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Subject: Re: BUG: bad usercopy in memdup_user
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From: Randy Dunlap <rdunlap@infradead.org>
Message-ID: <f03d0baa-6a4a-0b67-056b-f2d0acb7befe@infradead.org>
Date: Tue, 19 Dec 2017 14:09:45 -0800
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To: Al Viro <viro@ZenIV.linux.org.uk>, 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>

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

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