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

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Message-ID: <CAGXu5j+BZsmRXwXfXk1YcyuSRNMmre=K55e1CXaLJDUZ07rmvQ@mail.gmail.com>
Date:   Mon, 15 Oct 2018 09:18:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christian Brauner <christian@...uner.io>
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 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...

and it's still very heavily used:

$ git grep -E 'simple_strtoull?\(' | wc -l
745

> 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