[RFC PATCH v9 13/27] x86/mm: Shadow Stack page fault error checking

Yu-cheng Yu
2020-02-05T18:21:50Z

If a page fault is triggered by a Shadow Stack (SHSTK) access
(e.g. CALL/RET) or SHSTK management instructions (e.g. WRUSSQ), then bit[6]
of the page fault error code is set.

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

For a valid SHSTK 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 7ac26bbd0bef..8023d177fcd8 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -169,6 +169,7 @@ 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,
@@ -177,5 +178,6 @@ 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 304d31d8cbbc..9c1243302663 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1187,6 +1187,17 @@ 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)))
@@ -1344,6 +1355,13 @@ 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 (hw_error_code & X86_PF_SHSTK)
+ flags |= FAULT_FLAG_WRITE;
if (hw_error_code & X86_PF_WRITE)
flags |= FAULT_FLAG_WRITE;
if (hw_error_code & X86_PF_INSTR)
--
2.21.0