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Ilya Maximets 057dbc5b72 net: openvswitch: remove misbehaving actions length check 
[ Upstream commit a1e64addf3 ]

The actions length check is unreliable and produces different results
depending on the initial length of the provided netlink attribute and
the composition of the actual actions inside of it.  For example, a
user can add 4088 empty clone() actions without triggering -EMSGSIZE,
on attempt to add 4089 such actions the operation will fail with the
-EMSGSIZE verdict.  However, if another 16 KB of other actions will
be *appended* to the previous 4089 clone() actions, the check passes
and the flow is successfully installed into the openvswitch datapath.

The reason for a such a weird behavior is the way memory is allocated.
When ovs_flow_cmd_new() is invoked, it calls ovs_nla_copy_actions(),
that in turn calls nla_alloc_flow_actions() with either the actual
length of the user-provided actions or the MAX_ACTIONS_BUFSIZE.  The
function adds the size of the sw_flow_actions structure and then the
actually allocated memory is rounded up to the closest power of two.

So, if the user-provided actions are larger than MAX_ACTIONS_BUFSIZE,
then MAX_ACTIONS_BUFSIZE + sizeof(*sfa) rounded up is 32K + 24 -> 64K.
Later, while copying individual actions, we look at ksize(), which is
64K, so this way the MAX_ACTIONS_BUFSIZE check is not actually
triggered and the user can easily allocate almost 64 KB of actions.

However, when the initial size is less than MAX_ACTIONS_BUFSIZE, but
the actions contain ones that require size increase while copying
(such as clone() or sample()), then the limit check will be performed
during the reserve_sfa_size() and the user will not be allowed to
create actions that yield more than 32 KB internally.

This is one part of the problem.  The other part is that it's not
actually possible for the userspace application to know beforehand
if the particular set of actions will be rejected or not.

Certain actions require more space in the internal representation,
e.g. an empty clone() takes 4 bytes in the action list passed in by
the user, but it takes 12 bytes in the internal representation due
to an extra nested attribute, and some actions require less space in
the internal representations, e.g. set(tunnel(..)) normally takes
64+ bytes in the action list provided by the user, but only needs to
store a single pointer in the internal implementation, since all the
data is stored in the tunnel_info structure instead.

And the action size limit is applied to the internal representation,
not to the action list passed by the user.  So, it's not possible for
the userpsace application to predict if the certain combination of
actions will be rejected or not, because it is not possible for it to
calculate how much space these actions will take in the internal
representation without knowing kernel internals.

All that is causing random failures in ovs-vswitchd in userspace and
inability to handle certain traffic patterns as a result.  For example,
it is reported that adding a bit more than a 1100 VMs in an OpenStack
setup breaks the network due to OVS not being able to handle ARP
traffic anymore in some cases (it tries to install a proper datapath
flow, but the kernel rejects it with -EMSGSIZE, even though the action
list isn't actually that large.)

Kernel behavior must be consistent and predictable in order for the
userspace application to use it in a reasonable way.  ovs-vswitchd has
a mechanism to re-direct parts of the traffic and partially handle it
in userspace if the required action list is oversized, but that doesn't
work properly if we can't actually tell if the action list is oversized
or not.

Solution for this is to check the size of the user-provided actions
instead of the internal representation.  This commit just removes the
check from the internal part because there is already an implicit size
check imposed by the netlink protocol.  The attribute can't be larger
than 64 KB.  Realistically, we could reduce the limit to 32 KB, but
we'll be risking to break some existing setups that rely on the fact
that it's possible to create nearly 64 KB action lists today.

Vast majority of flows in real setups are below 100-ish bytes.  So
removal of the limit will not change real memory consumption on the
system.  The absolutely worst case scenario is if someone adds a flow
with 64 KB of empty clone() actions.  That will yield a 192 KB in the
internal representation consuming 256 KB block of memory.  However,
that list of actions is not meaningful and also a no-op.  Real world
very large action lists (that can occur for a rare cases of BUM
traffic handling) are unlikely to contain a large number of clones and
will likely have a lot of tunnel attributes making the internal
representation comparable in size to the original action list.
So, it should be fine to just remove the limit.

Commit in the 'Fixes' tag is the first one that introduced the
difference between internal representation and the user-provided action
lists, but there were many more afterwards that lead to the situation
we have today.

Fixes: 7d5437c709 ("openvswitch: Add tunneling interface.")
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Link: https://patch.msgid.link/20250308004609.2881861-1-i.maximets@ovn.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-04-10 14:31:49 +02:00
arch clockevents/drivers/i8253: Fix stop sequence for timer 0 2025-04-10 14:31:48 +02:00
block block: fix conversion of GPT partition name to 7-bit 2025-03-13 12:51:03 +01:00
certs certs/blacklist_hashes.c: fix const confusion in certs blacklist 2022-06-22 14:22:01 +02:00
crypto crypto: testmgr - some more fixes to RSA test vectors 2025-03-13 12:50:48 +01:00
Documentation sched/isolation: Prevent boot crash when the boot CPU is nohz_full 2025-04-10 14:31:48 +02:00
drivers net/mlx5: handle errors in mlx5_chains_create_table() 2025-04-10 14:31:49 +02:00
fs nilfs2: handle errors that nilfs_prepare_chunk() may return 2025-03-13 12:51:12 +01:00
include slab: Introduce kmalloc_size_roundup() 2025-04-10 14:31:49 +02:00
init initramfs: avoid filename buffer overrun 2024-12-14 19:50:42 +01:00
io_uring io_uring: fix possible deadlock in io_register_iowq_max_workers() 2024-11-17 15:06:25 +01:00
ipc ipc/sem: Fix dangling sem_array access in semtimedop race 2022-12-08 11:28:45 +01:00
kernel sched/fair: Fix potential memory corruption in child_cfs_rq_on_list 2025-03-13 12:51:07 +01:00
lib kfence: allow use of a deferrable timer 2025-03-13 12:50:49 +01:00
LICENSES LICENSES/dual/CC-BY-4.0: Git rid of "smart quotes" 2021-07-15 06:31:24 -06:00
mm slab: Introduce kmalloc_size_roundup() 2025-04-10 14:31:49 +02:00
net net: openvswitch: remove misbehaving actions length check 2025-04-10 14:31:49 +02:00
samples samples/landlock: Fix possible NULL dereference in parse_path() 2025-03-13 12:49:57 +01:00
scripts kbuild: Move -Wenum-enum-conversion to W=2 2025-03-13 12:50:29 +01:00
security tomoyo: don't emit warning in tomoyo_write_control() 2025-03-13 12:50:18 +01:00
sound ALSA: usx2y: validate nrpacks module parameter on probe 2025-03-13 12:51:05 +01:00
tools selftests: rtnetlink: update netdevsim ipsec output format 2025-03-13 12:50:46 +01:00
usr usr/include/Makefile: add linux/nfc.h to the compile-test coverage 2022-02-01 17:27:15 +01:00
virt KVM: Fix a data race on last_boosted_vcpu in kvm_vcpu_on_spin() 2024-10-22 15:40:41 +02:00
.clang-format clang-format: Update with the latest for_each macro list 2021-05-12 23:32:39 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore Remove *.orig pattern from .gitignore 2024-10-17 15:11:10 +02:00
.mailmap mailmap: add Andrej Shadura 2021-10-18 20:22:03 -10:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Daniel Drake to credits 2021-09-21 08:34:58 +03:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS trace: Relocate event helper files 2024-04-10 16:19:24 +02:00
Makefile Linux 5.15.179 2025-03-13 12:51:13 +01:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.