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

H. Peter Anvin
2025-07-02T14:39:20Z

On July 2, 2025 3:17:10 AM PDT, "Kirill A. Shutemov" <kirill.shutemov@linux.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@intel.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@linux.intel.com>
>> >> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
>> >> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.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?