linux-kernel - Re: [PATCH] kernel/sysctl.c: fix out of bounds access in fs.file-max

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Date:   Thu, 4 Apr 2019 16:09:33 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Matteo Croce <mcroce@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Zev Weiss <zev@...ilderbeest.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] kernel/sysctl.c: fix out of bounds access in fs.file-max

On Wed, Apr 03, 2019 at 07:08:47PM +0200, Matteo Croce wrote:
> On Wed, Apr 3, 2019 at 6:40 PM Matteo Croce <mcroce@...hat.com> wrote:
> >
> > On Wed, Apr 3, 2019 at 5:51 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Wed, Apr 03, 2019 at 05:24:26PM +0200, Matteo Croce wrote:
> > > > On Wed, Apr 3, 2019 at 4:02 PM Christian Brauner <christian@...uner.io> wrote:
> > > > > Yeah, maybe but it still feels cleaner and more obvious to just add:
> > > > >
> > > > > static long long_zero;
> > > > >
> > > > > given that most callers actually seem to want an (unsigned) int.
> > > > >
> > > > > I don't have a strong opinion though so if others feel that it's just a
> > > > > waste of space consider it acked.
> > > > >
> > > >
> > > > Well, given that the value is zero, in this expectional case we could
> > > > avoid duplicating the symbol and save 4 bytes.
> > > > What the maintainers think?
> > >
> > > If we care about saving four bytes, we could just pass the address of
> > > ZERO_PAGE(0).
> >
> > That would work, work too, maybe it's a bit overkill.
> > int zero is always there and it's static, so enlarging it to long
> > should be a straightforward fix.
> > Obviously we can't do it for other numbers, but we can alias it just
> > for the zero case..
> >
> > Regards,
> >
> > --
> > Matteo Croce
> > per aspera ad upstream
> 
> Anyway, I'm fine with both solutions, as I have other patches in the

I think Matthew's idea gets us best of both worlds so I'd suggest to use
it and resend the patch. You likely want to Cc stable@...r.kernel.org
since the original patch this fixes got backported by Greg quite a bit
since this was a rather long-standing issue. Please also Cc Andrew this
time since he's likely going to pick it up.

Thanks for the patch!
Christian