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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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.