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

[Paul Durrant]

On 01/02/2025 01:13, Sean Christopherson wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/679258d4.050a0220.2eae65.000a.GAE@xxxxxxxxxx
> Cc: Joao Martins <joao.m.martins@xxxxxxxxxx>
> Cc: Paul Durrant <paul@xxxxxxx>
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/xen.c | 8 ++++++++
>   1 file changed, 8 insertions(+)

Reviewed-by: Paul Durrant <paul@xxxxxxx>