Re: [RFC PATCH 1/1] virtio: write back features before verify

[Cornelia Huck <cohuck@redhat.com>]

On Thu, Sep 30 2021, Halil Pasic <pasic@linux.ibm.com> wrote:

> This patch fixes a regression introduced by commit 82e89ea077b9
> ("virtio-blk: Add validation for block size in config space") and
> enables similar checks in verify() on big endian platforms.
>
> The problem with checking multi-byte config fields in the verify
> callback, on big endian platforms, and with a possibly transitional
> device is the following. The verify() callback is called between
> config->get_features() and virtio_finalize_features(). That we have a
> device that offered F_VERSION_1 then we have the following options
> either the device is transitional, and then it has to present the legacy
> interface, i.e. a big endian config space until F_VERSION_1 is
> negotiated, or we have a non-transitional device, which makes
> F_VERSION_1 mandatory, and only implements the non-legacy interface and
> thus presents a little endian config space. Because at this point we
> can't know if the device is transitional or non-transitional, we can't
> know do we need to byte swap or not.
>
> The virtio spec explicitly states that the driver MAY read config
> between reading and writing the features so saying that first accessing
> the config before feature negotiation is done is not an option. The
> specification ain't clear about setting the features multiple times
> before FEATURES_OK, so I guess that should be fine.
>
> I don't consider this patch super clean, but frankly I don't think we
> have a ton of options. Another option that may or man not be cleaner,
> but is also IMHO much uglier is to figure out whether the device is
> transitional by rejecting _F_VERSION_1, then resetting it and proceeding
> according tho what we have figured out, hoping that the characteristics
> of the device didn't change.
>
> Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> Fixes: 82e89ea077b9 ("virtio-blk: Add validation for block size in config space")
> Reported-by: markver@us.ibm.com
> ---
>  drivers/virtio/virtio.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 0a5b54034d4b..9dc3cfa17b1c 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -249,6 +249,10 @@ static int virtio_dev_probe(struct device *_d)
>  		if (device_features & (1ULL << i))
>  			__virtio_set_bit(dev, i);
>  
> +	/* Write back features before validate to know endianness */
> +	if (device_features & (1ULL << VIRTIO_F_VERSION_1))
> +		dev->config->finalize_features(dev);

This really looks like a mess :(

We end up calling ->finalize_features twice: once before ->validate, and
once after, that time with the complete song and dance. The first time,
we operate on one feature set; after validation, we operate on another,
and there might be interdependencies between the two (like a that a bit
is cleared because of another bit, which would not happen if validate
had a chance to clear that bit before).

I'm not sure whether that is even a problem in the spec: while the
driver may read the config before finally accepting features, it does
not really make sense to do so before a feature bit as basic as
VERSION_1 which determines the endianness has been negotiated. For
VERSION_1, we can probably go ahead and just assume that we will accept
it if offered, but what about other (future) bits?

> +
>  	if (drv->validate) {
>  		err = drv->validate(dev);
>  		if (err)