Commit be04a6d1 authored Nov 14, 2017 by Will Deacon Committed by Catalin Marinas Jan 08, 2018

arm64: use RET instruction for exiting the trampoline

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>

parent 3b3b6810

arch/arm64/kernel/entry.S

+9 −1

Original line number	Diff line number	Diff line
		@@ -1029,6 +1029,14 @@ 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
		@@ -1041,7 +1049,7 @@ 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