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

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Message-ID: <08758240-fd38-4d90-9efa-354274e8bb8e@zytor.com>
Date: Fri, 20 Jun 2025 08:20:12 -0700
From: Xin Li <xin@...or.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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>
Cc: Jonathan Corbet <corbet@....net>, Sohil Mehta <sohil.mehta@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
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        Breno Leitao <leitao@...ian.org>,
        Rick Edgecombe
 <rick.p.edgecombe@...el.com>,
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        linux-mm@...ck.org
Subject: Re: [PATCHv6 13/16] x86/traps: Handle LASS thrown #SS

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.

> +
> +		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