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

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Date:   Tue, 19 Dec 2017 14:09:45 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Al Viro <viro@...IV.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "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 12/19/2017 01:48 PM, Al Viro wrote:
> 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

possibly poison values also?

> 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...


-- 
~Randy