[PATCH v3 1/2] kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add()

[PATCH v3 1/2] kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add()

[Dmitry Torokhov]

kvm_vfio_group_add() creates kvg instance, links it to kv->group_list,
and calls kvm_vfio_file_set_kvm() with kvg->file as an argument after
dropping kv->lock. If we race group addition and deletion calls, kvg
instance may get freed by the time we get around to calling
kvm_vfio_file_set_kvm().

Previous iterations of the code did not reference kvg->file outside of
the critical section, but used a temporary variable. Still, they had
similar problem of the file reference being owned by kvg structure and
potential for kvm_vfio_group_del() dropping it before
kvm_vfio_group_add() had a chance to complete.

Fix this by moving call to kvm_vfio_file_set_kvm() under the protection
of kv->lock. We already call it while holding the same lock when vfio
group is being deleted, so it should be safe here as well.

Fixes: 2fc1bec15883 ("kvm: set/clear kvm to/from vfio_group when group add/delete")
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---

v3: added Alex's reviewed-by

v2: updated commit description with the correct "Fixes" tag (per Alex),
    expanded commit description to mention issues with the earlier
    implementation of kvm_vfio_group_add().

 virt/kvm/vfio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
index 9584eb57e0ed..cd46d7ef98d6 100644
--- a/virt/kvm/vfio.c
+++ b/virt/kvm/vfio.c
@@ -179,10 +179,10 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
 	list_add_tail(&kvg->node, &kv->group_list);
 
 	kvm_arch_start_assignment(dev->kvm);
+	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
 
 	mutex_unlock(&kv->lock);
 
-	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
 	kvm_vfio_update_coherency(dev);
 
 	return 0;
-- 
2.41.0.255.g8b1d071c50-goog

[PATCH v3 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

[Dmitry Torokhov]

Stop taking kv->lock mutex in kvm_vfio_update_coherency() and instead
call it with this mutex held: the callers of the function usually
already have it taken (and released) before calling
kvm_vfio_update_coherency(). This avoid bouncing the lock up and down.

The exception is kvm_vfio_release() where we do not take the lock, but
it is being executed when the very last reference to kvm_device is being
dropped, so there are no concerns about concurrency.

Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---

v3: initialize "ret" with 0 (per Alex), added Alex's reviewed-by

v2: new patch.

 virt/kvm/vfio.c | 26 +++++++++-----------------
 1 file changed, 9 insertions(+), 17 deletions(-)

diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
index cd46d7ef98d6..dbf2b855cf78 100644
--- a/virt/kvm/vfio.c
+++ b/virt/kvm/vfio.c
@@ -122,8 +122,6 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
 	bool noncoherent = false;
 	struct kvm_vfio_group *kvg;
 
-	mutex_lock(&kv->lock);
-
 	list_for_each_entry(kvg, &kv->group_list, node) {
 		if (!kvm_vfio_file_enforced_coherent(kvg->file)) {
 			noncoherent = true;
@@ -139,8 +137,6 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
 		else
 			kvm_arch_unregister_noncoherent_dma(dev->kvm);
 	}
-
-	mutex_unlock(&kv->lock);
 }
 
 static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
@@ -148,7 +144,7 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
 	struct kvm_vfio *kv = dev->private;
 	struct kvm_vfio_group *kvg;
 	struct file *filp;
-	int ret;
+	int ret = 0;
 
 	filp = fget(fd);
 	if (!filp)
@@ -157,7 +153,7 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
 	/* Ensure the FD is a vfio group FD.*/
 	if (!kvm_vfio_file_is_group(filp)) {
 		ret = -EINVAL;
-		goto err_fput;
+		goto out_fput;
 	}
 
 	mutex_lock(&kv->lock);
@@ -165,30 +161,26 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
 	list_for_each_entry(kvg, &kv->group_list, node) {
 		if (kvg->file == filp) {
 			ret = -EEXIST;
-			goto err_unlock;
+			goto out_unlock;
 		}
 	}
 
 	kvg = kzalloc(sizeof(*kvg), GFP_KERNEL_ACCOUNT);
 	if (!kvg) {
 		ret = -ENOMEM;
-		goto err_unlock;
+		goto out_unlock;
 	}
 
-	kvg->file = filp;
+	kvg->file = get_file(filp);
 	list_add_tail(&kvg->node, &kv->group_list);
 
 	kvm_arch_start_assignment(dev->kvm);
 	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
-
-	mutex_unlock(&kv->lock);
-
 	kvm_vfio_update_coherency(dev);
 
-	return 0;
-err_unlock:
+out_unlock:
 	mutex_unlock(&kv->lock);
-err_fput:
+out_fput:
 	fput(filp);
 	return ret;
 }
@@ -224,12 +216,12 @@ static int kvm_vfio_group_del(struct kvm_device *dev, unsigned int fd)
 		break;
 	}
 
+	kvm_vfio_update_coherency(dev);
+
 	mutex_unlock(&kv->lock);
 
 	fdput(f);
 
-	kvm_vfio_update_coherency(dev);
-
 	return ret;
 }
 
-- 
2.41.0.255.g8b1d071c50-goog

RE: [PATCH v3 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

[Tian, Kevin]

> From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> Sent: Saturday, July 15, 2023 6:46 AM
>
> @@ -165,30 +161,26 @@ static int kvm_vfio_group_add(struct kvm_device
> *dev, unsigned int fd)
>  	list_for_each_entry(kvg, &kv->group_list, node) {
>  		if (kvg->file == filp) {
>  			ret = -EEXIST;
> -			goto err_unlock;
> +			goto out_unlock;
>  		}
>  	}
> 
>  	kvg = kzalloc(sizeof(*kvg), GFP_KERNEL_ACCOUNT);
>  	if (!kvg) {
>  		ret = -ENOMEM;
> -		goto err_unlock;
> +		goto out_unlock;
>  	}
> 
> -	kvg->file = filp;
> +	kvg->file = get_file(filp);

Why is another reference required here?

RE: [PATCH v3 1/2] kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add()

[Tian, Kevin]

> From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> Sent: Saturday, July 15, 2023 6:46 AM
> 
> kvm_vfio_group_add() creates kvg instance, links it to kv->group_list,
> and calls kvm_vfio_file_set_kvm() with kvg->file as an argument after
> dropping kv->lock. If we race group addition and deletion calls, kvg
> instance may get freed by the time we get around to calling
> kvm_vfio_file_set_kvm().
> 
> Previous iterations of the code did not reference kvg->file outside of
> the critical section, but used a temporary variable. Still, they had
> similar problem of the file reference being owned by kvg structure and
> potential for kvm_vfio_group_del() dropping it before
> kvm_vfio_group_add() had a chance to complete.
> 
> Fix this by moving call to kvm_vfio_file_set_kvm() under the protection
> of kv->lock. We already call it while holding the same lock when vfio
> group is being deleted, so it should be safe here as well.
> 
> Fixes: 2fc1bec15883 ("kvm: set/clear kvm to/from vfio_group when group
> add/delete")
> Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Reviewed-by: Kevin Tian <kevin.tian@intel.com>

Re: [PATCH v3 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

[Dmitry Torokhov]

On Wed, Jul 19, 2023 at 05:32:27AM +0000, Tian, Kevin wrote:
> > From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > Sent: Saturday, July 15, 2023 6:46 AM
> >
> > @@ -165,30 +161,26 @@ static int kvm_vfio_group_add(struct kvm_device
> > *dev, unsigned int fd)
> >  	list_for_each_entry(kvg, &kv->group_list, node) {
> >  		if (kvg->file == filp) {
> >  			ret = -EEXIST;
> > -			goto err_unlock;
> > +			goto out_unlock;
> >  		}
> >  	}
> > 
> >  	kvg = kzalloc(sizeof(*kvg), GFP_KERNEL_ACCOUNT);
> >  	if (!kvg) {
> >  		ret = -ENOMEM;
> > -		goto err_unlock;
> > +		goto out_unlock;
> >  	}
> > 
> > -	kvg->file = filp;
> > +	kvg->file = get_file(filp);
> 
> Why is another reference required here?

Because the function now has a single exit point and the original
reference is dropped unconditionally on exit. It looks cleaner than
checking for non-zero "ret" and deciding whether the reference should be
dropped or kept.

Thanks.

-- 
Dmitry

RE: [PATCH v3 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

[Tian, Kevin]

> From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> Sent: Wednesday, July 19, 2023 2:11 PM
> 
> On Wed, Jul 19, 2023 at 05:32:27AM +0000, Tian, Kevin wrote:
> > > From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > > Sent: Saturday, July 15, 2023 6:46 AM
> > >
> > > @@ -165,30 +161,26 @@ static int kvm_vfio_group_add(struct
> kvm_device
> > > *dev, unsigned int fd)
> > >  	list_for_each_entry(kvg, &kv->group_list, node) {
> > >  		if (kvg->file == filp) {
> > >  			ret = -EEXIST;
> > > -			goto err_unlock;
> > > +			goto out_unlock;
> > >  		}
> > >  	}
> > >
> > >  	kvg = kzalloc(sizeof(*kvg), GFP_KERNEL_ACCOUNT);
> > >  	if (!kvg) {
> > >  		ret = -ENOMEM;
> > > -		goto err_unlock;
> > > +		goto out_unlock;
> > >  	}
> > >
> > > -	kvg->file = filp;
> > > +	kvg->file = get_file(filp);
> >
> > Why is another reference required here?
> 
> Because the function now has a single exit point and the original
> reference is dropped unconditionally on exit. It looks cleaner than
> checking for non-zero "ret" and deciding whether the reference should be
> dropped or kept.
> 

A comment is appreciated. otherwise,

Reviewed-by: Kevin Tian <kevin.tian@intel.com>

Re: [PATCH v3 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

[Dmitry Torokhov]

On Thu, Jul 20, 2023 at 02:36:16AM +0000, Tian, Kevin wrote:
> > From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > Sent: Wednesday, July 19, 2023 2:11 PM
> > 
> > On Wed, Jul 19, 2023 at 05:32:27AM +0000, Tian, Kevin wrote:
> > > > From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > > > Sent: Saturday, July 15, 2023 6:46 AM
> > > >
> > > > @@ -165,30 +161,26 @@ static int kvm_vfio_group_add(struct
> > kvm_device
> > > > *dev, unsigned int fd)
> > > >  	list_for_each_entry(kvg, &kv->group_list, node) {
> > > >  		if (kvg->file == filp) {
> > > >  			ret = -EEXIST;
> > > > -			goto err_unlock;
> > > > +			goto out_unlock;
> > > >  		}
> > > >  	}
> > > >
> > > >  	kvg = kzalloc(sizeof(*kvg), GFP_KERNEL_ACCOUNT);
> > > >  	if (!kvg) {
> > > >  		ret = -ENOMEM;
> > > > -		goto err_unlock;
> > > > +		goto out_unlock;
> > > >  	}
> > > >
> > > > -	kvg->file = filp;
> > > > +	kvg->file = get_file(filp);
> > >
> > > Why is another reference required here?
> > 
> > Because the function now has a single exit point and the original
> > reference is dropped unconditionally on exit. It looks cleaner than
> > checking for non-zero "ret" and deciding whether the reference should be
> > dropped or kept.
> > 
> 
> A comment is appreciated. otherwise,
> 
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>

Thank you for the review! However I do not think any comment is needed,
if one is looking at the final source and not the patch form, the reason
for taking another reference is plain to see.

Thanks!

-- 
Dmitry

Re: [PATCH v3 1/2] kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add()

[Alex Williamson]

Hey Paolo,

I'll pull this through the vfio tree unless you have a particular
interest.  Thanks,

Alex

On Fri, 14 Jul 2023 15:45:32 -0700
Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:

> kvm_vfio_group_add() creates kvg instance, links it to kv->group_list,
> and calls kvm_vfio_file_set_kvm() with kvg->file as an argument after
> dropping kv->lock. If we race group addition and deletion calls, kvg
> instance may get freed by the time we get around to calling
> kvm_vfio_file_set_kvm().
> 
> Previous iterations of the code did not reference kvg->file outside of
> the critical section, but used a temporary variable. Still, they had
> similar problem of the file reference being owned by kvg structure and
> potential for kvm_vfio_group_del() dropping it before
> kvm_vfio_group_add() had a chance to complete.
> 
> Fix this by moving call to kvm_vfio_file_set_kvm() under the protection
> of kv->lock. We already call it while holding the same lock when vfio
> group is being deleted, so it should be safe here as well.
> 
> Fixes: 2fc1bec15883 ("kvm: set/clear kvm to/from vfio_group when group add/delete")
> Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> ---
> 
> v3: added Alex's reviewed-by
> 
> v2: updated commit description with the correct "Fixes" tag (per Alex),
>     expanded commit description to mention issues with the earlier
>     implementation of kvm_vfio_group_add().
> 
>  virt/kvm/vfio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
> index 9584eb57e0ed..cd46d7ef98d6 100644
> --- a/virt/kvm/vfio.c
> +++ b/virt/kvm/vfio.c
> @@ -179,10 +179,10 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
>  	list_add_tail(&kvg->node, &kv->group_list);
>  
>  	kvm_arch_start_assignment(dev->kvm);
> +	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
>  
>  	mutex_unlock(&kv->lock);
>  
> -	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
>  	kvm_vfio_update_coherency(dev);
>  
>  	return 0;

Re: [PATCH v3 1/2] kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add()

[Alex Williamson]

On Fri, 14 Jul 2023 15:45:32 -0700
Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:

> kvm_vfio_group_add() creates kvg instance, links it to kv->group_list,
> and calls kvm_vfio_file_set_kvm() with kvg->file as an argument after
> dropping kv->lock. If we race group addition and deletion calls, kvg
> instance may get freed by the time we get around to calling
> kvm_vfio_file_set_kvm().
> 
> Previous iterations of the code did not reference kvg->file outside of
> the critical section, but used a temporary variable. Still, they had
> similar problem of the file reference being owned by kvg structure and
> potential for kvm_vfio_group_del() dropping it before
> kvm_vfio_group_add() had a chance to complete.
> 
> Fix this by moving call to kvm_vfio_file_set_kvm() under the protection
> of kv->lock. We already call it while holding the same lock when vfio
> group is being deleted, so it should be safe here as well.
> 
> Fixes: 2fc1bec15883 ("kvm: set/clear kvm to/from vfio_group when group add/delete")
> Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> ---

Applied series to vfio next branch for v6.6.  There's a minor rebase
involved, so please double check the results:

https://github.com/awilliam/linux-vfio/commits/next

Thanks,
Alex

> 
> v3: added Alex's reviewed-by
> 
> v2: updated commit description with the correct "Fixes" tag (per Alex),
>     expanded commit description to mention issues with the earlier
>     implementation of kvm_vfio_group_add().
> 
>  virt/kvm/vfio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
> index 9584eb57e0ed..cd46d7ef98d6 100644
> --- a/virt/kvm/vfio.c
> +++ b/virt/kvm/vfio.c
> @@ -179,10 +179,10 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
>  	list_add_tail(&kvg->node, &kv->group_list);
>  
>  	kvm_arch_start_assignment(dev->kvm);
> +	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
>  
>  	mutex_unlock(&kv->lock);
>  
> -	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
>  	kvm_vfio_update_coherency(dev);
>  
>  	return 0;

Re: [PATCH v3 1/2] kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add()

[Dmitry Torokhov]

On Thu, Aug 03, 2023 at 01:38:12PM -0600, Alex Williamson wrote:
> On Fri, 14 Jul 2023 15:45:32 -0700
> Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:
> 
> > kvm_vfio_group_add() creates kvg instance, links it to kv->group_list,
> > and calls kvm_vfio_file_set_kvm() with kvg->file as an argument after
> > dropping kv->lock. If we race group addition and deletion calls, kvg
> > instance may get freed by the time we get around to calling
> > kvm_vfio_file_set_kvm().
> > 
> > Previous iterations of the code did not reference kvg->file outside of
> > the critical section, but used a temporary variable. Still, they had
> > similar problem of the file reference being owned by kvg structure and
> > potential for kvm_vfio_group_del() dropping it before
> > kvm_vfio_group_add() had a chance to complete.
> > 
> > Fix this by moving call to kvm_vfio_file_set_kvm() under the protection
> > of kv->lock. We already call it while holding the same lock when vfio
> > group is being deleted, so it should be safe here as well.
> > 
> > Fixes: 2fc1bec15883 ("kvm: set/clear kvm to/from vfio_group when group add/delete")
> > Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > ---
> 
> Applied series to vfio next branch for v6.6.  There's a minor rebase
> involved, so please double check the results:
> 
> https://github.com/awilliam/linux-vfio/commits/next

Looks good to me, thanks!

-- 
Dmitry