On Tue, Jun 21, 2016 at 12:47 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > On Tuesday, June 21, 2016 10:16:21 AM CEST Kees Cook wrote: >> On Tue, Jun 21, 2016 at 2:24 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote: >> > On Monday, June 20, 2016 4:43:30 PM CEST Andy Lutomirski wrote: >> >> >> >> On my laptop, this adds about 1.5µs of overhead to task creation, >> >> which seems to be mainly caused by vmalloc inefficiently allocating >> >> individual pages even when a higher-order page is available on the >> >> freelist. >> > >> > Would it help to have a fixed virtual address for the stack instead >> > and map the current stack to that during a task switch, similar to >> > how we handle fixmap pages? >> > >> > That would of course trade the allocation overhead for a task switch >> > overhead, which may be better or worse. It would also give "current" >> > a constant address, which may give a small performance advantage >> > but may also introduce a new attack vector unless we randomize it >> > again. >> >> Right: we don't want a fixed address. That makes attacks WAY easier. > > Do we care about making the address more random then? When I look > at /proc/vmallocinfo, I see that allocations are all using > consecutive addresses, so if you can figure out the virtual > address of the stack for one process that would give you a good > chance of guessing the address for the next pid. Quite possibly. We should seriously consider at least randomizing the *start* of the vmalloc area, at least on 64-bit architectures. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
