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

[H. Peter Anvin]

On July 1, 2025 6:35:40 PM PDT, Sohil Mehta <sohil.mehta@xxxxxxxxx> 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@xxxxxxxxxxxxxxx>
>> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> ---
>>  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;
>

An #SS can be generated by the kernel if RSP is corrupted. This is fatal, but as always we want to get a message out.