[PATCH v5 02/10] arm64: smccc: clear the Xn registers after SMC calls

[Abdellatif El Khlifi]

set to zero the x0-x17 registers

As per the SMCCC v1.2 spec, unused result and scratch registers
can leak information after an SMC call. We can mitigate against
this risk by returning zero in each register.

The leakage we are referring to is data leakage across exception
levels. The intent is to prevent lower exception levels (EL1/EL0)
from reading the SMC data exchanged at EL2.

Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi at arm.com>
Cc: Tom Rini <trini at konsulko.com>
Cc: Simon Glass <sjg at chromium.org>
Cc: Ilias Apalodimas <ilias.apalodimas at linaro.org>
Cc: Jens Wiklander <jens.wiklander at linaro.org>
---

Changelog:
===============

v4:

* move the clearing code into a new macro: clear_gp_regs

v3:

* clear the Xn registers after SMC calls


 arch/arm/cpu/armv8/smccc-call.S | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm/cpu/armv8/smccc-call.S b/arch/arm/cpu/armv8/smccc-call.S
index ec6f299bc9..32f3eb8eeb 100644
--- a/arch/arm/cpu/armv8/smccc-call.S
+++ b/arch/arm/cpu/armv8/smccc-call.S
@@ -50,6 +50,12 @@ ENDPROC(__arm_smccc_hvc)
 
 #ifdef CONFIG_ARM64
 
+	.macro	clear_gp_regs
+	.irp	n,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17
+	mov	x\n, xzr
+	.endr
+	.endm
+
 	.macro SMCCC_1_2 instr
 	/* Save `res` and free a GPR that won't be clobbered */
 	stp     x1, x19, [sp, #-16]!
@@ -84,6 +90,9 @@ ENDPROC(__arm_smccc_hvc)
 	stp	x14, x15, [x19, #ARM_SMCCC_1_2_REGS_X14_OFFS]
 	stp	x16, x17, [x19, #ARM_SMCCC_1_2_REGS_X16_OFFS]
 
+	/* x0-x17 registers can leak information after an SMC or HVC call. Let's clear them */
+	clear_gp_regs
+
 	/* Restore original x19 */
 	ldp     xzr, x19, [sp], #16
 	ret
-- 
2.17.1