Date: Tue, 19 Dec 2017 21:48:49 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>, "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:

> I suspect that an "offset and size within the kernel object" value
> might make sense.  But what does the _pointer_ tell you?

Well, for example seeing a 0xfffffffffffffff4 where a pointer to object
must have been is a pretty strong hint to start looking for a way for
that ERR_PTR(-ENOMEM) having ended up there...  Something like
0x6e69622f7273752f is almost certainly a misplaced "/usr/bin", i.e. a
pathname overwriting whatever it ends up in, etc.  And yes, I have run
into both of those in real life.

Debugging the situation when crap value has ended up in place of a
pointer is certainly a case where you do want to see what exactly has
ended up in there...