linux-kernel - [PATCH v10 12/15] x86/traps: Communicate a LASS violation in #GP message

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Message-ID: <20251007065119.148605-13-sohil.mehta@intel.com>
Date: Mon,  6 Oct 2025 23:51:16 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: x86@...nel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>
Cc: Jonathan Corbet <corbet@....net>,
	"H . Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	"Kirill A . Shutemov" <kas@...nel.org>,
	Sohil Mehta <sohil.mehta@...el.com>,
	Xin Li <xin@...or.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Sean Christopherson <seanjc@...gle.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	David Laight <david.laight.linux@...il.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Kees Cook <kees@...nel.org>,
	Tony Luck <tony.luck@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-efi@...r.kernel.org
Subject: [PATCH v10 12/15] x86/traps: Communicate a LASS violation in #GP message

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

A LASS violation typically results in a #GP. Provide a more helpful
message for a #GP when a kernel-side LASS violation is detected.

Currently, a kernel NULL pointer dereference triggers a #PF, which
prints a helpful message. Because LASS enforcement is pre-paging,
accesses to the first page frame would now be reported as a #GP, with an
LASS violation hint. Add a special case to print a friendly message
specifically for kernel NULL pointer dereferences.

Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Signed-off-by: Sohil Mehta <sohil.mehta@...el.com>
---
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 59bfbdf0a1a0..a5d10f7ae038 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -636,13 +636,23 @@ 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)
@@ -664,14 +674,27 @@ 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;
@@ -835,9 +858,7 @@ 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