Re: [PATCH v2 7/7] ARM: mm: Fix PXN process with LPAE feature

From: Kefeng Wang
Date: Thu Jun 03 2021 - 05:38:56 EST

On 2021/6/2 23:58, Russell King (Oracle) wrote:
On Wed, Jun 02, 2021 at 11:13:14PM +0800, Kefeng Wang wrote:
  IFSR format when using the Short-descriptor translation table format

    Domain fault      01001            First level   01011     Second level

    Permission fault 01101            First level   01111     Second level

  IFSR format when using the Long-descriptor translation table format

   0011LL Permission fault. LL bits indicate levelb.

After check the ARM spec, I think for the permission fault,  we should panic
with or without LPAE, will change to
As I explained in one of the previous patches, the page tables that get
used for mapping kernel space are the _tasks_ own page tables. Any new
kernel mappings are lazily copied to the task page tables - such as
when a module is loaded.

The first time we touch a page, we could end up with a page translation
fault. This will call do_page_fault(), and so with your proposal,
loading a module will potentially cause a kernel panic in this case,
probably leading to systems that panic early during userspace boot.

Could we add some FSR_FS check, only panic when the permission fault, eg,

+static inline bool is_permission_fault(unsigned int fsr)
+       int fs = fsr_fs(fsr);
+       if ((fs & FS_PERM_NOLL_MASK) == FS_PERM_NOLL)
+               return true;
+       if (fs == FS_L1_PERM || fs == )
+               return true;
+       return false;
 static int __kprobes
 do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
@@ -255,8 +268,7 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)

        if (fsr & FSR_LNX_PF) {
                vm_flags = VM_EXEC;
-               if (!user_mode(regs))
+               if (is_permission_fault && !user_mode(regs))
                        die_kernel_fault("execution of memory",
                                         mm, addr, fsr, regs);

diff --git a/arch/arm/mm/fault.h b/arch/arm/mm/fault.h
index 9ecc2097a87a..187954b4acca 100644
--- a/arch/arm/mm/fault.h
+++ b/arch/arm/mm/fault.h
@@ -14,6 +14,8 @@

 #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)
 #define FSR_FS_AEA             22
+#define FS_L1_PERM             0xD
+#define FS_L2_PERM             0xF

and suggestion or proper solution to solve the issue?