On Tue, Jul 01, 2025 at 06:35:40PM -0700, Sohil Mehta wrote:

On 7/1/2025 2:58 AM, Kirill A. Shutemov wrote:

quoted

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?

It can be generated by both kernel and userspace if RSP gets corrupted.

So far, do_error_trap() did the trick, handling what has to be handled.
LASS requires a bit more, though.

quoted

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 <redacted>
---
 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.

I think line split if justified. It is 120 characters long otherwise.
And with multi-line statement, brackets help readability.

I don't see a reason to change it.

quoted

+			printk(SSFSTR ", %s 0x%lx", kernel_exc_hint_help[hint],
+			       exc_addr);
+		}
+

quoted

+		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.

Okay, will fold into "x86/traps: Generalize #GP address decode and hint
code".

quoted

+		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;

-- 
  Kiryl Shutsemau / Kirill A. Shutemov