[PATCH 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial synthetic range

[Sean Christopherson <seanjc@google.com>]

Reject userspace attempts to set the Xen hypercall page MSR to an index
outside of the "standard" virtualization range [0x40000000, 0x4fffffff],
as KVM is not equipped to handle collisions with real MSRs, e.g. KVM
doesn't update MSR interception, conflicts with VMCS/VMCB fields, special
case writes in KVM, etc.

Allowing userspace to redirect any MSR write can also be used 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
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Paul Durrant <paul@xen.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/xen.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index a909b817b9c0..35ecafc410f0 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1324,6 +1324,14 @@ int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc)
 	     xhc->blob_size_32 || xhc->blob_size_64))
 		return -EINVAL;
 
+	/*
+	 * Restrict the MSR to the range that is unofficially reserved for
+	 * synthetic, virtualization-defined MSRs, e.g. to prevent confusing
+	 * KVM by colliding with a real MSR that requires special handling.
+	 */
+	if (xhc->msr && (xhc->msr < 0x40000000 || xhc->msr > 0x4fffffff))
+		return -EINVAL;
+
 	mutex_lock(&kvm->arch.xen.xen_lock);
 
 	if (xhc->msr && !kvm->arch.xen_hvm_config.msr)
-- 
2.48.1.362.g079036d154-goog