Date: Wed, 10 May 2023 16:21:31 -0300 From: Thadeu Lima de Souza Cascardo <cascardo@...onical.com> To: oss-security@...ts.openwall.com Cc: Turritopsis Dohrnii Teo En Ming <tdtemccnp@...il.com>, ceo@...-en-ming-corp.com, Piotr Krysiuk <piotras@...il.com> Subject: Re: New Linux kernel NetFilter flaw gives attackers root privileges On Wed, May 10, 2023 at 06:55:46PM +0200, Solar Designer wrote: > Hi, > > On Wed, May 10, 2023 at 11:52:58PM +0800, Turritopsis Dohrnii Teo En Ming wrote: > > I have just come across this article. Thought of sharing it. > > > > Article: New Linux kernel NetFilter flaw gives attackers root privileges > > Link: https://www.bleepingcomputer.com/news/security/new-linux-kernel-netfilter-flaw-gives-attackers-root-privileges/ > > We don't normally want in here links to news articles on something that > was already brought up in here in more detail. However, as a moderator, > I reluctantly approved this posting so that we can use the resulting > thread to discuss whether this issue got blown out of proportion and if > so what we can do to avoid that going forward. Here's the original > posting this refers to: > > https://www.openwall.com/lists/oss-security/2023/05/08/4 > > Another Linux kernel issue, in io_uring subsystem, was also disclosed in > here on the same day, but I think didn't gain such tech media attention: > > https://www.openwall.com/lists/oss-security/2023/05/08/3 > > Is the netfilter issue really worse than the io_uring issue? I doubt > it. So _maybe_ it was something in the wording that tripped someone > writing for one of those tech news websites, then others picked it up? > > Piotr's posting about the netfilter issue mentions intent to disclose an > exploit later (like it should have, thank you Piotr!) > > Tobias' posting directly links to an exploit (which is also fine). > > Is intent to disclose an exploit later more newsworthy than having done > so right away? I doubt it. > > So maybe it's just random, and there's nothing to see here, after all. > > Now as to the actual issue and its description, I think we should > clarify what exactly is meant by "unprivileged local users." Piotr, I > guess you actually meant not literally unprivileged, but users with > CAP_NET_ADMIN, which can be had via unprivileged user/net namespaces if > enabled in the distro / on the system, or when already in a container > with such capability granted to container root. Correct? I think going > forward we should always make this clear right away. Here's a former > netfilter core team leader also bringing this up: > > https://twitter.com/LaF0rge/status/1655867494152667140 > > LaForge - @LaF0rge@...os.social @LaF0rge: > > Really curious to see how CVS-223-32233 for #linux #netfilter nf_tables > > https://seclists.org/oss-sec/2023/q2/133 can be exploted fom > > "unprivileged local users". AFAICT, nf_tables_api goes through > > nfnetlink, and nfnetlink_rcv() checks for CAP_NET_ADMIN way before the > > code in nf_tables_api. > > and a reply: > > Alex Plaskett @alexjplaskett: > > Didn't look in depth at this one but you can trigger nf_tables_api > > operations from a user / network namespace and distros such as Ubuntu > > have unpriv user namespaces enabled. If users don't need user namespaces, they can disable it on Ubuntu kernels as a mitigation by doing: sysctl -w kernel.unprivileged_userns_clone=0 Or persisting the option by adding a .conf file at /etc/sysctl.d/ with the following line: kernel.unprivileged_userns_clone=0 Cascardo. > > As expected. Now, from a typical distro user's standpoint, > "unprivileged local users" may be just right. However, not all distros > have unprivileged user namespaces enabled by default. > > Alexander
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