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

Kirill A. Shutemov
2025-06-27T10:31:57Z

On Thu, Jun 26, 2025 at 10:57:47AM -0700, Xin Li wrote:
> 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]"?

I will add the link. It is important context.

> > fault on ERETU by jumping to fred_entrypoint_user") for more details.
> >
> > Co-developed-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > 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 | 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.

Naming is hard.

Maybe get_kernel_exc_address()/kernel_exc_hint_help/EXC_NO_HINT/... ?

> > +
> > + 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@zytor.com>

Thanks.

--
Kiryl Shutsemau / Kirill A. Shutemov