Subject: [PATCH v6 10/15] arm64: preserve x18 when CPU is suspended

From: Sami Tolvanen <[email-masked]>
Date: Fri, 6 Dec 2019 14:13:46 -0800

Don't lose the current task's shadow stack when the CPU is suspended.

Signed-off-by: Sami Tolvanen <[email-masked]>
Reviewed-by: Nick Desaulniers <[email-masked]>
Reviewed-by: Kees Cook <[email-masked]>
Reviewed-by: Mark Rutland <[email-masked]>
---
 arch/arm64/include/asm/suspend.h |  2 +-
 arch/arm64/mm/proc.S             | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/suspend.h b/arch/arm64/include/asm/suspend.h
index 8939c87c4dce..0cde2f473971 100644
--- a/arch/arm64/include/asm/suspend.h
+++ b/arch/arm64/include/asm/suspend.h
@@ -2,7 +2,7 @@
 #ifndef __ASM_SUSPEND_H
 #define __ASM_SUSPEND_H
 
-#define NR_CTX_REGS 12
+#define NR_CTX_REGS 13
 #define NR_CALLEE_SAVED_REGS 12
 
 /*
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index fdabf40a83c8..5c8219c55948 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -49,6 +49,8 @@
  * cpu_do_suspend - save CPU registers context
  *
  * x0: virtual address of context pointer
+ *
+ * This must be kept in sync with struct cpu_suspend_ctx in <asm/suspend.h>.
  */
 ENTRY(cpu_do_suspend)
 	mrs	x2, tpidr_el0
@@ -73,6 +75,11 @@ alternative_endif
 	stp	x8, x9, [x0, #48]
 	stp	x10, x11, [x0, #64]
 	stp	x12, x13, [x0, #80]
+	/*
+	 * Save x18 as it may be used as a platform register, e.g. by shadow
+	 * call stack.
+	 */
+	str	x18, [x0, #96]
 	ret
 ENDPROC(cpu_do_suspend)
 
@@ -89,6 +96,13 @@ ENTRY(cpu_do_resume)
 	ldp	x9, x10, [x0, #48]
 	ldp	x11, x12, [x0, #64]
 	ldp	x13, x14, [x0, #80]
+	/*
+	 * Restore x18, as it may be used as a platform register, and clear
+	 * the buffer to minimize the risk of exposure when used for shadow
+	 * call stack.
+	 */
+	ldr	x18, [x0, #96]
+	str	xzr, [x0, #96]
 	msr	tpidr_el0, x2
 	msr	tpidrro_el0, x3
 	msr	contextidr_el1, x4
-- 
2.24.0.393.g34dc348eaf-goog

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