Re: [PATCH v8 7/9] KVM: arm/arm64: context-switch ptrauth registers

Amit Daniel Kachhap <amit.kachhap@xxxxxxx> · Fri, 5 Apr 2019 16:30:58 +0530

Hi Kristina,

On 4/5/19 12:59 AM, Kristina Martsenko wrote:
On 02/04/2019 03:27, Amit Daniel Kachhap wrote:
From: Mark Rutland <mark.rutland@xxxxxxx>

When pointer authentication is supported, a guest may wish to use it.
This patch adds the necessary KVM infrastructure for this to work, with
a semi-lazy context switch of the pointer auth state.

[...]

diff --git a/arch/arm64/include/asm/kvm_ptrauth_asm.h b/arch/arm64/include/asm/kvm_ptrauth_asm.h
new file mode 100644
index 0000000..65f99e9
--- /dev/null
+++ b/arch/arm64/include/asm/kvm_ptrauth_asm.h
@@ -0,0 +1,106 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* arch/arm64/include/asm/kvm_ptrauth_asm.h: Guest/host ptrauth save/restore
+ * Copyright 2019 Arm Limited
+ * Author: Mark Rutland <mark.rutland@xxxxxxx>
+ *         Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
+ */
+
+#ifndef __ASM_KVM_PTRAUTH_ASM_H
+#define __ASM_KVM_PTRAUTH_ASM_H
+
+#ifndef __ASSEMBLY__
+
+#define __ptrauth_save_key(regs, key)						\
+({										\
+	regs[key ## KEYLO_EL1] = read_sysreg_s(SYS_ ## key ## KEYLO_EL1);	\
+	regs[key ## KEYHI_EL1] = read_sysreg_s(SYS_ ## key ## KEYHI_EL1);	\
+})
+
+#define __ptrauth_save_state(ctxt)						\
+({										\
+	__ptrauth_save_key(ctxt->sys_regs, APIA);				\
+	__ptrauth_save_key(ctxt->sys_regs, APIB);				\
+	__ptrauth_save_key(ctxt->sys_regs, APDA);				\
+	__ptrauth_save_key(ctxt->sys_regs, APDB);				\
+	__ptrauth_save_key(ctxt->sys_regs, APGA);				\
+})
+
+#else /* __ASSEMBLY__ */
+
+#include <asm/sysreg.h>
+
+#ifdef	CONFIG_ARM64_PTR_AUTH
+
+#define PTRAUTH_REG_OFFSET(x)	(x - CPU_APIAKEYLO_EL1)
+
+/*
+ * CPU_AP*_EL1 values exceed immediate offset range (512) for stp instruction
+ * so below macros takes CPU_APIAKEYLO_EL1 as base and calculates the offset of
+ * the keys from this base to avoid an extra add instruction. These macros
+ * assumes the keys offsets are aligned in a specific increasing order.
+ */
+.macro	ptrauth_save_state base, reg1, reg2
+	mrs_s	\reg1, SYS_APIAKEYLO_EL1
+	mrs_s	\reg2, SYS_APIAKEYHI_EL1
+	stp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APIAKEYLO_EL1)]
+	mrs_s	\reg1, SYS_APIBKEYLO_EL1
+	mrs_s	\reg2, SYS_APIBKEYHI_EL1
+	stp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APIBKEYLO_EL1)]
+	mrs_s	\reg1, SYS_APDAKEYLO_EL1
+	mrs_s	\reg2, SYS_APDAKEYHI_EL1
+	stp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APDAKEYLO_EL1)]
+	mrs_s	\reg1, SYS_APDBKEYLO_EL1
+	mrs_s	\reg2, SYS_APDBKEYHI_EL1
+	stp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APDBKEYLO_EL1)]
+	mrs_s	\reg1, SYS_APGAKEYLO_EL1
+	mrs_s	\reg2, SYS_APGAKEYHI_EL1
+	stp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APGAKEYLO_EL1)]
+.endm
+
+.macro	ptrauth_restore_state base, reg1, reg2
+	ldp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APIAKEYLO_EL1)]
+	msr_s	SYS_APIAKEYLO_EL1, \reg1
+	msr_s	SYS_APIAKEYHI_EL1, \reg2
+	ldp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APIBKEYLO_EL1)]
+	msr_s	SYS_APIBKEYLO_EL1, \reg1
+	msr_s	SYS_APIBKEYHI_EL1, \reg2
+	ldp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APDAKEYLO_EL1)]
+	msr_s	SYS_APDAKEYLO_EL1, \reg1
+	msr_s	SYS_APDAKEYHI_EL1, \reg2
+	ldp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APDBKEYLO_EL1)]
+	msr_s	SYS_APDBKEYLO_EL1, \reg1
+	msr_s	SYS_APDBKEYHI_EL1, \reg2
+	ldp	\reg1, \reg2, [\base, #PTRAUTH_REG_OFFSET(CPU_APGAKEYLO_EL1)]
+	msr_s	SYS_APGAKEYLO_EL1, \reg1
+	msr_s	SYS_APGAKEYHI_EL1, \reg2
+.endm
+
+.macro ptrauth_switch_to_guest g_ctxt, reg1, reg2, reg3
+	ldr	\reg1, [\g_ctxt, #CPU_HCR_EL2]
+	and	\reg1, \reg1, #(HCR_API | HCR_APK)
+	cbz	\reg1, 1f
+	add	\reg1, \g_ctxt, #CPU_APIAKEYLO_EL1
+	ptrauth_restore_state	\reg1, \reg2, \reg3
+1:

Nit: the label in assembly macros is usually a larger number (see
assembler.h or alternative.h for example). I think this is to avoid
future accidents like

	cbz	x0, 1f
	ptrauth_switch_to_guest	x1, x2, x3, x4
	add	x5, x5, x6
1:
	...

where the code would incorrectly branch to the label inside
ptrauth_switch_to_guest, instead of the one after it.
Yes agree that these kind of labels are problematic. I will update in 
the next iteration.

Thanks,
Amit Daniel

Thanks,
Kristina

+.endm
+
+.macro ptrauth_switch_to_host g_ctxt, h_ctxt, reg1, reg2, reg3
+	ldr	\reg1, [\g_ctxt, #CPU_HCR_EL2]
+	and	\reg1, \reg1, #(HCR_API | HCR_APK)
+	cbz	\reg1, 2f
+	add	\reg1, \g_ctxt, #CPU_APIAKEYLO_EL1
+	ptrauth_save_state	\reg1, \reg2, \reg3
+	add	\reg1, \h_ctxt, #CPU_APIAKEYLO_EL1
+	ptrauth_restore_state	\reg1, \reg2, \reg3
+	isb
+2:
+.endm
+
+#else /* !CONFIG_ARM64_PTR_AUTH */
+.macro ptrauth_switch_to_guest g_ctxt, reg1, reg2, reg3
+.endm
+.macro ptrauth_switch_to_host g_ctxt, h_ctxt, reg1, reg2, reg3
+.endm
+#endif /* CONFIG_ARM64_PTR_AUTH */
+#endif /* __ASSEMBLY__ */
+#endif /* __ASM_KVM_PTRAUTH_ASM_H */

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index 675fdc1..3a70213 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -24,6 +24,7 @@
  #include <asm/kvm_arm.h>
  #include <asm/kvm_asm.h>
  #include <asm/kvm_mmu.h>
+#include <asm/kvm_ptrauth_asm.h>
  
  #define CPU_GP_REG_OFFSET(x)	(CPU_GP_REGS + x)
  #define CPU_XREG_OFFSET(x)	CPU_GP_REG_OFFSET(CPU_USER_PT_REGS + 8*x)
@@ -64,6 +65,9 @@ ENTRY(__guest_enter)
  
  	add	x18, x0, #VCPU_CONTEXT
  
+	// Macro ptrauth_switch_to_guest(guest cxt, tmp1, tmp2, tmp3).
+	ptrauth_switch_to_guest x18, x0, x1, x2
+
  	// Restore guest regs x0-x17
  	ldp	x0, x1,   [x18, #CPU_XREG_OFFSET(0)]
  	ldp	x2, x3,   [x18, #CPU_XREG_OFFSET(2)]
@@ -118,6 +122,9 @@ ENTRY(__guest_exit)
  
  	get_host_ctxt	x2, x3
  
+	// Macro ptrauth_switch_to_host(guest cxt, host cxt, tmp1, tmp2, tmp3).
+	ptrauth_switch_to_host x1, x2, x3, x4, x5
+
  	// Now restore the host regs
  	restore_callee_saved_regs x2
  
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