Re: mlock(2), unprivileged users, and RLIMIT_MEMLOCK

On Sat, 2008-04-12 at 15:55 -0400, David Schultz wrote:
> On Sat, Apr 12, 2008, Joe Marcus Clarke wrote:
> > On Sat, 2008-04-12 at 15:09 -0400, Coleman Kane wrote:
> > > Hello,
> > > 
> > > Recently we've been having a discussion on the GNOME list about fixing
> > > the seahorse breakage introduced with the latest GNOME 2.22, rooted in
> > > the fact that FreeBSD's mlock(2) implementation is only usable if you
> > > have superuser privileges. Due to bugs in seahorse, the lack of mlock(2)
> > > causes many seahorse applications to die. I've posted a suggested patch
> > > to 
> [...]
> > > As a third idea, we could leave the per-process limit (to abide by
> > > historical documentation), but also add a sysctl that enforces a
> > > system-wide "max mlock pages" which can be tested by the mlock(2)
> > > syscall, refusing to mlock(2) more memory if the limit is hit.
> > 
> > I think this already exists in -CURRENT: vm.max_wired ("System-wide
> > limit to wired page count").  This is tested by mlock(2) in addition to
> > RLIMIT_MEMLOCK.
> 
> First of all, many other operating systems such as Solaris also
> restrict mlock(2) to the superuser, so this is a bug in seahorse.
> 
> That said, it seems like allowing ordinary users to mlock(2) small
> amounts of memory (e.g., vm_page_max_wired / 4 across all
> non-superuser processes by default) would fix your problem and be
> easy to implement.  Of course, per-user or per-process limits
> would be more flexible, but how many people really have lots of
> users who are trying to abuse the system?
> 

I did some math and came up with the following per-user limit on my
system. Using the default install, my maxproc is set to 5547:
   max_secure_mem = max_proc * memorylocked = 5547 * 16384 = 90882048 =
about 87MB

So, under my operating conditions (2GB System RAM), a user's maximum DoS
attempt would be capped at 87MB... which doesn't seem as serious
anymore. This is using the 16K memorylocked value that gnome-keyring &
friends seem to work fine with.

--
Coleman Kane