x86/traps: Communicate a LASS violation in #GP message

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>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20251118182911.2983253-7-sohil.mehta%40intel.com

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 6b22611e69cc8aa87716c3dcdaf44ba33db0fc2b..1b9177b9343312f78968d21b10315fa246bac7ab 100644 (file)
--- 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 +673,28 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
                return GP_NO_HINT;
 
 #ifdef CONFIG_X86_64
+       /* 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;
+
        /*
-        * Check that:
-        *  - the operand is not in the kernel half
-        *  - the last byte of the operand is not in the user canonical half
+        * A NULL pointer dereference usually causes a #PF. However, it
+        * can result in a #GP when LASS is active. Provide the same
+        * hint in the rare case that the condition is hit without LASS.
         */
-       if (*addr < ~__VIRTUAL_MASK &&
-           *addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
-               return GP_NON_CANONICAL;
+       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 +857,7 @@ 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