linux-kernel - Re: [PATCH v3 1/1] psi: Fix uaf issue when psi trigger is destroyed while being polled

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Date:   Wed, 12 Jan 2022 10:26:08 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Biggers <ebiggers@...nel.org>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        cgroups mailinglist <cgroups@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        syzbot <syzbot+cdb5dd11c97cc532efad@...kaller.appspotmail.com>
Subject: Re: [PATCH v3 1/1] psi: Fix uaf issue when psi trigger is destroyed
 while being polled

On Wed, Jan 12, 2022 at 10:16 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Wed, Jan 12, 2022 at 09:49:00AM -0800, Suren Baghdasaryan wrote:
> > > This happens with the following config:
> > >
> > > CONFIG_CGROUPS=n
> > > CONFIG_PSI=y
> > >
> > > With cgroups disabled these functions are defined as non-static but
> > > are not defined in the header
> > > (https://elixir.bootlin.com/linux/latest/source/include/linux/psi.h#L28)
> > > since the only external user cgroup.c is disabled. The cleanest way to
> > > fix these I think is by doing smth like this in psi.c:
>
> A cleaner way to solve these is simply:
>
> #ifndef CONFIG_CGROUPS
> static struct psi_trigger *psi_trigger_create(...);
> ...
> #endif
>
> I tested this works:
>
> $ cat foo5.c
> static int psi(void *);
>
> int psi(void *x)
> {
>         return (int)(long)x;
> }
>
> int bar(void *x)
> {
>         return psi(x);
> }
> $ gcc -W -Wall -O2 -c -o foo5.o foo5.c
> $ readelf -s foo5.o
>
> Symbol table '.symtab' contains 4 entries:
>    Num:    Value          Size Type    Bind   Vis      Ndx Name
>      0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
>      1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS foo5.c
>      2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 .text
>      3: 0000000000000000     3 FUNC    GLOBAL DEFAULT    1 bar
>

Thanks Matthew!
That looks much cleaner. I'll post a separate patch to fix these. My
main concern was whether it's worth adding more code to satisfy this
warning but with this approach the code changes are minimal, so I'll
go ahead and post it shortly.