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virtio: acknowledge all features before access

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The feature negotiation was designed in a way that
makes it possible for devices to know which config
fields will be accessed by drivers.

This is broken since commit 404123c2db ("virtio: allow drivers to
validate features") with fallout in at least block and net.  We have a
partial work-around in commit 2f9a174f91 ("virtio: write back
F_VERSION_1 before validate") which at least lets devices find out which
format should config space have, but this is a partial fix: guests
should not access config space without acknowledging features since
otherwise we'll never be able to change the config space format.

To fix, split finalize_features from virtio_finalize_features and
call finalize_features with all feature bits before validation,
and then - if validation changed any bits - once again after.

Since virtio_finalize_features no longer writes out features
rename it to virtio_features_ok - since that is what it does:
checks that features are ok with the device.

As a side effect, this also reduces the amount of hypervisor accesses -
we now only acknowledge features once unless we are clearing any
features when validating (which is uncommon).

IRC I think that this was more or less always the intent in the spec but
unfortunately the way the spec is worded does not say this explicitly, I
plan to address this at the spec level, too.

Acked-by: Jason Wang <jasowang@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 404123c2db ("virtio: allow drivers to validate features")
Fixes: 2f9a174f91 ("virtio: write back F_VERSION_1 before validate")
Cc: "Halil Pasic" <pasic@linux.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Michael S. Tsirkin 2022-01-14 14:58:41 -05:00
parent 838d6d3461
commit 4fa59ede95
2 changed files with 24 additions and 18 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

39
drivers/virtio/virtio.c
View File

@ -166,14 +166,13 @@ void virtio_add_status(struct virtio_device *dev, unsigned int status)
}
EXPORT_SYMBOL_GPL(virtio_add_status);
static int virtio_finalize_features(struct virtio_device *dev)
/* Do some validation, then set FEATURES_OK */
static int virtio_features_ok(struct virtio_device *dev)
{
int ret = dev->config->finalize_features(dev);
unsigned status;
int ret;
might_sleep();
if (ret)
return ret;
ret = arch_has_restricted_virtio_memory_access();
if (ret) {
@ -244,17 +243,6 @@ static int virtio_dev_probe(struct device *_d)
driver_features_legacy = driver_features;
}
/*
* Some devices detect legacy solely via F_VERSION_1. Write
* F_VERSION_1 to force LE config space accesses before FEATURES_OK for
* these when needed.
*/
if (drv->validate && !virtio_legacy_is_little_endian()
&& device_features & BIT_ULL(VIRTIO_F_VERSION_1)) {
dev->features = BIT_ULL(VIRTIO_F_VERSION_1);
dev->config->finalize_features(dev);
}
if (device_features & (1ULL << VIRTIO_F_VERSION_1))
dev->features = driver_features & device_features;
else
@ -265,13 +253,26 @@ static int virtio_dev_probe(struct device *_d)
if (device_features & (1ULL << i))
__virtio_set_bit(dev, i);
err = dev->config->finalize_features(dev);
if (err)
goto err;
if (drv->validate) {
u64 features = dev->features;
err = drv->validate(dev);
if (err)
goto err;
/* Did validation change any features? Then write them again. */
if (features != dev->features) {
err = dev->config->finalize_features(dev);
if (err)
goto err;
}
}
err = virtio_finalize_features(dev);
err = virtio_features_ok(dev);
if (err)
goto err;
@ -495,7 +496,11 @@ int virtio_device_restore(struct virtio_device *dev)
/* We have a driver! */
virtio_add_status(dev, VIRTIO_CONFIG_S_DRIVER);
ret = virtio_finalize_features(dev);
ret = dev->config->finalize_features(dev);
if (ret)
goto err;
ret = virtio_features_ok(dev);
if (ret)
goto err;

3
include/linux/virtio_config.h
View File

@ -64,8 +64,9 @@ struct virtio_shm_region {
* Returns the first 64 feature bits (all we currently need).
* @finalize_features: confirm what device features we'll be using.
* vdev: the virtio_device
* This gives the final feature bits for the device: it can change
* This sends the driver feature bits to the device: it can change
* the dev->feature bits if it wants.
* Note: despite the name this can be called any number of times.
* Returns 0 on success or error status
* @bus_name: return the bus name associated with the device (optional)
* vdev: the virtio_device