linux-kernel - Re: BUG: bad usercopy in memdup

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Message-ID: <20171219221625.GB22696@bombadil.infradead.org>
Date:   Tue, 19 Dec 2017 14:16:25 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Tobin C. Harding" <me@...in.cc>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux-MM <linux-mm@...ck.org>,
        syzbot 
        <bot+719398b443fd30155f92f2a888e749026c62b427@...kaller.appspotmail.com>,
        David Windsor <dave@...lcore.net>, keun-o.park@...kmatter.ae,
        Laura Abbott <labbott@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...nel.org>,
        syzkaller-bugs@...glegroups.com, Will Deacon <will.deacon@....com>
Subject: Re: BUG: bad usercopy in memdup_user

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@...radead.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.