KVM: VMX: Zero host's SYSENTER_ESP iff SYSENTER is NOT used

Zero vmcs.HOST_IA32_SYSENTER_ESP when initializing *constant* host state
if and only if SYSENTER cannot be used, i.e. the kernel is a 64-bit
kernel and is not emulating 32-bit syscalls.  As the name suggests,
vmx_set_constant_host_state() is intended for state that is *constant*.
When SYSENTER is used, SYSENTER_ESP isn't constant because stacks are
per-CPU, and the VMCS must be updated whenever the vCPU is migrated to a
new CPU.  The logic in vmx_vcpu_load_vmcs() doesn't differentiate between
"never loaded" and "loaded on a different CPU", i.e. setting SYSENTER_ESP
on VMCS load also handles setting correct host state when the VMCS is
first loaded.

Because a VMCS must be loaded before it is initialized during vCPU RESET,
zeroing the field in vmx_set_constant_host_state() obliterates the value
that was written when the VMCS was loaded.  If the vCPU is run before it
is migrated, the subsequent VM-Exit will zero out MSR_IA32_SYSENTER_ESP,
leading to a #DF on the next 32-bit syscall.

  double fault: 0000 [#1] SMP
  CPU: 0 PID: 990 Comm: stable Not tainted 5.16.0+ #97
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  EIP: entry_SYSENTER_32+0x0/0xe7
  Code: <9c> 50 eb 17 0f 20 d8 a9 00 10 00 00 74 0d 25 ff ef ff ff 0f 22 d8
  EAX: 000000a2 EBX: a8d1300c ECX: a8d13014 EDX: 00000000
  ESI: a8f87000 EDI: a8d13014 EBP: a8d12fc0 ESP: 00000000
  DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068 EFLAGS: 00210093
  CR0: 80050033 CR2: fffffffc CR3: 02c3b000 CR4: 00152e90

Fixes: 6ab8a40 ("KVM: VMX: Avoid to rdmsrl(MSR_IA32_SYSENTER_ESP)")
Cc: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220122015211.1468758-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -4094,10 +4094,14 @@ void vmx_set_constant_host_state(struct vcpu_vmx *vmx)
 	vmcs_write32(HOST_IA32_SYSENTER_CS, low32);
 
 	/*
-	 * If 32-bit syscall is enabled, vmx_vcpu_load_vcms rewrites
-	 * HOST_IA32_SYSENTER_ESP.
+	 * SYSENTER is used for 32-bit system calls on either 32-bit or
+	 * 64-bit kernels.  It is always zero If neither is allowed, otherwise
+	 * vmx_vcpu_load_vmcs loads it with the per-CPU entry stack (and may
+	 * have already done so!).
 	 */
-	vmcs_writel(HOST_IA32_SYSENTER_ESP, 0);
+	if (!IS_ENABLED(CONFIG_IA32_EMULATION) && !IS_ENABLED(CONFIG_X86_32))
+		vmcs_writel(HOST_IA32_SYSENTER_ESP, 0);
+
 	rdmsrl(MSR_IA32_SYSENTER_EIP, tmpl);
 	vmcs_writel(HOST_IA32_SYSENTER_EIP, tmpl);   /* 22.2.3 */