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

[David Woodhouse <dwmw2@infradead.org>]

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On Fri, 2025-01-31 at 17:13 -0800, 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@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>

With macros for the magic numbers as discussed (and a corresponding
update to the documentation), and with the Reported-by: and Closes:
tags dropped because they should move to the commit which makes the
hypercall page only trigger for !host_initiated writes and resolves it
in a more future-proof way for the general case,

Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>

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