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

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Message-ID: <4DE45AFD-C1E0-4FB8-BE01-44A72C5C6E1E@zytor.com>
Date: Tue, 01 Jul 2025 19:06:10 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Sohil Mehta <sohil.mehta@...el.com>,
        "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,
        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>, Ingo Molnar <mingo@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        Kai Huang <kai.huang@...el.com>, Sandipan Das <sandipan.das@....com>,
        Breno Leitao <leitao@...ian.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
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        Eric Biggers <ebiggers@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>,
        "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Huang Shijie <shijie@...amperecomputing.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Namhyung Kim <namhyung@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>, linux-doc@...r.kernel.org,
        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 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.