Adding Linus On Tue, Dec 19, 2017 at 03:12:05PM +0100, Dmitry Vyukov wrote: > On Tue, Dec 19, 2017 at 3:08 PM, Tetsuo Handa > <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote: > > Dmitry Vyukov wrote: > >> On Tue, Dec 19, 2017 at 2:22 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > >> >> > >> This BUG is reporting > >> >> > >> > >> >> > >> [ 26.089789] usercopy: kernel memory overwrite attempt detected to 0000000022a5b430 (kmalloc-1024) (1024 bytes) > >> >> > >> > >> >> > >> line. But isn't 0000000022a5b430 strange for kmalloc(1024, GFP_KERNEL)ed kernel address? > >> >> > > > >> >> > > The address is hashed (see the %p threads for 4.15). > >> >> > > >> >> > > >> >> > +Tobin, is there a way to disable hashing entirely? The only > >> >> > designation of syzbot is providing crash reports to kernel developers > >> >> > with as much info as possible. It's fine for it to leak whatever. > >> >> > >> >> We have new specifier %px to print addresses in hex if leaking info is > >> >> not a worry. > >> > > >> > Could we have a way to know that the printed address is hashed and not just > >> > a pointer getting completely scrogged? Perhaps prefix it with ... a hash! > >> > So this line would look like: > >> > > >> > [ 26.089789] usercopy: kernel memory overwrite attempt detected to #0000000022a5b430 (kmalloc-1024) (1024 bytes) > >> > > >> > Or does that miss the point of hashing the address, so the attacker > >> > thinks its a real address? > >> > >> If we do something with this, I would suggest that we just disable > >> hashing. Any of the concerns that lead to hashed pointers are not > >> applicable in this context, moreover they are harmful, cause confusion > >> and make it harder to debug these bugs. That perfectly can be an > >> opt-in CONFIG_DEBUG_INSECURE_BLA_BLA_BLA. > >> > > Why not a kernel command line option? Hashing by default. > > > Would work for continuous testing systems too. > I just thought that since it has security implications, a config would > be more reliable. Say if a particular distribution builds kernel > without this config, then there is no way to enable it on the fly, > intentionally or not. I wasn't the architect behind the hashing, I've cc'd Linus in the event he wants to correct me. I believe that some of the benefit of hashing was to shake things up and force people to think about this issue. If we implement a method of disabling hashing (command-line parameter or CONFIG_) at this stage then we risk loosing this benefit since one has to assume that people will just take the easy option and disable it. Though perhaps after things settle a bit we could implement this without the risk? thanks, Tobin. -- 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>
