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:

[...]

quoted

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

quoted

+	}
+
  	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