linux-kernel - [PATCH v4 03/17] arm64: kvm: stop treating register x18 as caller save

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Date:   Fri,  1 Nov 2019 15:11:36 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH v4 03/17] arm64: kvm: stop treating register x18 as caller save

From: Ard Biesheuvel <ard.biesheuvel@...aro.org>

In preparation of reserving x18, stop treating it as caller save in
the KVM guest entry/exit code. Currently, the code assumes there is
no need to preserve it for the host, given that it would have been
assumed clobbered anyway by the function call to __guest_enter().
Instead, preserve its value and restore it upon return.

Link: https://patchwork.kernel.org/patch/9836891/
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
[Sami: updated commit message, switched from x18 to x29 for the guest context]
Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
Reviewed-by: Kees Cook <keescook@...omium.org>
---
 arch/arm64/kvm/hyp/entry.S | 41 +++++++++++++++++++-------------------
 1 file changed, 20 insertions(+), 21 deletions(-)

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index e5cc8d66bf53..c3c2d842c609 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -23,6 +23,7 @@
 	.pushsection	.hyp.text, "ax"
 
 .macro save_callee_saved_regs ctxt
+	str	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
 	stp	x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
 	stp	x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
 	stp	x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
@@ -32,6 +33,8 @@
 .endm
 
 .macro restore_callee_saved_regs ctxt
+	// We assume \ctxt is not x18-x28
+	ldr	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
 	ldp	x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
 	ldp	x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
 	ldp	x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
@@ -48,7 +51,7 @@ ENTRY(__guest_enter)
 	// x0: vcpu
 	// x1: host context
 	// x2-x17: clobbered by macros
-	// x18: guest context
+	// x29: guest context
 
 	// Store the host regs
 	save_callee_saved_regs x1
@@ -67,31 +70,28 @@ alternative_else_nop_endif
 	ret
 
 1:
-	add	x18, x0, #VCPU_CONTEXT
+	add	x29, x0, #VCPU_CONTEXT
 
 	// Macro ptrauth_switch_to_guest format:
 	// 	ptrauth_switch_to_guest(guest cxt, tmp1, tmp2, tmp3)
 	// The below macro to restore guest keys is not implemented in C code
 	// as it may cause Pointer Authentication key signing mismatch errors
 	// when this feature is enabled for kernel code.
-	ptrauth_switch_to_guest x18, x0, x1, x2
+	ptrauth_switch_to_guest x29, x0, x1, x2
 
 	// Restore guest regs x0-x17
-	ldp	x0, x1,   [x18, #CPU_XREG_OFFSET(0)]
-	ldp	x2, x3,   [x18, #CPU_XREG_OFFSET(2)]
-	ldp	x4, x5,   [x18, #CPU_XREG_OFFSET(4)]
-	ldp	x6, x7,   [x18, #CPU_XREG_OFFSET(6)]
-	ldp	x8, x9,   [x18, #CPU_XREG_OFFSET(8)]
-	ldp	x10, x11, [x18, #CPU_XREG_OFFSET(10)]
-	ldp	x12, x13, [x18, #CPU_XREG_OFFSET(12)]
-	ldp	x14, x15, [x18, #CPU_XREG_OFFSET(14)]
-	ldp	x16, x17, [x18, #CPU_XREG_OFFSET(16)]
-
-	// Restore guest regs x19-x29, lr
-	restore_callee_saved_regs x18
-
-	// Restore guest reg x18
-	ldr	x18,      [x18, #CPU_XREG_OFFSET(18)]
+	ldp	x0, x1,   [x29, #CPU_XREG_OFFSET(0)]
+	ldp	x2, x3,   [x29, #CPU_XREG_OFFSET(2)]
+	ldp	x4, x5,   [x29, #CPU_XREG_OFFSET(4)]
+	ldp	x6, x7,   [x29, #CPU_XREG_OFFSET(6)]
+	ldp	x8, x9,   [x29, #CPU_XREG_OFFSET(8)]
+	ldp	x10, x11, [x29, #CPU_XREG_OFFSET(10)]
+	ldp	x12, x13, [x29, #CPU_XREG_OFFSET(12)]
+	ldp	x14, x15, [x29, #CPU_XREG_OFFSET(14)]
+	ldp	x16, x17, [x29, #CPU_XREG_OFFSET(16)]
+
+	// Restore guest regs x18-x29, lr
+	restore_callee_saved_regs x29
 
 	// Do not touch any register after this!
 	eret
@@ -114,7 +114,7 @@ ENTRY(__guest_exit)
 	// Retrieve the guest regs x0-x1 from the stack
 	ldp	x2, x3, [sp], #16	// x0, x1
 
-	// Store the guest regs x0-x1 and x4-x18
+	// Store the guest regs x0-x1 and x4-x17
 	stp	x2, x3,   [x1, #CPU_XREG_OFFSET(0)]
 	stp	x4, x5,   [x1, #CPU_XREG_OFFSET(4)]
 	stp	x6, x7,   [x1, #CPU_XREG_OFFSET(6)]
@@ -123,9 +123,8 @@ ENTRY(__guest_exit)
 	stp	x12, x13, [x1, #CPU_XREG_OFFSET(12)]
 	stp	x14, x15, [x1, #CPU_XREG_OFFSET(14)]
 	stp	x16, x17, [x1, #CPU_XREG_OFFSET(16)]
-	str	x18,      [x1, #CPU_XREG_OFFSET(18)]
 
-	// Store the guest regs x19-x29, lr
+	// Store the guest regs x18-x29, lr
 	save_callee_saved_regs x1
 
 	get_host_ctxt	x2, x3
-- 
2.24.0.rc1.363.gb1bccd3e3d-goog