On Thu, Sep 24, 2015 at 12:10 PM, Dave Hansen <dave@xxxxxxxx> wrote: > 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. > This may mean that we want to have a way for binaries to indicate that they want their --x segments to be loaded with a particular protection key. The right way to do that might be using an ELF note, and I also want to use ELF notes to allow turning off vsyscalls, so maybe it's time to write an ELF note parser in the kernel. --Andy -- 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>