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Subject[PATCH] KVM: VMX: Add a trampoline to fix VMREAD error handling
Date
Add a hand coded assembly trampoline to preserve volatile registers
across vmread_error(), and to handle the calling convention differences
between 64-bit and 32-bit due to asmlinkage on vmread_error(). Pass
@field and @fault on the stack when invoking the trampoline to avoid
clobbering volatile registers in the context of the inline assembly.

Calling vmread_error() directly from inline assembly is partially broken
on 64-bit, and completely broken on 32-bit. On 64-bit, it will clobber
%rdi and %rsi (used to pass @field and @fault) and any volatile regs
written by vmread_error(). On 32-bit, asmlinkage means vmread_error()
expects the parameters to be passed on the stack, not via regs.

Opportunistically zero out the result in the trampoline to save a few
bytes of code for every VMREAD. A happy side effect of the trampoline
is that the inline code footprint is reduced by three bytes on 64-bit
due to PUSH/POP being more efficent (in terms of opcode bytes) than MOV.

Fixes: 6e2020977e3e6 ("KVM: VMX: Add error handling to VMREAD helper")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---

Becuase there just isn't enough custom assembly in VMX :-)

Simply reverting isn't a great option because we'd lose error reporting
for VMREAD failure, i.e. it'd return garbage with no other indication that
something went awry.

Tested all paths (fail, fault w/o rebooting, fault w/ rebooting) on both
64-bit and 32-bit.

arch/x86/kvm/vmx/ops.h | 28 +++++++++++++-----
arch/x86/kvm/vmx/vmenter.S | 58 ++++++++++++++++++++++++++++++++++++++
2 files changed, 79 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/vmx/ops.h b/arch/x86/kvm/vmx/ops.h
index 45eaedee2ac0..09b0937d56b1 100644
--- a/arch/x86/kvm/vmx/ops.h
+++ b/arch/x86/kvm/vmx/ops.h
@@ -12,7 +12,8 @@

#define __ex(x) __kvm_handle_fault_on_reboot(x)

-asmlinkage void vmread_error(unsigned long field, bool fault);
+__attribute__((regparm(0))) void vmread_error_trampoline(unsigned long field,
+ bool fault);
void vmwrite_error(unsigned long field, unsigned long value);
void vmclear_error(struct vmcs *vmcs, u64 phys_addr);
void vmptrld_error(struct vmcs *vmcs, u64 phys_addr);
@@ -70,15 +71,28 @@ static __always_inline unsigned long __vmcs_readl(unsigned long field)
asm volatile("1: vmread %2, %1\n\t"
".byte 0x3e\n\t" /* branch taken hint */
"ja 3f\n\t"
- "mov %2, %%" _ASM_ARG1 "\n\t"
- "xor %%" _ASM_ARG2 ", %%" _ASM_ARG2 "\n\t"
- "2: call vmread_error\n\t"
- "xor %k1, %k1\n\t"
+
+ /*
+ * VMREAD failed. Push '0' for @fault, push the failing
+ * @field, and bounce through the trampoline to preserve
+ * volatile registers.
+ */
+ "push $0\n\t"
+ "push %2\n\t"
+ "2:call vmread_error_trampoline\n\t"
+
+ /*
+ * Unwind the stack. Note, the trampoline zeros out the
+ * memory for @fault so that the result is '0' on error.
+ */
+ "pop %2\n\t"
+ "pop %1\n\t"
"3:\n\t"

+ /* VMREAD faulted. As above, except push '1' for @fault. */
".pushsection .fixup, \"ax\"\n\t"
- "4: mov %2, %%" _ASM_ARG1 "\n\t"
- "mov $1, %%" _ASM_ARG2 "\n\t"
+ "4: push $1\n\t"
+ "push %2\n\t"
"jmp 2b\n\t"
".popsection\n\t"
_ASM_EXTABLE(1b, 4b)
diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 81ada2ce99e7..861ae40e7144 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -234,3 +234,61 @@ SYM_FUNC_START(__vmx_vcpu_run)
2: mov $1, %eax
jmp 1b
SYM_FUNC_END(__vmx_vcpu_run)
+
+/**
+ * vmread_error_trampoline - Trampoline from inline asm to vmread_error()
+ * @field: VMCS field encoding that failed
+ * @fault: %true if the VMREAD faulted, %false if it failed
+
+ * Save and restore volatile registers across a call to vmread_error(). Note,
+ * all parameters are passed on the stack.
+ */
+SYM_FUNC_START(vmread_error_trampoline)
+ push %_ASM_BP
+ mov %_ASM_SP, %_ASM_BP
+
+ push %_ASM_AX
+ push %_ASM_CX
+ push %_ASM_DX
+#ifdef CONFIG_X86_64
+ push %rdi
+ push %rsi
+ push %r8
+ push %r9
+ push %r10
+ push %r11
+#endif
+#ifdef CONFIG_X86_64
+ /* Load @field and @fault to arg1 and arg2 respectively. */
+ mov 3*WORD_SIZE(%rbp), %_ASM_ARG2
+ mov 2*WORD_SIZE(%rbp), %_ASM_ARG1
+#else
+ /* Parameters are passed on the stack for 32-bit (see asmlinkage). */
+ push 3*WORD_SIZE(%ebp)
+ push 2*WORD_SIZE(%ebp)
+#endif
+
+ call vmread_error
+
+#ifndef CONFIG_X86_64
+ add $8, %esp
+#endif
+
+ /* Zero out @fault, which will be popped into the result register. */
+ _ASM_MOV $0, 3*WORD_SIZE(%_ASM_BP)
+
+#ifdef CONFIG_X86_64
+ pop %r11
+ pop %r10
+ pop %r9
+ pop %r8
+ pop %rsi
+ pop %rdi
+#endif
+ pop %_ASM_DX
+ pop %_ASM_CX
+ pop %_ASM_AX
+ pop %_ASM_BP
+
+ ret
+SYM_FUNC_END(vmread_error_trampoline)
--
2.24.1
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