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

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Message-ID: <lvutnc4et6r4a5eayoweb5butpspvop2m2pjioiudjwa3mkpo7@drcsdchj66w5>
Date: Fri, 27 Jun 2025 13:31:44 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Xin Li <xin@...or.com>
Cc: Andy Lutomirski <luto@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
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	"Paul E. McKenney" <paulmck@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Xiongwei Song <xiongwei.song@...driver.com>, Xin Li <xin3.li@...el.com>, 
	"Mike Rapoport (IBM)" <rppt@...nel.org>, Brijesh Singh <brijesh.singh@....com>, 
	Michael Roth <michael.roth@....com>, Tony Luck <tony.luck@...el.com>, 
	Alexey Kardashevskiy <aik@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
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	linux-mm@...ck.org
Subject: Re: [PATCHv7 13/16] x86/traps: Handle LASS thrown #SS

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@...ux.intel.com>
> > Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.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@...or.com>

Thanks.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov