Re: Regression: Commit "netfilter: nf_tables: disallow rule addition to bound chain via NFTA_RULE_CHAIN_ID" breaks ruleset loading in linux-stable

[Timo Sigurdsson]

Hi,

Salvatore Bonaccorso schrieb am 12.09.2023 21:13 (GMT +02:00):

> Hi Timo,
> 
> On Tue, Sep 12, 2023 at 01:39:59PM +0200, Timo Sigurdsson wrote:
>> Hi Pablo,
>> 
>> Pablo Neira Ayuso schrieb am 12.09.2023 00:57 (GMT +02:00):
>> 
>> > Hi Timo,
>> > 
>> > On Mon, Sep 11, 2023 at 11:37:50PM +0200, Timo Sigurdsson wrote:
>> >> Hi,
>> >> 
>> >> recently, Debian updated their stable kernel from 6.1.38 to 6.1.52
>> >> which broke nftables ruleset loading on one of my machines with lots
>> >> of "Operation not supported" errors. I've reported this to the
>> >> Debian project (see link below) and Salvatore Bonaccorso and I
>> >> identified "netfilter: nf_tables: disallow rule addition to bound
>> >> chain via NFTA_RULE_CHAIN_ID" (0ebc1064e487) as the offending commit
>> >> that introduced the regression. Salvatore also found that this issue
>> >> affects the 5.10 stable tree as well (observed in 5.10.191), but he
>> >> cannot reproduce it on 6.4.13 and 6.5.2.
>> >> 
>> >> The issue only occurs with some rulesets. While I can't trigger it
>> >> with simple/minimal rulesets that I use on some machines, it does
>> >> occur with a more complex ruleset that has been in use for months
>> >> (if not years, for large parts of it). I'm attaching a somewhat
>> >> stripped down version of the ruleset from the machine I originally
>> >> observed this issue on. It's still not a small or simple ruleset,
>> >> but I'll try to reduce it further when I have more time.
>> >> 
>> >> The error messages shown when trying to load the ruleset don't seem
>> >> to be helpful. Just two simple examples: Just to give two simple
>> >> examples from the log when nftables fails to start:
>> >> /etc/nftables.conf:99:4-44: Error: Could not process rule: Operation not
>> >> supported
>> >>                         tcp option maxseg size 1-500 counter drop
>> >>                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> >> /etc/nftables.conf:308:4-27: Error: Could not process rule: Operation not
>> >> supported
>> >>                         tcp dport sip-tls accept
>> >>                         ^^^^^^^^^^^^^^^^^^^^^^^^
>> > 
>> > I can reproduce this issue with 5.10.191 and 6.1.52 and nftables v1.0.6,
>> > this is not reproducible with v1.0.7 and v1.0.8.
>> > 
>> >> Since the issue only affects some stable trees, Salvatore thought it
>> >> might be an incomplete backport that causes this.
>> >> 
>> >> If you need further information, please let me know.
>> > 
>> > Userspace nftables v1.0.6 generates incorrect bytecode that hits a new
>> > kernel check that rejects adding rules to bound chains. The incorrect
>> > bytecode adds the chain binding, attach it to the rule and it adds the
>> > rules to the chain binding. I have cherry-picked these three patches
>> > for nftables v1.0.6 userspace and your ruleset restores fine.
>> 
>> hmm, that doesn't explain why Salvatore didn't observe this with
>> more recent kernels.
>> 
>> Salvatore, did you use newer userspace components when you tested
>> your 6.4.13 and 6.5.2 builds?
> 
> It does explain now because understanding the issue better. While one
> while experinting should only change each one constraint for the
> 6.4.13 and 6.5.2 testing I indeed switched to a Debian unstable
> system, which has newer userpace nftables and so not triggering the
> issue. This was missleading for the report.
> 
>> As for the regression and how it be dealt with: Personally, I don't
>> really care whether the regression is solved in the kernel or
>> userspace. If everybody agrees that this is the best or only viable
>> option and Debian decides to push a nftables update to fix this,
>> that works for me. But I do feel the burden to justify this should
>> be high. A kernel change that leaves users without a working packet
>> filter after upgrading their machines is serious, if you ask me. And
>> since it affects several stable/longterm trees, I would assume this
>> will hit other stable (non-rolling) distributions as well, since
>> they will also use older userspace components (unless this is
>> behavior specific to nftables 1.0.6 but not older versions). They
>> probably should get a heads up then.
> 
> So if it is generally believed on kernel side there should not happen
> any further changes to work with older userland, I guess in Debian we
> will need to patch nftables. I'm CC'ing Arturo Borrero Gonzalez
> <arturo@xxxxxxxxxx>, maintainer for the package. The update should go
> ideally in the next point releases from October (and maybe released
> earlier as well trough the stable-updates mechanism).

So, I built nftables 1.0.6-2+deb12u1 with the three cherry-picked patches from Pablo and can confirm that they resolve the issue for me on bookworm. I can now run linux 6.1.52-1 and load my original nftables ruleset again.
 
> FWIW: In Debian bullseye we have 0.9.8 based nftables, in bookworm
> 1.0.6, so both will need those fixes.
> 
> As 0ebc1064e487 is to address CVE-2023-4147 other distros picking the
> fix will likely encounter the problem at some point. It looks Red Hat
> has taken it (some RHSA's were released), I assume Ubuntu will shortly
> as well release USN's containing a fix.

SUSE has also picked this patch for SLES/SLED. I hope maintainers follow the mailing lists cc'ed here or that someone gives them a heads up before this hits more production systems.

Thanks and regards,

Timo