[PATCH v11 7/9] x86/traps: Communicate a LASS violation in #GP message

Sohil Mehta
2025-10-29T21:06:16Z

From: Alexander Shishkin <alexander.shishkin@linux.intel.com>

A LASS violation typically results in a #GP. With LASS active, any
invalid access to user memory (including the first page frame) would be
reported as a #GP, instead of a #PF.

Unfortunately, the #GP error messages provide limited information about
the cause of the fault. This could be confusing for kernel developers
and users who are accustomed to the friendly #PF messages.

To make the transition easier, enhance the #GP Oops message to include a
hint about LASS violations. Also, add a special hint for kernel NULL
pointer dereferences to match with the existing #PF message.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
---
v11:
- Improve commit log.

v10:
- Minor improvement to code comments and hints.
---
arch/x86/kernel/traps.c | 45 ++++++++++++++++++++++++++++++-----------
1 file changed, 33 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 6b22611e69cc..30d5c690f9a1 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -635,13 +635,23 @@ DEFINE_IDTENTRY(exc_bounds)
enum kernel_gp_hint {
GP_NO_HINT,
GP_NON_CANONICAL,
- GP_CANONICAL
+ GP_CANONICAL,
+ GP_LASS_VIOLATION,
+ GP_NULL_POINTER,
+};
+
+static const char * const kernel_gp_hint_help[] = {
+ [GP_NON_CANONICAL] = "probably for non-canonical address",
+ [GP_CANONICAL] = "maybe for address",
+ [GP_LASS_VIOLATION] = "probably LASS violation for address",
+ [GP_NULL_POINTER] = "kernel NULL pointer dereference",
};

/*
* When an uncaught #GP occurs, try to determine the memory address accessed by
* the instruction and return that address to the caller. Also, try to figure
- * out whether any part of the access to that address was non-canonical.
+ * out whether any part of the access to that address was non-canonical or
+ * across privilege levels.
*/
static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
unsigned long *addr)
@@ -663,14 +673,27 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
return GP_NO_HINT;

#ifdef CONFIG_X86_64
- /*
- * Check that:
- * - the operand is not in the kernel half
- * - the last byte of the operand is not in the user canonical half
- */
- if (*addr < ~__VIRTUAL_MASK &&
- *addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
+ /* Operand is in the kernel half */
+ if (*addr >= ~__VIRTUAL_MASK)
+ return GP_CANONICAL;
+
+ /* The last byte of the operand is not in the user canonical half */
+ if (*addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
return GP_NON_CANONICAL;
+
+ /*
+ * If LASS is active, a NULL pointer dereference generates a #GP
+ * instead of a #PF.
+ */
+ if (*addr < PAGE_SIZE)
+ return GP_NULL_POINTER;
+
+ /*
+ * Assume that LASS caused the exception, because the address is
+ * canonical and in the user half.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_LASS))
+ return GP_LASS_VIOLATION;
#endif

return GP_CANONICAL;
@@ -833,9 +856,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)

if (hint != GP_NO_HINT)
snprintf(desc, sizeof(desc), GPFSTR ", %s 0x%lx",
- (hint == GP_NON_CANONICAL) ? "probably for non-canonical address"
- : "maybe for address",
- gp_addr);
+ kernel_gp_hint_help[hint], gp_addr);

/*
* KASAN is interested only in the non-canonical case, clear it
--
2.43.0