Messages in this thread | | | Date | Wed, 20 Dec 2017 07:33:57 +1100 | From | "Tobin C. Harding" <> | Subject | Re: BUG: bad usercopy in memdup_user |
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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@chromium.org> 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.
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