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

Yu-cheng Yu
2018-11-19T21:47:57Z

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@intel.com>
---
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