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

Kirill A. Shutemov
2025-07-01T09:59:24Z

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.

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) {
+ 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);
+ 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;
--
2.47.2