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

[Halil Pasic]

On Thu, 30 Sep 2021 13:31:04 +0200
Cornelia Huck <cohuck@xxxxxxxxxx> wrote:

> On Thu, Sep 30 2021, Halil Pasic <pasic@xxxxxxxxxxxxx> wrote:
> 
> > On Thu, 30 Sep 2021 11:28:23 +0200
> > Cornelia Huck <cohuck@xxxxxxxxxx> wrote:
> >  
> >> On Thu, Sep 30 2021, Halil Pasic <pasic@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> >> > Fixes: 82e89ea077b9 ("virtio-blk: Add validation for block size in config space")
> >> > Reported-by: markver@xxxxxxxxxx
> >> > ---
> >> >  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).  
> >
> > Basically the second set is a subset of the first set.  
> 
> I don't think that's clear.

Validate can only remove features, or? So I guess after validate
is a subset of before validate.


> 
> >  
> >> 
> >> I'm not sure whether that is even a problem in the spec: while the
> >> driver may read the config before finally accepting features  
> >
> > I'm not sure I'm following you. Let me please qoute the specification:
> > """
> > 4. Read device feature bits, and write the subset of feature bits
> > understood by the OS and driver to the device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration fields to check that it can support the device before accepting it. 
> > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step. 
> > """
> > https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-930001  
> 
> Yes, exactly, it MAY read before accepting features. How does the device
> know whether the config space is little-endian or not?
> 

Well that is what we are talking about. One can try to infer things from
the spec. This reset dance I called ugly is probably the cleanest,
because the spec says that re-nego should work.

> >  
> >> , 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.   
> >
> > Are you suggesting that ->verify() should be after
> > virtio_finalize_features()?  
> 
> No, that would defeat the entire purpose of verify. After
> virtio_finalize_features(), we are done with feature negotiation.
>

Exactly!
 
> > Wouldn't
> > that mean that verify() can't reject feature bits? But that is the whole
> > point of commit 82e89ea077b9 ("virtio-blk: Add validation for block size
> > in config space"). Do you think that the commit in question is
> > conceptually flawed? My understanding of the verify is, that it is supposed
> > to fence features and feature bits we can't support, e.g. because of
> > config space things, but I may be wrong.  
> 
> No, that commit is not really flawed on its own, I think the whole
> procedure may be problematic.
> 

I agree! But that regression really hurts us. Maybe the best band-aid is
to conditional-compile it (not compile the check if s390).

> >
> > The trouble is, feature bits are not negotiated one by one, but basically all
> > at once. I suppose, I did the next best thing to first negotiating
> > VERSION_1.  
> 
> We probably need to special-case VERSION_1 to move at least forward;
> i.e. proceed as if we accepted it when reading the config space.
> 
> The problem is that we do not know what the device assumes when we read
> the config space prior to setting FEATURES_OK. It may assume
> little-endian if it offered VERSION_1, or it may not. The spec does not
> really say what happens before feature negotiation has finished.
> 
No it does not, but I hope, the implementations we care the most about do
little endian if VERSION_1 is set but FEATURES_OK is not yet done. A
transitional device would have to act upon a feature that is set,
because for legacy there is no FEATURES_OK. Where we can run into
trouble is minimum required feature set, e.g. mandatory features.

I will do some testing.

> >
> >  
> >> For
> >> VERSION_1, we can probably go ahead and just assume that we will accept
> >> it if offered, but what about other (future) bits?  
> >
> > I don't quite understand.  
> 
> There might be other bits in the future that change how the config space
> works. We cannot assume that any of those bits will be accepted if
> offered; i.e. we need a special hack for VERSION_1.

I tend to agree. What I didn't consider in this patch is that, setting
bits does not only set bits, but may also change the device in a way,
that clearing the bit would not change it back.

> 
> >
> > Anyway, how do you think we should solve this problem?  
> 
> This is a mess. For starters, we need to think about if we should do
> something in the spec, and if yes, what.. Then, we can probably think
> about how to implement that properly.
>

I agree.

 
> As we have an error right now that is basically a regression, we
> probably need a band-aid to keep going. Not sure if your patch is the
> right approach, maybe we really need to special-case VERSION_1 (the
> "assume we accepted it" hack mentioned above.) This will likely fix the
> reported problem (I assume that is s390x on QEMU); do we know about
> other VMMs? Any other big-endian architectures?

I didn't quite get it. Would this hack take place in QEMU or in the guest
kernel?

> 
> Anyone have any better suggestions?
> 

There is the conditional compile, as an option but I would not say it is
better.

Regards,
Halil