirill A. Shutemov wrote:
> From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> 
> 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@linux.intel.com>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>   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

On Fri, Jun 20, 2025 at 08:20:12AM -0700, Xin Li wrote:
> On 6/20/2025 6:53 AM, Kirill A. Shutemov wrote:
> > From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > 
> > 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@linux.intel.com>
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >   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.

Okay, will do.

> > +
> > +		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.

Yep, thanks for catching that!

-- 
  Kiryl Shutsemau / Kirill A. Shutemov