arm64: efi: Restore register x18 if it was corrupted

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 <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Will Deacon <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

Author: samitolvanen

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:
+	/*
+	 * 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
 ENDPROC(__efi_rt_asm_wrapper)