linux-kernel - Re: [PATCHv6 13/16] x86/traps: Handle LASS thrown #SS

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Message-ID: <p6nykttpomxtjw4py65ozxpuirviubhmhqu5paa75mnzk75kck@jb43wyhf5sn3>
Date: Fri, 20 Jun 2025 20:53:36 +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>, 
	Peter Zijlstra <peterz@...radead.org>, Ard Biesheuvel <ardb@...nel.org>, 
	"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>, 
	Jonathan Corbet <corbet@....net>, Sohil Mehta <sohil.mehta@...el.com>, 
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	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCHv6 13/16] x86/traps: Handle LASS thrown #SS

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@...ux.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@...ux.intel.com>
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.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