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    SubjectRe: [PATCH v2 1/4] drivers: hwspinlock: add generic framework
    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.
    --
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