[RFC PATCH v6 14/26] x86/mm: Shadow stack page fault error checking

[Yu-cheng Yu]

If a page fault is triggered by a shadow stack access (e.g. call/ret)
or shadow stack management instructions (e.g. wrussq), then bit[6] of
the page fault error code is set.

In access_error(), verify a shadow stack page fault is within a
shadow stack memory area.  It is always an error otherwise.

For a valid shadow stack access, set FAULT_FLAG_WRITE to effect
copy-on-write.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
 arch/x86/include/asm/traps.h |  2 ++
 arch/x86/mm/fault.c          | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index 20841d301e93..50e8a670624a 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -157,6 +157,7 @@ enum {
  *   bit 3 ==				1: use of reserved bit detected
  *   bit 4 ==				1: fault was an instruction fetch
  *   bit 5 ==				1: protection keys block access
+ *   bit 6 ==				1: shadow stack access fault
  */
 enum x86_pf_error_code {
 	X86_PF_PROT	=		1 << 0,
@@ -165,5 +166,6 @@ enum x86_pf_error_code {
 	X86_PF_RSVD	=		1 << 3,
 	X86_PF_INSTR	=		1 << 4,
 	X86_PF_PK	=		1 << 5,
+	X86_PF_SHSTK	=		1 << 6,
 };
 #endif /* _ASM_X86_TRAPS_H */
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 71d4b9d4d43f..c3368fed706c 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1117,6 +1117,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
 				       (error_code & X86_PF_INSTR), foreign))
 		return 1;
 
+	/*
+	 * Verify X86_PF_SHSTK is within a shadow stack VMA.
+	 * It is always an error if there is a shadow stack
+	 * fault outside a shadow stack VMA.
+	 */
+	if (error_code & X86_PF_SHSTK) {
+		if (!(vma->vm_flags & VM_SHSTK))
+			return 1;
+		return 0;
+	}
+
 	if (error_code & X86_PF_WRITE) {
 		/* write, present and write, not present: */
 		if (unlikely(!(vma->vm_flags & VM_WRITE)))
@@ -1313,6 +1324,13 @@ void do_user_addr_fault(struct pt_regs *regs,
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
 
+	/*
+	 * If the fault is caused by a shadow stack access,
+	 * i.e. CALL/RET/SAVEPREVSSP/RSTORSSP, then set
+	 * FAULT_FLAG_WRITE to effect copy-on-write.
+	 */
+	if (sw_error_code & X86_PF_SHSTK)
+		flags |= FAULT_FLAG_WRITE;
 	if (sw_error_code & X86_PF_WRITE)
 		flags |= FAULT_FLAG_WRITE;
 	if (sw_error_code & X86_PF_INSTR)
-- 
2.17.1