• Jann Horn's avatar
    kvm: fix kvm_ioctl_create_device() reference counting (CVE-2019-6974) · e02d0a24
    Jann Horn authored
    commit cfa39381 upstream.
    
    kvm_ioctl_create_device() does the following:
    
    1. creates a device that holds a reference to the VM object (with a borrowed
       reference, the VM's refcount has not been bumped yet)
    2. initializes the device
    3. transfers the reference to the device to the caller's file descriptor table
    4. calls kvm_get_kvm() to turn the borrowed reference to the VM into a real
       reference
    
    The ownership transfer in step 3 must not happen before the reference to the VM
    becomes a proper, non-borrowed reference, which only happens in step 4.
    After step 3, an attacker can close the file descriptor and drop the borrowed
    reference, which can cause the refcount of the kvm object to drop to zero.
    
    This means that we need to grab a reference for the device before
    anon_inode_getfd(), otherwise the VM can disappear from under us.
    
    Fixes: 852b6d57 ("kvm: add device control API")
    Cc: stable@kernel.org
    Signed-off-by: default avatarJann Horn <jannh@google.com>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    e02d0a24
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