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

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Tue, Jul 01, 2025 at 06:35:40PM -0700, Sohil Mehta wrote:
> 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?

It can be generated by both kernel and userspace if RSP gets corrupted.

So far, do_error_trap() did the trick, handling what has to be handled.
LASS requires a bit more, though.

> 
> > 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@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 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.

I think line split if justified. It is 120 characters long otherwise.
And with multi-line statement, brackets help readability.

I don't see a reason to change it.

> > +			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.

Okay, will fold into "x86/traps: Generalize #GP address decode and hint
code".

> > +		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;
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov