linux-kernel - Re: [PATCH v1 1/2] sysctl: cap to ULONG_MAX in proc_get

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Message-ID: <CAHrFyr4LLxeVFMaP+H=QTLUcVsnNp0JEP4T0L02nKVXU5OwwUA@mail.gmail.com>
Date:   Mon, 15 Oct 2018 21:01:35 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v1 1/2] sysctl: cap to ULONG_MAX in proc_get_long()

On Mon, Oct 15, 2018 at 6:30 PM Christian Brauner <christian@...uner.io> wrote:
>
> On Mon, Oct 15, 2018 at 09:18:40AM -0700, Kees Cook wrote:
> > On Mon, Oct 15, 2018 at 3:55 AM, Christian Brauner <christian@...uner.io> wrote:
> > > proc_get_long() is a funny function. It uses simple_strtoul() and for a
> > > good reason. proc_get_long() wants to always succeed the parse and return
> > > the maybe incorrect value and the trailing characters to check against a
> > > pre-defined list of acceptable trailing values.
> > > However, simple_strtoul() explicitly ignores overflows which can cause
> >
> > What depends on simple_strtoul() ignoring overflows? Can we just cap
> > it to ULONG_MAX instead?
> >
> > I note that both simple_strtoul() and simple_strtoull() are marked as
> > obsolete (more below).
> >
> > > funny things like the following to happen:
> > >
> > > echo 18446744073709551616 > /proc/sys/fs/file-max
> > > cat /proc/sys/fs/file-max
> > > 0
> > >
> > > (Which will cause your system to silently die behind your back.)
> > >
> > > On the other hand kstrtoul() does do overflow detection but fails the parse
> > > in this case, does not return the trailing characters, and also fails the
> > > parse when anything other than '\n' is a trailing character whereas
> > > proc_get_long() wants to be more lenient.
> >
> > This parsing strictness difference makes it seem like the simple_*()
> > shouldn't be considered obsolete...

So what I would prefer in this case - if people agree that such a function
is useful - is to add a new function to lib/kstrtox.c and include/linux/kernel.h
e.g.:

int kstrtoul_bounded(const char *s, unsigned int base, char
**trailing, unsigned long long *res)

which aligns with the other kstrto*() functions, caps at <TYPE>_MAX,
doesn't fail the parse,
and returns the trailing characters in char **trailing. Then people
can start switching users of
simple_strtoul() over to this function without regressing anything.

Christian

> >
> > and it's still very heavily used:
> >
> > $ git grep -E 'simple_strtoull?\(' | wc -l
> > 745
>
> Maybe, but the intention is probably to fade it out and to not use it in
> new code because it doesn't handle overflow.
> Tbh, I'm weary to change that to suddenly return a ULONG_MAX on overflow
> instead of what it is doing now. I have absolutely no idea what this
> might break given how much it is still used in the kernel...
>
> >
> > > Now, before adding another kstrtoul() function let's simply add a static
> > > parse strtoul_cap_erange() which does:
> > > - returns ULONG_MAX on ERANGE
> > > - returns the trailing characters to the caller
> > > This guarantees that we don't regress userspace in any way but also caps
> > > any parsed value to ULONG_MAX and prevents things like file-max to become 0
> > > on overflow.
> >
> > -Kees
> >
> > --
> > Kees Cook
> > Pixel Security