Re: BUG: bad usercopy in memdup_user

[Tobin C. Harding]

On Tue, Dec 19, 2017 at 05:22:46AM -0800, Matthew Wilcox wrote:
> On Tue, Dec 19, 2017 at 07:37:46PM +1100, Tobin C. Harding wrote:
> > On Tue, Dec 19, 2017 at 09:12:58AM +0100, Dmitry Vyukov wrote:
> > > On Tue, Dec 19, 2017 at 1:57 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > > > On Mon, Dec 18, 2017 at 6:22 AM, Tetsuo Handa
> > > >> 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)

This poses the risk of breaking userland tools that parse the
address. The zeroing of the first 32 bits was a design compromise to
keep the address format while making _kind of_ explicit that some funny
business was going on.

> Or does that miss the point of hashing the address, so the attacker
> thinks its a real address?

No subterfuge intended.

Bonus points Wily, I had to go to 'The New Hackers Dictionary' to look
up 'scrogged' :)

thanks,
Tobin.