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

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Date:   Tue, 19 Dec 2017 14:24:08 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.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,
        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 01:36 PM, 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.
> 
> So no, it won't work.
> 
> When we find something like this, we should either remove it, fix the
> permissions, or switch to %px.
> 
> In this case, there's obviously no permission issue: it's an error
> report. So it's either "remove it, or switch to %px".
> 
> I'm personally not clear on whether the pointer really makes any sense
> at all. But if it does, it should just be changed to %px, since it's a
> bug report.
> 
> But honestly, what do people expect that the pointer value will
> actually tell you if it is unhashed?
> 
> I suspect that an "offset and size within the kernel object" value
> might make sense.  But what does the _pointer_ tell you?
> 
> I've noticed this with pretty much every report. People get upset
> about the hashing, but don't seem to actually be able to ever tell
> what the f*ck they would use the non-hashed pointer value for.
> 
> I've asked for this before: whenever somebody complains about the
> hashing, you had better tell exactly what the unhashed value would
> have given you, and how it would have helped debug the problem.
> 
> Because if you can't tell that, then dammit, then we should just
> _remove_ the stupid %p.
> 
> Instead, people ask for "can I get everything unhashed" even when they
> can't give a reason for it.
> 
>              Linus
> 

It's most useful in the "I really have no idea what's going on" case.
Sometimes just narrowing down if a pointer was kernel or vmalloc or
kmap or whatever is the only starting point. I agree that
size plus offset from object would be helpful.

Thanks,
Laura