KVM: x86/xen: Only write Xen hypercall page for guest writes to MSR

The Xen hypercall page MSR is write-only. When the guest writes an address
to the MSR, the hypervisor populates the referenced page with hypercall
functions.

There is no reason for the host ever to write to the MSR, and it isn't
even readable.

Allowing host writes to trigger the hypercall page allows userspace to
attack the kernel, as kvm_xen_write_hypercall_page() takes multiple
locks and writes to guest memory.  E.g. if userspace sets the MSR to
MSR_IA32_XSS, KVM's write to MSR_IA32_XSS during vCPU creation will
trigger an SRCU violation due to writing guest memory:

  =============================
  WARNING: suspicious RCU usage
  6.13.0-rc3
  -----------------------------
  include/linux/kvm_host.h:1046 suspicious rcu_dereference_check() usage!

  stack backtrace:
  CPU: 6 UID: 1000 PID: 1101 Comm: repro Not tainted 6.13.0-rc3
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  Call Trace:
   <TASK>
   dump_stack_lvl+0x7f/0x90
   lockdep_rcu_suspicious+0x176/0x1c0
   kvm_vcpu_gfn_to_memslot+0x259/0x280
   kvm_vcpu_write_guest+0x3a/0xa0
   kvm_xen_write_hypercall_page+0x268/0x300
   kvm_set_msr_common+0xc44/0x1940
   vmx_set_msr+0x9db/0x1fc0
   kvm_vcpu_reset+0x857/0xb50
   kvm_arch_vcpu_create+0x37e/0x4d0
   kvm_vm_ioctl+0x669/0x2100
   __x64_sys_ioctl+0xc1/0xf0
   do_syscall_64+0xc5/0x210
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x7feda371b539

While the MSR index isn't strictly ABI, i.e. can theoretically float to
any value, in practice no known VMM sets the MSR index to anything other
than 0x40000000 or 0x40000200.

Reported-by: syzbot+cdeaeec70992eca2d920@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/679258d4.050a0220.2eae65.000a.GAE@google.com
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/de0437379dfab11e431a23c8ce41a29234c06cbf.camel@infradead.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: dwmw2

arch/x86/kvm/x86.c

@@ -3733,7 +3733,13 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	u32 msr = msr_info->index;
 	u64 data = msr_info->data;
 
-	if (msr && msr == vcpu->kvm->arch.xen_hvm_config.msr)
+	/*
+	 * Do not allow host-initiated writes to trigger the Xen hypercall
+	 * page setup; it could incur locking paths which are not expected
+	 * if userspace sets the MSR in an unusual location.
+	 */
+	if (msr && msr == vcpu->kvm->arch.xen_hvm_config.msr &&
+	    !msr_info->host_initiated)
 		return kvm_xen_write_hypercall_page(vcpu, data);
 
 	switch (msr) {