ntpd doesn't like ASLR on stable/12 post-r350672

[Konstantin Belousov]

On Tue, Sep 10, 2019 at 10:50:33AM -0600, Ian Lepore wrote:
> On Sun, 2019-08-25 at 15:03 +0300, Konstantin Belousov wrote:
> > On Sun, Aug 25, 2019 at 12:40:22AM +0200, Trond Endrestøl wrote:
> > > On Sun, 25 Aug 2019 01:28+0300, Konstantin Belousov wrote:
> > > 
> > > > On Sun, Aug 25, 2019 at 12:19:43AM +0200, Trond Endrestøl wrote:
> > > > > On Sat, 24 Aug 2019 23:41+0300, Konstantin Belousov wrote:
> > > > > > > I tried changing command="/usr/sbin/${name}" to 
> > > > > > > command="/usr/bin/proccontrol -m aslr -s disable /usr/sbin/${name}" in 
> > > > > > > /etc/rc.d/ntpd, but that didn't go well.
> > > > > > 
> > > > > > If you set kern.elf64.aslr.stack_gap to zero, does it help ?
> > > > > 
> > > > > That helped. Thank you again.
> > > > 
> > > > Can you verify is ntpd sets new rlimit(RLIMIT_STACK) for the main thread,
> > > > and if yes, what this new limit is ?
> > > 
> > > (gdb)
> > > 5265                            if (-1 == setrlimit(RLIMIT_STACK, &rl)) {
> > > (gdb) print rl
> > > $1 = {rlim_cur = 204800, rlim_max = 536870912}
> > 
> > So they set the stack limit to 200K, am I right ?  I suspect they do
> > that because ntpd wires entire process address space, so 512M blows off
> > all limits on wiring.
> > 
> > I do not have a good idea how to make this behaviour compatible with
> > the gap.  Might be we can change the gap sizing parameter to KBs instead
> > of percentage, and set the defaults in 64KB range.
> > 
> > > 
> > > > aslr.stack_gap is the percentage for the gap on that stack, and since
> > > > default size of the main stack limit is quite large 512M, even 3%
> > > > (default gap upper limit) are whole 15M. If the new limit is less than
> > > > 15M, there is a likely probability that only the gap is left after the
> > > > rlimit(2) call, leaving no space for the program frames.
> > > > 
> > > > At least this looks like a nice theory.
> 
> So is the problem here that before ntpd is running and has the chance
> to call setrlimit(), aslr has already created a large stack gap?  If
> so, it seems to me that aslr and setrlimit(RLIMIT_STACK, ...) are never
> going to work right together.  Even if the default stack gap were much
> smaller, code using RLIMIT_STACK is going to end up with a stack
> smaller than it asked for because the gap it has no way of knowing
> about uses up some part (or all of) the limited space.
Sort of, yes.  There is a UI problem with the control for the gap,
and I am not sure how to fix it.

> 
> If the default gap were 64K or less, things would be much more likely
> to work accidentally (and we might never have noticed this situtation),
> but they still wouldn't be working correctly.  Is it possible for the
> code on the kernel side to add the requested limit to the gap size to
> generate a result that gives the caller the usable stack size they
> asked for?
I do not see a way to account for the gap in RLIMIT_STACK adjustment.
It should be handled in cooperation with the user code.

When program does adjust RLIMIT_STACK in so radical way, e.g. setting
it to 64k, it must know a lot about execution environment.