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Date:   Thu, 30 Aug 2018 07:38:53 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omiun.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: [RFC PATCH v3 13/24] x86/mm: Shadow stack page fault error checking

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(), we check if a shadow stack page fault
is within a shadow stack memory area.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.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 5196050ff3d5..58ea2f5722e9 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 b9123c497e0a..3842353fb4a3 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1162,6 +1162,17 @@ 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)))
@@ -1300,6 +1311,13 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
 
 	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 (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)
-- 
2.17.1

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