CVE-2025-37865

In the Linux kernel, the following vulnerability has been resolved:

net: dsa: mv88e6xxx: fix -ENOENT when deleting VLANs and MST is unsupported

Russell King reports that on the ZII dev rev B, deleting a bridge VLAN from a user port fails with -ENOENT: https://lore.kernel.org/netdev/Z_lQXNP0s5-IiJzd@shell.armlinux.org.uk/

This comes from mv88e6xxxportvlanleave() -> mv88e6xxxmst_put(), which tries to find an MST entry in &chip->msts associated with the SID, but fails and returns -ENOENT as such.

But we know that this chip does not support MST at all, so that is not surprising. The question is why does the guard in mv88e6xxxmstput() not exit early:

if (!sid)
    return 0;

And the answer seems to be simple: the sid comes from vlan.sid which supposedly was previously populated by mv88e6xxxvtuget(). But some chip->info->ops->vtugetnext() implementations do not populate vlan.sid, for example see mv88e6185g1vtugetnext(). In that case, later in mv88e6xxxportvlan_leave() we are using a garbage sid which is just residual stack memory.

Testing for sid == 0 covers all cases of a non-bridge VLAN or a bridge VLAN mapped to the default MSTI. For some chips, SID 0 is valid and installed by mv88e6xxxstusetup(). A chip which does not support the STU would implicitly only support mapping all VLANs to the default MSTI, so although SID 0 is not valid, it would be sufficient, if we were to zero-initialize the vlan structure, to fix the bug, due to the coincidence that a test for vlan.sid == 0 already exists and leads to the same (correct) behavior.

Another option which would be sufficient would be to add a test for mv88e6xxxhasstu() inside mv88e6xxxmstput(), symmetric to the one which already exists in mv88e6xxxmstget(). But that placement means the caller will have to dereference vlan.sid, which means it will access uninitialized memory, which is not nice even if it ignores it later.

So we end up making both modifications, in order to not rely just on the sid == 0 coincidence, but also to avoid having uninitialized structure fields which might get temporarily accessed.