[v4.9.y,15/27] arm64: use RET instruction for exiting the trampoline

Message ID 20180403110923.43575-16-mark.rutland@arm.com
State New
Headers show
  • arm64 meltdown patches
Related show

Commit Message

Mark Rutland April 3, 2018, 11:09 a.m.
From: Will Deacon <will.deacon@arm.com>

commit be04a6d1126b upstream.

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: Alex Shi <alex.shi@linaro.org> [v4.9 backport]

Signed-off-by: Mark Rutland <mark.rutland@arm.com> [v4.9 backport]

 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 805dc76517c3..f35ca1e54b5a 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -880,6 +880,14 @@  __ni_sys_trace:
 	.if	\regsize == 64
 	msr	tpidrro_el0, x30	// Restored in kernel_ventry
+	/*
+	 * 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	.
 	tramp_map_kernel	x30
 	adr	x30, tramp_vectors + PAGE_SIZE
@@ -892,7 +900,7 @@  __ni_sys_trace:
 	msr	vbar_el1, x30
 	add	x30, x30, #(1b - tramp_vectors)
-	br	x30
+	ret
 	.macro tramp_exit, regsize = 64