net/sched: act_api: deny mismatched skip_sw/skip_hw flags for actions created by...

net/sched: act_api: deny mismatched skip_sw/skip_hw flags for actions created by classifiers

tcf_action_init() has logic for checking mismatches between action and
filter offload flags (skip_sw/skip_hw). AFAIU, this is intended to run
on the transition between the new tc_act_bind(flags) returning true (aka
now gets bound to classifier) and tc_act_bind(act->tcfa_flags) returning
false (aka action was not bound to classifier before). Otherwise, the
check is skipped.

For the case where an action is not standalone, but rather it was
created by a classifier and is bound to it, tcf_action_init() skips the
check entirely, and this means it allows mismatched flags to occur.

Taking the matchall classifier code path as an example (with mirred as
an action), the reason is the following:

 1 | mall_change()
 2 | -> mall_replace_hw_filter()
 3 |   -> tcf_exts_validate_ex()
 4 |      -> flags |= TCA_ACT_FLAGS_BIND;
 5 |      -> tcf_action_init()
 6 |         -> tcf_action_init_1()
 7 |            -> a_o->init()
 8 |               -> tcf_mirred_init()
 9 |                  -> tcf_idr_create_from_flags()
10 |                     -> tcf_idr_create()
11 |                        -> p->tcfa_flags = flags;
12 |         -> tc_act_bind(flags))
13 |         -> tc_act_bind(act->tcfa_flags)

When invoked from tcf_exts_validate_ex() like matchall does (but other
classifiers validate their extensions as well), tcf_action_init() runs
in a call path where "flags" always contains TCA_ACT_FLAGS_BIND (set by
line 4). So line 12 is always true, and line 13 is always true as well.
No transition ever takes place, and the check is skipped.

The code was added in this form in commit c86e0209dc77 ("flow_offload:
validate flags of filter and actions"), but I'm attributing the blame
even earlier in that series, to when TCA_ACT_FLAGS_SKIP_HW and
TCA_ACT_FLAGS_SKIP_SW were added to the UAPI.

Following the development process of this change, the check did not
always exist in this form. A change took place between v3 [1] and v4 [2],
AFAIU due to review feedback that it doesn't make sense for action flags
to be different than classifier flags. I think I agree with that
feedback, but it was translated into code that omits enforcing this for
"classic" actions created at the same time with the filters themselves.

There are 3 more important cases to discuss. First there is this command:

$ tc qdisc add dev eth0 clasct
$ tc filter add dev eth0 ingress matchall skip_sw \
	action mirred ingress mirror dev eth1

which should be allowed, because prior to the concept of dedicated
action flags, it used to work and it used to mean the action inherited
the skip_sw/skip_hw flags from the classifier. It's not a mismatch.

Then we have this command:

$ tc qdisc add dev eth0 clasct
$ tc filter add dev eth0 ingress matchall skip_sw \
	action mirred ingress mirror dev eth1 skip_hw

where there is a mismatch and it should be rejected.

Finally, we have:

$ tc qdisc add dev eth0 clasct
$ tc filter add dev eth0 ingress matchall skip_sw \
	action mirred ingress mirror dev eth1 skip_sw

where the offload flags coincide, and this should be treated the same as
the first command based on inheritance, and accepted.

[1]: https://lore.kernel.org/netdev/20211028110646[email protected]/
[2]: https://lore.kernel.org/netdev/20211118130805[email protected]/
Fixes: 7adc57651211 ("flow_offload: add skip_hw and skip_sw to control if offload the action")
Signed-off-by: Vladimir Oltean <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Tested-by: Ido Schimmel <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>