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SubjectRe: [PATCH v1 2/2] sysctl: handle overflow for file-max
On Mon, Oct 15, 2018 at 3:55 AM, Christian Brauner <christian@brauner.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@chromium.org>
> Signed-off-by: Christian Brauner <christian@brauner.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

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