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 <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>

diff --git a/arch/arm64/kernel/efi-rt-wrapper.S b/arch/arm64/kernel/efi-rt-wrapper.S
index 3fc71106cb2b45eb0dd7091c7da824856bae4ea9..6ca6c0dc11a1b30e8fff7bf7f42317baa36b6891 100644 (file)
--- 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:
+       /*
+        * 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)