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Date: Mon, 27 Apr 2020 09:00:13 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Will Deacon <will@...nel.org>, Catalin Marinas <catalin.marinas@....com>, 
	James Morse <james.morse@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ard Biesheuvel <ard.biesheuvel@...aro.org>, Mark Rutland <mark.rutland@....com>, 
	Masahiro Yamada <masahiroy@...nel.org>, Michal Marek <michal.lkml@...kovi.net>, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dave Martin <Dave.Martin@....com>, Kees Cook <keescook@...omium.org>, 
	Laura Abbott <labbott@...hat.com>, Marc Zyngier <maz@...nel.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Jann Horn <jannh@...gle.com>, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, 
	clang-built-linux@...glegroups.com, kernel-hardening@...ts.openwall.com, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH v13 07/12] arm64: efi: restore 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@...gle.com>
Reviewed-by: Kees Cook <keescook@...omium.org>
Acked-by: Will Deacon <will@...nel.org>
---
 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..6ca6c0dc11a1 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:
+	/*
+	 * 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)
-- 
2.26.2.303.gf8c07b1a785-goog

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