[PATCHv7 13/16] x86/traps: Handle LASS thrown #SS

[Kirill A. Shutemov]

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[1]. 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 e2ad760b17ea..f1f92e1ba524 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_gp_hint hint;
+		unsigned long gp_addr;
+
+		hint = get_kernel_gp_address(regs, &gp_addr);
+		if (hint != GP_NO_HINT) {
+			printk(SSFSTR ", %s 0x%lx", kernel_gp_hint_help[hint],
+			       gp_addr);
+		}
+
+		if (hint != GP_NON_CANONICAL)
+			gp_addr = 0;
+
+		die_addr(SSFSTR, regs, error_code, gp_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