* Dave Hansen <dave@xxxxxxxx> wrote: > +Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature > +which will be found on future Intel CPUs. > + > +Memory Protection Keys provides a mechanism for enforcing page-based > +protections, but without requiring modification of the page tables > +when an application changes protection domains. It works by > +dedicating 4 previously ignored bits in each page table entry to a > +"protection key", giving 16 possible keys. Wondering how user-space is supposed to discover the number of protection keys, is that CPUID leaf based, or hardcoded on the CPU feature bit? > +There is also a new user-accessible register (PKRU) with two separate > +bits (Access Disable and Write Disable) for each key. Being a CPU > +register, PKRU is inherently thread-local, potentially giving each > +thread a different set of protections from every other thread. > + > +There are two new instructions (RDPKRU/WRPKRU) for reading and writing > +to the new register. The feature is only available in 64-bit mode, > +even though there is theoretically space in the PAE PTEs. These > +permissions are enforced on data access only and have no effect on > +instruction fetches. Another question, related to enumeration as well: I'm wondering whether there's any way for the kernel to allocate a bit or two for its own purposes - such as protecting crypto keys? Or is the facility fundamentally intended for user-space use only? Just a quick example: let's assume the kernel has an information leak hole, a way to read any kernel address and pass that to the kernel attacker. Let's also assume that the main crypto-keys of the kernel are protected by protection-keys. The code exposing the information leak will very likely have protection-key protected areas masked out, so the scope of the information leak is mitigated to a certain degree, the crypto keys are not readable. Similarly, the pmem (persistent memory) driver could employ protection keys to keep terabytes of data 'masked out' most of the time - protecting data from kernel space memory corruption bugs. Thanks, Ingo -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>