On 2024/6/28 23:18, Yan Zhao wrote:
In the device cdev path, adjust the handling of the KVM reference count to
only increment with the first vfio_df_open() and decrement after the final
vfio_df_close(). This change addresses a KVM reference leak that occurs
when a device cdev file is opened multiple times and attempts to bind to
iommufd repeatedly.
Currently, vfio_df_get_kvm_safe() is invoked prior to each vfio_df_open()
in the cdev path during iommufd binding. The corresponding
vfio_device_put_kvm() is executed either when iommufd is unbound or if an
error occurs during the binding process.
However, issues arise when a device binds to iommufd more than once. The
second vfio_df_open() will fail during iommufd binding, and
vfio_device_put_kvm() will be triggered, setting device->kvm to NULL.
Consequently, when iommufd is unbound from the first successfully bound
device, vfio_device_put_kvm() becomes ineffective, leading to a leak in the
KVM reference count.
Good catch!!!
Below is the calltrace that will be produced in this scenario when the KVM
module is unloaded afterwards, reporting "BUG kvm_vcpu (Tainted: G S):
Objects remaining in kvm_vcpu on __kmem_cache_shutdown()".
Call Trace:
<TASK>
dump_stack_lvl+0x80/0xc0
slab_err+0xb0/0xf0
? __kmem_cache_shutdown+0xc1/0x4e0
? rcu_is_watching+0x11/0x50
? lock_acquired+0x144/0x3c0
__kmem_cache_shutdown+0x1b7/0x4e0
kmem_cache_destroy+0xa6/0x260
kvm_exit+0x80/0xc0 [kvm]
vmx_exit+0xe/0x20 [kvm_intel]
__x64_sys_delete_module+0x143/0x250
? ktime_get_coarse_real_ts64+0xd3/0xe0
? syscall_trace_enter+0x143/0x210
do_syscall_64+0x6f/0x140
entry_SYSCALL_64_after_hwframe+0x76/0x7e
Fixes: 5fcc26969a16 ("vfio: Add VFIO_DEVICE_BIND_IOMMUFD")
Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
---
drivers/vfio/device_cdev.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/drivers/vfio/device_cdev.c b/drivers/vfio/device_cdev.c
index bb1817bd4ff3..3b85d01d1b27 100644
--- a/drivers/vfio/device_cdev.c
+++ b/drivers/vfio/device_cdev.c
@@ -65,6 +65,7 @@ long vfio_df_ioctl_bind_iommufd(struct vfio_device_file *df,
{
struct vfio_device *device = df->device;
struct vfio_device_bind_iommufd bind;
+ bool put_kvm = false;
unsigned long minsz;
int ret;
@@ -101,12 +102,15 @@ long vfio_df_ioctl_bind_iommufd(struct vfio_device_file *df,
}
/*
- * Before the device open, get the KVM pointer currently
+ * Before the device's first open, get the KVM pointer currently
* associated with the device file (if there is) and obtain
- * a reference. This reference is held until device closed.
+ * a reference. This reference is held until device's last closed.
* Save the pointer in the device for use by drivers.
*/
- vfio_df_get_kvm_safe(df);
+ if (device->open_count == 0) {
+ vfio_df_get_kvm_safe(df);
+ put_kvm = true;
+ }
ret = vfio_df_open(df);
if (ret)
@@ -129,7 +133,8 @@ long vfio_df_ioctl_bind_iommufd(struct vfio_device_file *df,
out_close_device:
vfio_df_close(df);
out_put_kvm:
- vfio_device_put_kvm(device);
+ if (put_kvm)
you may use if (device->open_count == 0) as well here to save a bool.
Otherwise looks good to me.
Reviewed-by: Yi Liu <yi.l.liu@xxxxxxxxx>
+ vfio_device_put_kvm(device);
iommufd_ctx_put(df->iommufd);
df->iommufd = NULL;
out_unlock:
base-commit: 6ba59ff4227927d3a8530fc2973b80e94b54d58f
BTW. The vfio_device_get_kvm_safe() is not supposed to be invoked multiple
times by design as the second call would override the device->put_kvm and
device->kvm. This does not change the put_kvm nor the kvm though. But not a
good thing anyghow. maybe worth a warn like below.
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index ee72c0b61795..a4bead0e5820 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -408,6 +408,8 @@ void vfio_device_get_kvm_safe(struct vfio_device
*device, struct kvm *kvm)
if (!kvm)
return;
+ WARN_ON(device->put_kvm || device->kvm);
+
pfn = symbol_get(kvm_put_kvm);
if (WARN_ON(!pfn))
return;
--
Regards,
Yi Liu
