From 7cf3f4c7e9d37fd364640af0d9f3c596ff736dea Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon@arm.com>
Date: Tue, 14 Nov 2017 16:15:59 +0000
Subject: [PATCH 06/24] arm64: use RET instruction for exiting the trampoline

Git-commit: be04a6d1126b02c6a28741155b899d648739fc5b
Patch-mainline: v4.16-rc1
References: bsc#1068032

Speculation attacks against the entry trampoline can potentially resteer
the speculative instruction stream through the indirect branch and into
arbitrary gadgets within the kernel.

This patch defends against these attacks by forcing a misprediction
through the return stack: a dummy BL instruction loads an entry into
the stack, so that the predicted program flow of the subsequent RET
instruction is to a branch-to-self instruction which is finally resolved
as a branch to the kernel vectors with speculation suppressed.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@suse.com>
---
 arch/arm64/kernel/entry.S | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index ddf318d7e36e..cab88b4250aa 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -996,6 +996,14 @@ alternative_else_nop_endif
 	.if	\regsize == 64
 	msr	tpidrro_el0, x30	// Restored in kernel_ventry
 	.endif
+	/*
+	 * Defend against branch aliasing attacks by pushing a dummy
+	 * entry onto the return stack and using a RET instruction to
+	 * enter the full-fat kernel vectors.
+	 */
+	bl	2f
+	b	.
+2:
 	tramp_map_kernel	x30
 #ifdef CONFIG_RANDOMIZE_BASE
 	adr	x30, tramp_vectors + PAGE_SIZE
@@ -1008,7 +1016,7 @@ alternative_insn isb, nop, ARM64_WORKAROUND_QCOM_FALKOR_E1003
 	msr	vbar_el1, x30
 	add	x30, x30, #(1b - tramp_vectors)
 	isb
-	br	x30
+	ret
 	.endm
 
 	.macro tramp_exit, regsize = 64
-- 
2.11.0