KVM: x86: Serialize vendor module initialization (hardware setup)

Acquire a new mutex, vendor_module_lock, in kvm_x86_vendor_init() while
doing hardware setup to ensure that concurrent calls are fully serialized.
KVM rejects attempts to load vendor modules if a different module has
already been loaded, but doesn't handle the case where multiple vendor
modules are loaded at the same time, and module_init() doesn't run under
the global module_mutex.

Note, in practice, this is likely a benign bug as no platform exists that
supports both SVM and VMX, i.e. barring a weird VM setup, one of the
vendor modules is guaranteed to fail a support check before modifying
common KVM state.

Alternatively, KVM could perform an atomic CMPXCHG on .hardware_enable,
but that comes with its own ugliness as it would require setting
.hardware_enable before success is guaranteed, e.g. attempting to load
the "wrong" could result in spurious failure to load the "right" module.

Introduce a new mutex as using kvm_lock is extremely deadlock prone due
to kvm_lock being taken under cpus_write_lock(), and in the future, under
under cpus_read_lock().  Any operation that takes cpus_read_lock() while
holding kvm_lock would potentially deadlock, e.g. kvm_timer_init() takes
cpus_read_lock() to register a callback.  In theory, KVM could avoid
such problematic paths, i.e. do less setup under kvm_lock, but avoiding
all calls to cpus_read_lock() is subtly difficult and thus fragile.  E.g.
updating static calls also acquires cpus_read_lock().

Inverting the lock ordering, i.e. always taking kvm_lock outside
cpus_read_lock(), is not a viable option as kvm_lock is taken in various
callbacks that may be invoked under cpus_read_lock(), e.g. x86's
kvmclock_cpufreq_notifier().

The lockdep splat below is dependent on future patches to take
cpus_read_lock() in hardware_enable_all(), but as above, deadlock is
already is already possible.

  ======================================================
  WARNING: possible circular locking dependency detected
  6.0.0-smp--7ec93244f194-init2 #27 Tainted: G           O
  ------------------------------------------------------
  stable/251833 is trying to acquire lock:
  ffffffffc097ea28 (kvm_lock){+.+.}-{3:3}, at: hardware_enable_all+0x1f/0xc0 [kvm]

               but task is already holding lock:
  ffffffffa2456828 (cpu_hotplug_lock){++++}-{0:0}, at: hardware_enable_all+0xf/0xc0 [kvm]

               which lock already depends on the new lock.

               the existing dependency chain (in reverse order) is:

               -> #1 (cpu_hotplug_lock){++++}-{0:0}:
         cpus_read_lock+0x2a/0xa0
         __cpuhp_setup_state+0x2b/0x60
         __kvm_x86_vendor_init+0x16a/0x1870 [kvm]
         kvm_x86_vendor_init+0x23/0x40 [kvm]
         0xffffffffc0a4d02b
         do_one_initcall+0x110/0x200
         do_init_module+0x4f/0x250
         load_module+0x1730/0x18f0
         __se_sys_finit_module+0xca/0x100
         __x64_sys_finit_module+0x1d/0x20
         do_syscall_64+0x3d/0x80
         entry_SYSCALL_64_after_hwframe+0x63/0xcd

               -> #0 (kvm_lock){+.+.}-{3:3}:
         __lock_acquire+0x16f4/0x30d0
         lock_acquire+0xb2/0x190
         __mutex_lock+0x98/0x6f0
         mutex_lock_nested+0x1b/0x20
         hardware_enable_all+0x1f/0xc0 [kvm]
         kvm_dev_ioctl+0x45e/0x930 [kvm]
         __se_sys_ioctl+0x77/0xc0
         __x64_sys_ioctl+0x1d/0x20
         do_syscall_64+0x3d/0x80
         entry_SYSCALL_64_after_hwframe+0x63/0xcd

               other info that might help us debug this:

   Possible unsafe locking scenario:

         CPU0                    CPU1
         ----                    ----
    lock(cpu_hotplug_lock);
                                 lock(kvm_lock);
                                 lock(cpu_hotplug_lock);
    lock(kvm_lock);

                *** DEADLOCK ***

  1 lock held by stable/251833:
   #0: ffffffffa2456828 (cpu_hotplug_lock){++++}-{0:0}, at: hardware_enable_all+0xf/0xc0 [kvm]

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221130230934.1014142-16-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>