On Fri, Feb 12, 2016 at 1:02 PM, Dave Hansen <dave@xxxxxxxx> wrote: > > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> > > 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?) -Kees -- Kees Cook Chrome OS & Brillo Security -- 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>