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

[Christian Brauner]

On Tue, Oct 16, 2018 at 03:16:21PM +0200, Christian Brauner wrote:
> On Mon, Oct 15, 2018 at 02:20:15PM -0700, Kees Cook wrote:
> > On Mon, Oct 15, 2018 at 9:28 AM, Christian Brauner <christian@xxxxxxxxxx> wrote:
> > > On Mon, Oct 15, 2018 at 09:11:51AM -0700, Kees Cook wrote:
> > >> On Mon, Oct 15, 2018 at 3:55 AM, Christian Brauner <christian@xxxxxxxxxx> 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@xxxxxxxxxxxx>
> > >> > Signed-off-by: Christian Brauner <christian@xxxxxxxxxx>
> > >> > ---
> > >> > 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;
> > >
> > > Does that check even make sense?
> > > Commit 518de9b39e854542de59bfb8b9f61c8f7ecf808b made get_max_files()
> > > return a long to bump the number of allowed files to more than 2^31.
> > >
> > > But assuming a platform where an unsigned long is 64bit which is what
> > > get_max_files() returns and atomic_long_read() is 64bit too this is
> > > guaranteed to overflow, no?  So I'm not clear what this is trying to do.
> > > Seems this should simply be:
> > >
> > > if (atomic_long_read(&unix_nr_socks) > get_max_files())
> > >         goto out;
> > >
> > > or am I missing a crucial point?
> > >
> > >>
> > >> Seems like max-files should be  SLONG_MAX / 2 or something instead?
> > >
> > > Hm. Isn't that a bit low? Iiuc, this would mean cutting the maximum
> > > number of open files in half? If at all shouldn't it be LONG_MAX?
> > 
> > LONG_MAX would align us with the values in the percpu stuff. I'm
> > really not sure what's happening in the sock check, but it's prone to
> > an unsigned multiplication overflow, if I'm reading it right. Probably
> > should just be a separate bug fix:
> > 
> > -         if (atomic_long_read(&unix_nr_socks) > 2 * get_max_files())
> > +         if (atomic_long_read(&unix_nr_socks) / 2 > get_max_files())
> 
> Yeah, as I said before this is just waiting for an overflow and it seems
> this bug has existed since the switch to git. The intention apparently
> was indeed to allow the number of sockets to be double the open file
> limit. However, as it is right now this just overflows. I'll send a
> separate patch for this but I'd sugest to do something like:
> 

Forget that. I'm obviously overcomplicating things that are supposed to
be simple.

> 
> > 
> > -Kees
> > 
> > >
> > >>
> > >> >         },
> > >> >         {
> > >> >                 .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
> > 
> > 
> > 
> > -- 
> > Kees Cook
> > Pixel Security