On Feb 17, 2016 1:27 PM, "Kees Cook" <[email protected]> wrote: > > On Fri, Feb 12, 2016 at 1:02 PM, Dave Hansen <[email protected]> wrote: > > > > From: Dave Hansen <[email protected]> > > > > Protection keys provide new page-based protection in hardware. > > But, they have an interesting attribute: they only affect data > > accesses and never affect instruction fetches. That means that > > if we set up some memory which is set as "access-disabled" via > > protection keys, we can still execute from it. > > > > This patch uses protection keys to set up mappings to do just that. > > If a user calls: > > > > mmap(..., PROT_EXEC); > > or > > mprotect(ptr, sz, PROT_EXEC); > > > > (note PROT_EXEC-only without PROT_READ/WRITE), the kernel will > > notice this, and set a special protection key on the memory. It > > also sets the appropriate bits in the Protection Keys User Rights > > (PKRU) register so that the memory becomes unreadable and > > unwritable. > > > > I haven't found any userspace that does this today. With this > > facility in place, we expect userspace to move to use it > > eventually. Userspace _could_ start doing this today. Any > > PROT_EXEC calls get converted to PROT_READ inside the kernel, and > > would transparently be upgraded to "true" PROT_EXEC with this > > code. IOW, userspace never has to do any PROT_EXEC runtime > > detection. > > Random thought while skimming email: > > Is there a way to detect this feature's availability without userspace > having to set up a segv handler and attempting to read a > PROT_EXEC-only region? (i.e. cpu flag for protection keys, or a way to > check the protection to see if PROT_READ got added automatically, > etc?) >
We could add an HWCAP. --Andy
