Am 26.10.2014 um 01:16 schrieb Andy Lutomirski: > On Sat, Oct 25, 2014 at 3:26 PM, Richard Weinberger > <richard.weinber...@gmail.com> wrote: >> On Sat, Oct 25, 2014 at 2:22 AM, Andy Lutomirski <l...@amacapital.net> wrote: >>> Is there any good reason not to use vmalloc for x86_64 stacks? >>> >>> The tricky bits I've thought of are: >>> >>> - On any context switch, we probably need to probe the new stack >>> before switching to it. That way, if it's going to fault due to an >>> out-of-sync pgd, we still have a stack available to handle the fault. >>> >>> - Any time we change cr3, we may need to check that the pgd >>> corresponding to rsp is there. If now, we need to sync it over. >>> >>> - For simplicity, we probably want all stack ptes to be present all >>> the time. This is fine; vmalloc already works that way. >>> >>> - If we overrun the stack, we double-fault. This should be easy to >>> detect: any double-fault where rsp is less than 20 bytes from the >>> bottom of the stack is a failure to deliver a non-IST exception due to >>> a stack overflow. The question is: what do we do if this happens? >>> We could just panic (guaranteed to work). We could also try to >>> recover by killing the offending task, but that might be a bit >>> challenging, since we're in IST context. We could do something truly >>> awful: increment RSP by a few hundred bytes, point RIP at do_exit, and >>> return from the double fault. >>> >>> Thoughts? This shouldn't be all that much code. >> >> FWIW, grsecurity has this already. >> Maybe we can reuse their GRKERNSEC_KSTACKOVERFLOW feature. >> It allocates the kernel stack using vmalloc() and installs guard pages. >> > > On brief inspection, grsecurity isn't actually vmallocing the stack. > It seems to be allocating it the normal way and then vmapping it. > That allows it to modify sg_set_buf to work on stack addresses (sigh).
Oh, you're right. They have changed it. (But not the Kconfig help of course) Last time I looked they did a vmalloc(). I'm not sure which version of the patch was but I think it was code like that one: http://www.grsecurity.net/~spender/kstackovf32.diff Thanks, //richard -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
