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

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Message-ID: <DF2E7B41-46C9-4DAC-A671-EC8D6F53F496@zytor.com>
Date: Wed, 02 Jul 2025 07:37:12 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC: Sohil Mehta <sohil.mehta@...el.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,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
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        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>, Ingo Molnar <mingo@...nel.org>,
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        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCHv8 14/17] x86/traps: Handle LASS thrown #SS

On July 2, 2025 3:17:10 AM PDT, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
>On Tue, Jul 01, 2025 at 07:06:10PM -0700, H. Peter Anvin wrote:
>> On July 1, 2025 6:35:40 PM PDT, Sohil Mehta <sohil.mehta@...el.com> 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?
>> >
>> >> 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@...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 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.
>> >
>> >> +			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.
>> >
>> >> +		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;
>> >
>> 
>> Note: for a FRED system, ERETU can generate #SS for a non-canonical user space RSP even in the absence of LASS, so if that is not currently handled that is an active bug.
>
>It is handled by fixup code inside do_error_trap(). We need to add
>explicit fixup before LASS handling to avoid treating bad userspace RSP as
>kernel LASS violation.
>

Great. I was pretty sure, but I wanted to address Sohil's question directly. Thanks for verifying.

A LASS violation of any kind in the kernel (unless handled by fixup, including user access fixup) ought to be fatal, correct?