Re: BUG: bad usercopy in memdup_user

[Tetsuo Handa]

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.