linux-kernel - Re: [PATCH v1 2/2] sysctl: handle overflow for file-max

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Date:   Mon, 15 Oct 2018 09:11:51 -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 2/2] sysctl: handle overflow for file-max

On Mon, Oct 15, 2018 at 3:55 AM, Christian Brauner <christian@...uner.io> wrote:
> Currently, when writing
>
> echo 18446744073709551616 > /proc/sys/fs/file-max
>
> /proc/sys/fs/file-max will overflow and be set to 0. That quickly
> crashes the system.
> This commit explicitly caps the value for file-max to ULONG_MAX.
>
> Note, this isn't technically necessary since proc_get_long() will already
> return ULONG_MAX. However, two reason why we still should do this:
> 1. it makes it explicit what the upper bound of file-max is instead of
>    making readers of the code infer it from proc_get_long() themselves
> 2. other tunebles than file-max may want to set a lower max value than
>    ULONG_MAX and we need to enable __do_proc_doulongvec_minmax() to handle
>    such cases too
>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Christian Brauner <christian@...uner.io>
> ---
> v0->v1:
> - if max value is < than ULONG_MAX use max as upper bound
> - (Dominik) remove double "the" from commit message
> ---
>  kernel/sysctl.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 97551eb42946..226d4eaf4b0e 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -127,6 +127,7 @@ static int __maybe_unused one = 1;
>  static int __maybe_unused two = 2;
>  static int __maybe_unused four = 4;
>  static unsigned long one_ul = 1;
> +static unsigned long ulong_max = ULONG_MAX;
>  static int one_hundred = 100;
>  static int one_thousand = 1000;
>  #ifdef CONFIG_PRINTK
> @@ -1696,6 +1697,7 @@ static struct ctl_table fs_table[] = {
>                 .maxlen         = sizeof(files_stat.max_files),
>                 .mode           = 0644,
>                 .proc_handler   = proc_doulongvec_minmax,
> +               .extra2         = &ulong_max,

Don't we want this capped lower? The percpu comparisons, for example,
are all signed long. And there is at least this test, which could
overflow:

        if (atomic_long_read(&unix_nr_socks) > 2 * get_max_files())
                goto out;

Seems like max-files should be  SLONG_MAX / 2 or something instead?

>         },
>         {
>                 .procname       = "nr_open",
> @@ -2795,6 +2797,8 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
>                                 break;
>                         if (neg)
>                                 continue;
> +                       if (max && val > *max)
> +                               val = *max;
>                         val = convmul * val / convdiv;
>                         if ((min && val < *min) || (max && val > *max))
>                                 continue;
> --
> 2.17.1
>

-Kees

-- 
Kees Cook
Pixel Security