Hi, David.
Some comments below.
On 3/3/22 15:33, David Matlack wrote:
Tie the lifetime the KVM module to the lifetime of each VM via
kvm.users_count. This way anything that grabs a reference to the VM via
kvm_get_kvm() cannot accidentally outlive the KVM module.
Prior to this commit, the lifetime of the KVM module was tied to the
lifetime of /dev/kvm file descriptors, VM file descriptors, and vCPU
file descriptors by their respective file_operations "owner" field.
This approach is insufficient because references grabbed via
kvm_get_kvm() do not prevent closing any of the aforementioned file
descriptors.
This fixes a long standing theoretical bug in KVM that at least affects
async page faults. kvm_setup_async_pf() grabs a reference via
kvm_get_kvm(), and drops it in an asynchronous work callback. Nothing
prevents the VM file descriptor from being closed and the KVM module
from being unloaded before this callback runs.
Fixes: af585b921e5d ("KVM: Halt vcpu if page it tries to access is swapped out")
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: Ben Gardon <bgardon@xxxxxxxxxx>
[ Based on a patch from Ben implemented for Google's kernel. ]
Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
---
virt/kvm/kvm_main.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 35ae6d32dae5..b59f0a29dbd5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -117,6 +117,8 @@ EXPORT_SYMBOL_GPL(kvm_debugfs_dir);
static const struct file_operations stat_fops_per_vm;
+static struct file_operations kvm_chardev_ops;
+
static long kvm_vcpu_ioctl(struct file *file, unsigned int ioctl,
unsigned long arg);
#ifdef CONFIG_KVM_COMPAT
@@ -1131,6 +1133,11 @@ static struct kvm *kvm_create_vm(unsigned long type)
preempt_notifier_inc();
kvm_init_pm_notifier(kvm);
+ if (!try_module_get(kvm_chardev_ops.owner)) {
+ r = -ENODEV;
+ goto out_err;
+ }
+
Doesn't this problem also affects the other functions called from
kvm_dev_ioctl()?
Is it possible that the module is removed while other ioctl's are still running,
e.g. KVM_GET_API_VERSION and KVM_CHECK_EXTENSION, even though they don't use
struct kvm?
I wonder if this try_module_get() (along with module_put() in the out path of
the function) shouldn't be placed in the upper function kvm_dev_ioctl() so it
would cover all the other ioctl's.
return kvm;
out_err:
@@ -1220,6 +1227,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
preempt_notifier_dec();
hardware_disable_all();
mmdrop(mm);
+ module_put(kvm_chardev_ops.owner);
}
void kvm_get_kvm(struct kvm *kvm)
base-commit: b13a3befc815eae574d87e6249f973dfbb6ad6cd
prerequisite-patch-id: 38f66d60319bf0bc9bf49f91f0f9119e5441629b
prerequisite-patch-id: 51aa921d68ea649d436ea68e1b8f4aabc3805156
--
Murilo
