SRU request submitted to the Ubuntu kernel team mailing list for impish,
hirsute and focal:
https://lists.ubuntu.com/archives/kernel-team/2021-November/thread.html#125581
Changing status to 'In Progress' for impish, hirsute and focal.
** Changed in: linux (Ubuntu Impish)
Status: New => In Progress
** Changed in: linux (Ubuntu Hirsute)
Status: New => In Progress
** Changed in: linux (Ubuntu Focal)
Status: New => In Progress
** Changed in: linux (Ubuntu Impish)
Assignee: (unassigned) => Canonical Kernel Team (canonical-kernel-team)
** Changed in: linux (Ubuntu Hirsute)
Assignee: (unassigned) => Canonical Kernel Team (canonical-kernel-team)
** Changed in: linux (Ubuntu Focal)
Assignee: (unassigned) => Canonical Kernel Team (canonical-kernel-team)
** Description changed:
+ 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-pa...@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.
+
+ __________
+
Description: kernel: unable to read partitions on virtio-block dasd (kvm)
Symptom: unable to read partitions on virtio-block dasd (kvm)
Problem: 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.
+ 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.
Solution: 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.
+ 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.
** Changed in: ubuntu-z-systems
Status: Triaged => In Progress
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You received this bug notification because you are a member of Kernel
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https://bugs.launchpad.net/bugs/1950144
Title:
[UBUNTU 20.04] kernel: unable to read partitions on virtio-block dasd
(kvm)
Status in Ubuntu on IBM z Systems:
In Progress
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Focal:
In Progress
Status in linux source package in Hirsute:
In Progress
Status in linux source package in Impish:
In Progress
Status in linux source package in Jammy:
Fix Released
Bug description:
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-pa...@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.
__________
Description: kernel: unable to read partitions on virtio-block dasd (kvm)
Symptom: unable to read partitions on virtio-block dasd (kvm)
Problem: 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.
Solution: 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.
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