APPLIED[H/I]/Cmnt: [SRU][I][H][F][PATCH 0/1] kernel: unable to read partitions on virtio-block dasd (kvm) (LP: 1950144)

[Stefan Bader]

On 09.11.21 13:47, frank.heimes at canonical.com wrote:
> BugLink: https://bugs.launchpad.net/bugs/1950144
> 
> SRU Justification:
> 
> [Impact]
> 
> * The kernel is unable to read partitions on virtio-block DASD (on KVM).
>    That's a severe situation, since it prevents Ubuntu from starting, if installed on a DASD.
>    This issue can either occur after a fresh installation or after an upgrade.
> 
> * The virtio specification virtio-v1.1-cs01 states: "Transitional devices
>    MUST detect Legacy drivers by detecting that VIRTIO_F_VERSION_1 has not
>    been acknowledged by the driver."
>    And this is what QEMU as of 6.1 has done relying solely on
>    VIRTIO_F_VERSION_1 for detecting that.
> 
> * But the specification also says: "... the driver MAY read (but MUST
>    NOT write) the device-specific configuration fields to check that it can
>    support the device ..." before setting FEATURES_OK.
>      
> * In this case, any transitional device relying solely on VIRTIO_F_VERSION_1
>    for detecting legacy drivers will return data in legacy format.
>    In particular, this implies that it's in big endian format for
>    big endian guests. This naturally confuses the driver that expects
>    little endian in the modern mode.
> 
> * VIRTIO_F_VERSION_1 can only be relied on after the feature negotiation is done.
> 
> * 'verify' is called before virtio_finalize_features(), so a transitional
>    s390 virtio device still serves native endian (i.e. big endian) config space,
>    while the driver knows that it is going to accept VERSION_1,
>    so when reading the config space, it assumes it got little endian, and byteswaps.
> 
> * For QEMU, we can work around the issue by writing out the feature bits with
>    VIRTIO_F_VERSION_1 bit set. We (ab)use the finalize_features config op for
>    this. This isn't enough to address all vhost devices since these do not get
>    the features until FEATURES_OK, however it looks like the affected devices
>    actually never handled the endianness for legacy mode correctly, so at least
>    that's not a regression.
> 
> [Fix]
> 
> * 2f9a174f918e29608564c7a4e8329893ab604fb4 2f9a174f918e "virtio: write back F_VERSION_1 before validate"
> 
> [Test Case]
> 
> * Setup an IBM Z or LinuxONE LPAR with Ubuntu Server 20.04 as KVM host.
> 
> * This Ubuntu KVM host can either be installed on FCP or DASD storage,
>    but at least one DASD disk need to be reserved for a KVM guest.
> 
> * Now hand over the reserved DASD disk   (low-level formatted using dasdfmt
>    and partitioned using fdasd) using 'virtio-block' to a KVM virtual machine
>    (e.g. using a virsh VM config).
> 
> * Try to install an Ubuntu KVM virtual machine using this DASD disk,
>    that includes the check and read of the partition table.
> 
> [Where problems could occur]
> 
> * First of all requested commit contains one additional if statement;
>    and is due tothat relatively traceable.
> 
> * But the change is in /drivers/virtio/virtio.c, means in common code!
> 
> * This issue obviously affects big endian systems only.
> 
> * But if done wrong, it may effect in worst-case little endian systems, too!
> 
> * But the if statement explicitly checks for '!virtio_legacy_is_little_endian()'.
> 
> * Only virtio net and virtio blk devices seem to be affected.
> 
> * And the commit/solutions was in-depth discussed upstream here:
>    https://lore.kernel.org/all/20211011053921.1198936-1-pasic@linux.ibm.com/t/#u
> 
> [Other]
> 
> * Patches are upstream accepted with since 5.15-rc6
>    and tagged for upstream stable #v4.11.
>    Hence jammy is not affected.
> 
> * Request was to add the patches to focal / 20.04,
>    but to avoid potential regressions on upgrades,
>    the patches need to be added to impish and hirsute, too.
> 
> * Fortunately cherry-picking the commit works cleanly
>    from all the affected Ubuntu releases.
> 
> Halil Pasic (1):
>    virtio: write back F_VERSION_1 before validate
> 
>   drivers/virtio/virtio.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 

Applied to hirsute,impish:linux/master-next. For focal this patch was just 
applied for v5.4.155 upstream stable. I have added the bug reference for this 
SRU to it. Thanks.

-Stefan

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