[PATCHv9 11/16] x86/traps: Communicate a LASS violation in #GP message

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

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 | 41 +++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 59bfbdf0a1a0..4a4194e1d119 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 * const 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",
 };
 
 /*
@@ -664,14 +673,23 @@ 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 enabled, NULL pointer dereference generates
+	 * #GP instead of #PF.
+	 */
+	if (*addr < PAGE_SIZE)
+		return GP_NULL_POINTER;
+
+	if (cpu_feature_enabled(X86_FEATURE_LASS))
+		return GP_LASS_VIOLATION;
 #endif
 
 	return GP_CANONICAL;
@@ -833,11 +851,10 @@ 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