[BUG] net/sched: TEQL qdisc can modify init-net teql0 from another netns

[BUG] net/sched: TEQL qdisc can modify init-net teql0 from another netns

[shaos]

Hello,

A process running as UID/GID 65534 creates a user and network namespace, sets a namespace-local dummy device to MTU 60000, and installs a root qdisc of kind teql0. TEQL reuses its global master and changes the init network namespace teql0 device from MTU 1500 to 60000.

HEAD commit:    c03c1a14507dfaf8b856c2608f9ac216021fc222
git tree:       30fd2ef82912c26cf3a58ad0ba6afc375fa851ac
source version: Linux 7.3.0, local master
compiler:       gcc 11.4.0, GNU ld 2.38

console output:
https://gist.githubusercontent.com/shaos26/5a9460785f381e5feb8decc5dead00b8/raw/a87637a25fcc759de83709b4fb0fe8f7cb03d089/console-output.txt
kernel config:
https://gist.githubusercontent.com/shaos26/5a9460785f381e5feb8decc5dead00b8/raw/8039a72c1de72497f040f40b7507fd8d2103425d/kernel-config.txt
reproducer:
https://gist.githubusercontent.com/shaos26/c800422c572d258de956971217d3b6dc/raw/53eca8ea2f399fd3de75aa2b38423c3928c9825a/poc.c

Build the tested kernel with the attached configuration, compile the reproducer, and boot it in a disposable QEMU VM with TEQL enabled. The reproducer creates user and network namespaces as an unprivileged UID, then sends the netlink requests.

Impact and prerequisites:
This crosses a network-namespace boundary: an unprivileged user-namespace process changes a globally visible network device. The reproducer also shows that a direct unprivileged MTU change in the initial namespace is denied and a pfifo control does not change teql0.

Validation: Two fresh QEMU boots at the exact v7.3 revision reproduced the change. The pfifo and direct-EPERM controls passed.

Thanks,

shaos

Re: [BUG] net/sched: TEQL qdisc can modify init-net teql0 from another netns

[Willy Tarreau]

Hello,

On Thu, Oct 22, 2026 at 02:48:30PM +0000, shaos wrote:
> Hello,
> 
> A process running as UID/GID 65534 creates a user and network namespace, sets
> a namespace-local dummy device to MTU 60000, and installs a root qdisc of
> kind teql0. TEQL reuses its global master and changes the init network
> namespace teql0 device from MTU 1500 to 60000.
(...)

I know this was mentioned elsewhere, but since you're sending so many
reports at once:

   please oh please stop CCing the security list for messages that
   you're already sending to public lists. It serves no purposes
   and adds extra work for several of us (review, triage, response).

Ccing the security list for a public message won't get your message
handled faster and might even cause the opposite by showing that you
don't read docs and seem to post in a hurry.

Thanks,
Willy