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Subject[PATCH v6 03/15] arm64: kvm: stop treating register x18 as caller save
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From: Ard Biesheuvel <ard.biesheuvel@linaro.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@linaro.org>
[Sami: updated commit message, switched from x18 to x29 for the guest context]
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
---
arch/arm64/kvm/hyp/entry.S | 45 ++++++++++++++++++++------------------
1 file changed, 24 insertions(+), 21 deletions(-)

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index e5cc8d66bf53..0c6832ec52b1 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -22,7 +22,12 @@
.text
.pushsection .hyp.text, "ax"

+/*
+ * We treat x18 as callee-saved as the host may use it as a platform
+ * register (e.g. for shadow call stack).
+ */
.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 +37,8 @@
.endm

.macro restore_callee_saved_regs ctxt
+ // We require \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 +55,7 @@ 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 +74,28 @@ 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 +118,7 @@ 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 +127,8 @@ 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
--
2.24.0.393.g34dc348eaf-goog
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