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

On Thu, Sep 30 2021, Halil Pasic [off-list ref] wrote:

On Thu, 30 Sep 2021 11:28:23 +0200
Cornelia Huck [off-list ref] wrote:

quoted

On Thu, Sep 30 2021, Halil Pasic [off-list ref] wrote:

quoted

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).

Basically the second set is a subset of the first set.

I don't think that's clear.

quoted

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?

quoted

, 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.

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.

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.

quoted

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.

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.

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?

Anyone have any better suggestions?

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