On Mon, Mar 10, 2014 at 8:11 AM, Linus Torvalds <[email protected]> wrote: > > On Mar 10, 2014 8:01 AM, "H. Peter Anvin" <[email protected]> wrote: >> >> I have mentioned in the past wanting to move the fixmap to the low part >> of the kernel space, because the top isn't really fixed... > > How about the high part of the user address space, just above the stack? > Leave a unmapped page in between, or something. The stack is already > randomized, isn't it?
For the !compat_vdso case, I don't like it -- this will put the vdso (which is executable) at a constant offset from the stack, which will make it much easier to use the vdso to defeat ASLR. For the compat_vdso case, this only works if the address is *not* random, unless we're going to start giving each process its very own relocated vdso. > > That would actually be preferable in a few ways, notably not having to mark > page directories user accessible in the kennel space area. Is that where the rabid pte dogs live? We can already avoid making fixmap pages user-accessible in the !compat_vdso case for 32-bit tasks -- the vdso lives in a couple of more-or-less ordinary vmas. For 64-bit, this is an entirely different story. The vsyscall page is stuck in the fixmap forever, although I want to add a way for userspace to opt out. The vvar page, hpet, etc could move into vmas, though. I kind of want to do that anyway to allow processes to turn off the ability to read the clock. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
