Re: [PATCH v23 14/28] x86/mm: Shadow Stack page fault error checking

[Yu, Yu-cheng]

On 3/22/2021 3:38 AM, Kirill A. Shutemov wrote:
On Tue, Mar 16, 2021 at 08:10:40AM -0700, Yu-cheng Yu wrote:
[...]
  
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index a73347e2cdfc..4316732a18c6 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1100,6 +1100,17 @@ access_error(unsigned long error_code, struct 
vm_area_struct *vma)
                                       (error_code & X86_PF_INSTR), foreign))
                return 1;
  
+	/*
+        * Verify a shadow stack access is within a shadow stack VMA.
+        * It is always an error otherwise.  Normal data access to a
+        * shadow stack area is checked in the case followed.
+        */
+       if (error_code & X86_PF_SHSTK) {
+               if (!(vma->vm_flags & VM_SHSTK))
+                       return 1;
+               return 0;

Any reason to return 0 here? I would rather keep the single return 0 in
the function, after all checks are done.


For shadow stack fault, X86_PF_SHSTK and X86_PF_WRITE both can be set. 
So for shadow stack fault, return from here and don't go into the normal 
write fault case.

Thanks,
Yu-cheng

+       }
+
        if (error_code & X86_PF_WRITE) {
                /* write, present and write, not present: */
                if (unlikely(!(vma->vm_flags & VM_WRITE)))
@@ -1293,6 +1304,14 @@ void do_user_addr_fault(struct pt_regs *regs,
  
  	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
  
+	/*
+        * Clearing _PAGE_DIRTY is used to detect shadow stack access.
+        * This method cannot distinguish shadow stack read vs. write.
+        * For valid shadow stack accesses, set FAULT_FLAG_WRITE to effect
+        * copy-on-write.
+        */
+       if (error_code & X86_PF_SHSTK)
+               flags |= FAULT_FLAG_WRITE;
        if (error_code & X86_PF_WRITE)
                flags |= FAULT_FLAG_WRITE;
        if (error_code & X86_PF_INSTR)
--
2.21.0