[PATCHv6 12/16] x86/traps: Communicate a LASS violation in #GP message

Kirill A. Shutemov
2025-06-20T13:53:52Z

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

Provide a more helpful message on #GP when a kernel side LASS violation
is detected.

A NULL pointer dereference is reported if a LASS violation occurs due to
accessing the first page frame.

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 | 24 +++++++++++++++++++-----
1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 333c45df80fc..0f558d3369a3 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -636,7 +636,16 @@ 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 *kernel_gp_hint_help[] = {
+ [GP_NON_CANONICAL] = "probably for non-canonical address",
+ [GP_CANONICAL] = "maybe for address",
+ [GP_LASS_VIOLATION] = "LASS prevented access to address",
+ [GP_NULL_POINTER] = "kernel NULL pointer dereference",
};

/*
@@ -672,6 +681,12 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
if (*addr < ~__VIRTUAL_MASK &&
*addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
return GP_NON_CANONICAL;
+ else if (*addr < ~__VIRTUAL_MASK &&
+ cpu_feature_enabled(X86_FEATURE_LASS)) {
+ if (*addr < PAGE_SIZE)
+ return GP_NULL_POINTER;
+ return GP_LASS_VIOLATION;
+ }
#endif

return GP_CANONICAL;
@@ -833,11 +848,10 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
else
hint = get_kernel_gp_address(regs, &gp_addr);

- if (hint != GP_NO_HINT)
+ 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
--
2.47.2