Re: [PATCH v2 1/4] drivers: hwspinlock: add generic framework

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Fri, Nov 26, 2010 at 12:16:39PM +0200, Ohad Ben-Cohen wrote:
> On Fri, Nov 26, 2010 at 11:18 AM, Russell King - ARM Linux
> <linux@arm.linux.org.uk> wrote:
> > On Fri, Nov 26, 2010 at 10:53:10AM +0200, Ohad Ben-Cohen wrote:
> >> >> +int __hwspin_trylock(struct hwspinlock *hwlock, int mode, unsigned long *flags)
> >> >> +{
> >> >> +     int ret;
> >> >> +
> >> >> +     if (unlikely(!hwlock)) {
> >> >> +             pr_err("invalid hwlock\n");
> >> >
> >> > These kind of errors can get very spammy for buggy drivers.
> >>
> >> Yeah, but that's the purpose - I want to catch such egregious drivers
> >> who try to crash the kernel.
> >
> > That can be better - because you get a backtrace, and it causes people
> > to report the problem rather than just ignore it.  It may also prevent
> > the driver author releasing his code (as it won't work on their
> > initial testing.)
> >
> ...
> >
> > If it's "extremely buggy behaviour" then the drivers deserve to crash.
> > Such stuff should cause them not to get out the door.  A simple printk
> > with an error return can just be ignored.
> 
> I like this approach too, but recently we had a few privilege
> escalation exploits which involved NULL dereference kernel bugs
> (process context mapped address 0 despite a positive mmap_min_addr).

That's not a concern on ARM.  The prevention of having a user page mapped
at virtual address 0 does not rely on mmap_min_addr - in fact, we can't
use this as it's tuneable to enforce this requirement.

It's highly illegal on ARM - as ARM CPUs without vector remap place the
hardware vectors at virtual address 0, and as such allowing the user to
map a page there will take the system down.  So we have this code in the
mmap path:

#define arch_mmap_check(addr, len, flags) \
        (((flags) & MAP_FIXED && (addr) < FIRST_USER_ADDRESS) ? -EINVAL : 0)

which prevents any attempt what so ever, irrespective of the mmap_min_addr
setting, to create a userspace induced mapping at address 0.