On 09/24/2015 02:49 AM, Ingo Molnar wrote: > * Dave Hansen <dave@xxxxxxxx> wrote: >>> 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? >> >> No, that's not possible with the current setup. > > Ok, then another question, have you considered the following usecase: > > AFAICS pkeys only affect data loads and stores. Instruction fetches are notably > absent from the documentation. Can you clarify that instructions can be fetched > and executed from PTE_READ but pkeys-all-access-disabled pags? That is my understanding. I don't have a test for it, but I'll go make one. > If yes then this could be a significant security feature / usecase for pkeys: > executable sections of shared libraries and binaries could be mapped with pkey > access disabled. If I read the Intel documentation correctly then that should be > possible. Agreed. I've even heard from some researchers who are interested in this: https://www.infsec.cs.uni-saarland.de/wp-content/uploads/sites/2/2014/10/nuernberger2014ccs_disclosure.pdf > I.e. AFAICS pkeys could be used to create true '--x' permissions for executable > (user-space) pages. Just remember that all of the protections are dependent on the contents of PKRU. If an attacker controls the Access-Disable bit in PKRU for the executable-only region, you're sunk. But, that either requires being able to construct and execute arbitrary code *or* call existing code that sets PKRU to the desired values. Which, I guess, gets harder to do if all of the the wrpkru's are *in* the execute-only area. -- 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>