[PATCH] ARM: mm: Provide better fault message for permission fault

[Kefeng Wang]

If there is a permission fault in __do_kernel_fault(), we only
print the generic "paging request" message which don't show
read, write or execute information, let's provide better fault
message for them.

Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
---
 arch/arm/mm/fault.c | 35 +++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index 46cccd6bf705..d1b848f0f7ed 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -95,6 +95,19 @@ void show_pte(const char *lvl, struct mm_struct *mm, unsigned long addr)
 
 	pr_cont("\n");
 }
+
+static inline bool is_permission_fault(unsigned int fsr)
+{
+	int fs = fsr_fs(fsr);
+#ifdef CONFIG_ARM_LPAE
+	if ((fs & FS_PERM_NOLL_MASK) == FS_PERM_NOLL)
+		return true;
+#else
+	if (fs == FS_L1_PERM || fs == FS_L2_PERM)
+		return true;
+#endif
+	return false;
+}
 #else					/* CONFIG_MMU */
 void show_pte(const char *lvl, struct mm_struct *mm, unsigned long addr)
 { }
@@ -137,7 +150,14 @@ __do_kernel_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
 	/*
 	 * No handler, we'll have to terminate things with extreme prejudice.
 	 */
-	if (addr < PAGE_SIZE) {
+	if (is_permission_fault(fsr)) {
+		if (fsr & FSR_WRITE)
+			msg = "write to read-only memory";
+		else if (fsr & FSR_LNX_PF)
+			msg = "execute from non-executable memory";
+		else
+			msg = "read from unreadable memory";
+	} else if (addr < PAGE_SIZE) {
 		msg = "NULL pointer dereference";
 	} else {
 		if (kfence_handle_page_fault(addr, is_write_fault(fsr), regs))
@@ -204,19 +224,6 @@ void do_bad_area(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 #define VM_FAULT_BADMAP		((__force vm_fault_t)0x010000)
 #define VM_FAULT_BADACCESS	((__force vm_fault_t)0x020000)
 
-static inline bool is_permission_fault(unsigned int fsr)
-{
-	int fs = fsr_fs(fsr);
-#ifdef CONFIG_ARM_LPAE
-	if ((fs & FS_PERM_NOLL_MASK) == FS_PERM_NOLL)
-		return true;
-#else
-	if (fs == FS_L1_PERM || fs == FS_L2_PERM)
-		return true;
-#endif
-	return false;
-}
-
 static vm_fault_t __kprobes
 __do_page_fault(struct mm_struct *mm, unsigned long addr, unsigned int flags,
 		unsigned long vma_flags, struct pt_regs *regs)
-- 
2.35.3