[PATCH v3 6/6] ARM: mm: Fix PXN process with LPAE feature

[Kefeng Wang]

When user code execution with privilege mode, it will lead to
infinite loop in the page fault handler if ARM_LPAE enabled,

The issue could be reproduced with
  "echo EXEC_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT"

As Permission fault shows in ARM spec,
  IFSR format when using the Short-descriptor translation table format
    Permission fault:       01101 First level      01111 Second level
  IFSR format when using the Long-descriptor translation table format
    Permission fault:       0011LL LL bits indicate levelb.

Add is_permission_fault() function to check permission fault and die
if permission fault occurred under instruction fault in do_page_fault().

Fixes: 1d4d37159d01 ("ARM: 8235/1: Support for the PXN CPU feature on ARMv7")
Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
---
 arch/arm/mm/fault.c | 20 +++++++++++++++++++-
 arch/arm/mm/fault.h |  4 ++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index 82bcfe57de20..bc8779d54a64 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -194,6 +194,19 @@ void do_bad_area(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 #define VM_FAULT_BADMAP		0x010000
 #define VM_FAULT_BADACCESS	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)
@@ -253,9 +266,14 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		vm_flags = VM_WRITE;
 	}
 
-	if (fsr & FSR_LNX_PF)
+	if (fsr & FSR_LNX_PF) {
 		vm_flags = VM_EXEC;
 
+		if (is_permission_fault(fsr) && !user_mode(regs))
+			die_kernel_fault("execution of memory",
+					 mm, addr, fsr, regs);
+	}
+
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
 
 	/*
diff --git a/arch/arm/mm/fault.h b/arch/arm/mm/fault.h
index 9ecc2097a87a..83b5ab32d7a4 100644
--- a/arch/arm/mm/fault.h
+++ b/arch/arm/mm/fault.h
@@ -14,6 +14,8 @@
 
 #ifdef CONFIG_ARM_LPAE
 #define FSR_FS_AEA		17
+#define FS_PERM_NOLL		0xC
+#define FS_PERM_NOLL_MASK	0x3C
 
 static inline int fsr_fs(unsigned int fsr)
 {
@@ -21,6 +23,8 @@ static inline int fsr_fs(unsigned int fsr)
 }
 #else
 #define FSR_FS_AEA		22
+#define FS_L1_PERM             0xD
+#define FS_L2_PERM             0xF
 
 static inline int fsr_fs(unsigned int fsr)
 {
-- 
2.26.2