From: Sami Tolvanen <samitolvanen@google.com>
Date: Mon, 27 Apr 2020 09:00:13 -0700
Subject: arm64: efi: Restore register x18 if it was corrupted
Patch-mainline: v5.8-rc1
Git-commit: e73f02c6eb15729164b9dd5e19214b54446823ab
References: jsc#SLE-16407

If we detect a corrupted x18, restore the register before jumping back
to potentially SCS 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 <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
Acked-by: Lee, Chun-Yi <jlee@suse.com>
---
 arch/arm64/kernel/efi-rt-wrapper.S |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/arch/arm64/kernel/efi-rt-wrapper.S
+++ b/arch/arm64/kernel/efi-rt-wrapper.S
@@ -34,5 +34,14 @@ SYM_FUNC_START(__efi_rt_asm_wrapper)
 	ldp	x29, x30, [sp], #32
 	b.ne	0f
 	ret
-0:	b	efi_handle_corrupted_x18	// tail call
+0:
+	/*
+	 * With CONFIG_SHADOW_CALL_STACK, the kernel uses x18 to store a
+	 * shadow stack pointer, which we need to restore before returning to
+	 * potentially 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
+	b	efi_handle_corrupted_x18	// tail call
 SYM_FUNC_END(__efi_rt_asm_wrapper)