linux-kernel - Re: [PATCH] vfio: Get/put KVM only for the first/last vfio_df

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Message-ID: <ZoJBuHuusbzeGoXZ@yzhao56-desk.sh.intel.com>
Date: Mon, 1 Jul 2024 13:42:16 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Yi Liu <yi.l.liu@...el.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<alex.williamson@...hat.com>, <kevin.tian@...el.com>, <jgg@...dia.com>
Subject: Re: [PATCH] vfio: Get/put KVM only for the first/last
 vfio_df_open/close in cdev path

On Mon, Jul 01, 2024 at 01:08:30PM +0800, Yi Liu wrote:
> 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@...el.com>
> > ---
> >   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.
Upon here, device->open_count is not necessarily 0 even for the first open.
The failure can be after a successful increment of device->open_count.

Maybe renaming "bool put_kvm" to "bool is_first_open" to save an assignment
in most common case?
 
> Reviewed-by: Yi Liu <yi.l.liu@...el.com>
Thanks:)

> 
> > +		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
"kvm" may also be changed if the second bind is from a different VM.

> 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);
Yes, better.

>  	pfn = symbol_get(kvm_put_kvm);
>  	if (WARN_ON(!pfn))
>  		return;
> 
> -- 
> Regards,
> Yi Liu