linux-kernel - Re: linux 5.14.3: free_user_ns causes NULL pointer dereference

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Date:   Wed, 15 Sep 2021 22:42:21 +0000
From:   Jordan Glover <Golden_Miller83@...tonmail.ch>
To:     ebiederm@...ssion.com
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "linux-mm\\@kvack.org" <linux-mm@...ck.org>,
        "legion\\@kernel.org" <legion@...nel.org>,
        "containers\\@lists.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>
Subject: Re: linux 5.14.3: free_user_ns causes NULL pointer dereference

On Wednesday, September 15th, 2021 at 9:02 PM, <ebiederm@...ssion.com> wrote:

> Jordan Glover Golden_Miller83@...tonmail.ch writes:
>
> > Hi, recently I hit system freeze after I was closing few containerized apps on my system. As for now it occurred only once on linux 5.14.3. I think it maybe be related to "Count rlimits in each user namespace" patchset merged during 5.14 window
> >
> > https://lore.kernel.org/all/257aa5fb1a7d81cf0f4c34f39ada2320c4284771.1619094428.git.legion@kernel.org/T/#u
>
> So that warning comes from:
>
> void dec_ucount(struct ucounts *ucounts, enum ucount_type type)
>
> {
>
> struct ucounts *iter;
>
> for (iter = ucounts; iter; iter = iter->ns->ucounts) {
>
>     	long dec = atomic_long_dec_if_positive(&iter->ucount[type]);
>
>     	WARN_ON_ONCE(dec < 0);
>     }
>     put_ucounts(ucounts);
>
>
> }
>
> Which certainly looks like a reference count bug. It could also be a
>
> memory stomp somewhere close.
>
> Do you have any idea what else was going on? This location is the
>
> symptom but not the actual cause.
>
> Eric

I had about 2 containerized (flatpak/bubblewrap) apps (browser + music player) running . I quickly closed them with intent to shutdown the system but instead get the freeze and had to use magic sysrq to reboot. System logs end with what I posted and before there is nothing suspicious.

Maybe it's some random fluke. I'll reply if I hit it again.

Jordan