Date: Tue, 19 Dec 2017 14:16:25 -0800
From: Matthew Wilcox <willy@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Tobin C. Harding" <me@tobin.cc>, Dmitry Vyukov <dvyukov@google.com>,
        Kees Cook <keescook@chromium.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Linux-MM <linux-mm@kvack.org>,
        syzbot 
        <bot+719398b443fd30155f92f2a888e749026c62b427@syzkaller.appspotmail.com>,
        David Windsor <dave@nullcore.net>, keun-o.park@darkmatter.ae,
        Laura Abbott <labbott@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>, Ingo Molnar <mingo@kernel.org>,
        syzkaller-bugs@googlegroups.com, Will Deacon <will.deacon@arm.com>
Subject: Re: BUG: bad usercopy in memdup_user
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On Tue, Dec 19, 2017 at 01:36:46PM -0800, Linus Torvalds wrote:
> On Tue, Dec 19, 2017 at 5:22 AM, Matthew Wilcox <willy@infradead.org> wrote:
> >
> > 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:
> 
> The problem with that is that it will break tools that parse things.

Yeah, but the problem is that until people know to expect hashes, it
breaks people.  I spent most of a day last week puzzling over a value
coming from a VM_BUG_ON that was explicitly tested for and couldn't
happen.

> When we find something like this, we should either remove it, fix the
> permissions, or switch to %px.

Right; I sent a patch to fix VM_BUG_ON earlier today after reading
this thread.

> But honestly, what do people expect that the pointer value will
> actually tell you if it is unhashed?

It would have been meaningful to me.  For a start, I would have seen
that the bottom two bits were clear, so this was actually a pointer and
not something masquerading as a pointer.