On 24 Apr 2017 at 10:32, Peter Zijlstra wrote: > On Fri, Apr 21, 2017 at 03:09:39PM -0700, Kees Cook wrote: > > This patch ports the x86-specific atomic overflow handling from PaX's > > PAX_REFCOUNT to the upstream refcount_t API. This is an updated version > > from PaX that eliminates the saturation race condition by resetting the > > atomic counter back to the INT_MAX saturation value on both overflow and > > underflow. To win a race, a system would have to have INT_MAX threads > > simultaneously overflow before the saturation handler runs. note that the above is wrong (and even contradicting itself and the code). > And is this impossible? Highly unlikely I'll grant you, but absolutely > impossible? here's my analysis from a while ago: http://www.openwall.com/lists/kernel-hardening/2017/01/05/19 > Also, you forgot nr_cpus in your bound. Afaict the worst case here is > O(nr_tasks + 3*nr_cpus). what does nr_cpus have to do with winning the race? > Because PaX does it, is not a correctness argument. And this really > wants one. heh, do you want to tell me about how checking for a 0 refcount prevents exploiting a bug?
