Subject: [PATCH v6 11/15] arm64: efi: restore x18 if it was corrupted

From: Sami Tolvanen <[email-masked]>
Date: Fri, 6 Dec 2019 14:13:47 -0800

If we detect a corrupted x18 and SCS is enabled, restore the register
before jumping back to instrumented code. This is safe, because the
wrapper is called with preemption disabled and a separate shadow stack
is used for interrupt handling.

Signed-off-by: Sami Tolvanen <[email-masked]>
Reviewed-by: Kees Cook <[email-masked]>
---
 arch/arm64/kernel/efi-rt-wrapper.S | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/efi-rt-wrapper.S b/arch/arm64/kernel/efi-rt-wrapper.S
index 3fc71106cb2b..62f0260f5c17 100644
--- a/arch/arm64/kernel/efi-rt-wrapper.S
+++ b/arch/arm64/kernel/efi-rt-wrapper.S
@@ -34,5 +34,14 @@ ENTRY(__efi_rt_asm_wrapper)
 	ldp	x29, x30, [sp], #32
 	b.ne	0f
 	ret
-0:	b	efi_handle_corrupted_x18	// tail call
+0:
+#ifdef CONFIG_SHADOW_CALL_STACK
+	/*
+	 * Restore x18 before returning to instrumented code. This is
+	 * safe because the wrapper is called with preemption disabled and
+	 * a separate shadow stack is used for interrupts.
+	 */
+	mov	x18, x2
+#endif
+	b	efi_handle_corrupted_x18	// tail call
 ENDPROC(__efi_rt_asm_wrapper)
-- 
2.24.0.393.g34dc348eaf-goog

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