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

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Message-ID: <hPgvCJ2KbKeauk78uWJEsuKJ5VfMqknPJ_oyOZe6M78-6eG7qnj0t0UKC-joPVowo_nOikIsEWP-ZDioARfI-Cl6zrHjCHPJST3drpi5ALE=@protonmail.ch>
Date:   Wed, 29 Sep 2021 21:39:06 +0000
From:   Jordan Glover <Golden_Miller83@...tonmail.ch>
To:     Alexey Gladkov <legion@...nel.org>
Cc:     ebiederm@...ssion.com, LKML <linux-kernel@...r.kernel.org>,
        "linux-mm\\\\@kvack.org" <linux-mm@...ck.org>,
        "containers\\\\@lists.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>,
        Yu Zhao <yuzhao@...gle.com>
Subject: Re: linux 5.14.3: free_user_ns causes NULL pointer dereference

On Wednesday, September 29th, 2021 at 5:36 PM, Alexey Gladkov <legion@...nel.org> wrote:

> On Tue, Sep 28, 2021 at 01:40:48PM +0000, Jordan Glover wrote:
>
> > On Thursday, September 16th, 2021 at 5:30 PM, ebiederm@...ssion.com wrote:
> >
> > > Jordan Glover Golden_Miller83@...tonmail.ch writes:
> > >
> > > > On Wednesday, September 15th, 2021 at 10:42 PM, Jordan Glover Golden_Miller83@...tonmail.ch wrote:
> > > >
> > > > > 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.
> > > >
> > > > Heh, it jut happened again. This time closing firefox alone had such
> > > >
> > > > effect:
> > >
> > > Ok. It looks like he have a couple of folks seeing issues here.
> > >
> > > I thought we had all of the issues sorted out for the release of v5.14,
> > >
> > > but it looks like there is still some little bug left.
> > >
> > > If Alex doesn't beat me to it I will see if I can come up with a
> > >
> > > debugging patch to make it easy to help track down where the reference
> > >
> > > count is going wrong. It will be a little bit as my brain is mush at
> > >
> > > the moment.
> > >
> > > Eric
> >
> > As the issue persist in 5.14.7 I would be very interested in such patch.
> >
> > For now the thing is mostly reproducible when I close several tabs in ff then
> >
> > close the browser in short period of time. When I close tabs then wait out
> >
> > a bit then close the browser it doesn't happen so I guess some interrupted
> >
> > cleanup triggers it.
>
> I'm still investigating, but I would like to rule out one option.
>
> Could you check out the patch?


Thx, I added it to my kernel and will report in few days.
Does this patch try to fix the issue or make it easier to track?

Jordan

> diff --git a/kernel/ucount.c b/kernel/ucount.c
>
> index bb51849e6375..f23f906f4f62 100644
>
> --- a/kernel/ucount.c
>
> +++ b/kernel/ucount.c
>
> @@ -201,11 +201,14 @@ void put_ucounts(struct ucounts *ucounts)
>
> {
>
> unsigned long flags;
>
> -         if (atomic_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, flags)) {
>
>
>
> -         spin_lock_irqsave(&ucounts_lock, flags);
>
>
> -         if (atomic_dec_and_test(&ucounts->count)) {
>
>                   hlist_del_init(&ucounts->node);
>
>                   spin_unlock_irqrestore(&ucounts_lock, flags);
>                   kfree(ucounts);
>
>
> -                 return;
>           }
>
>
> -         spin_unlock_irqrestore(&ucounts_lock, flags);
>
>
>
> }
>
> static inline bool atomic_long_inc_below(atomic_long_t *v, int u)
>
> ---------------------------------------------------------------------
>
> Rgrds, legion