linux-kernel - Re: [GIT PULL] hash addresses printed with %p

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Date:   Wed, 29 Nov 2017 13:31:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Tobin C. Harding" <me@...in.cc>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] hash addresses printed with %p

On Wed, Nov 29, 2017 at 11:39 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Nov 29, 2017 at 11:22 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> What I didn't realize until after pulling this and testing, is that it
>> completely breaks '%pK'.
>>
>> We've marked various sensitive pointers with %pK, but that is now
>> _less_ secure than %p is, since it doesn't do the hashing because of
>> how you refactored the %pK code out of 'pointer()' into its own
>> function.
>>
>> So now %pK ends up using the plain "number()" function. Reading
>> through the series I hadn't noticed that the refactoring ended up
>> messing with that.
>>
>> I'll fix it up somehow.
>
> I ended up just doing this:
>
>             case 'K':
>     +               if (!kptr_restrict)
>     +                       break;
>                     return restricted_pointer(buf, end, ptr, spec);
>
> which basically says that "if kptr_restrict isn't set, %pK is the same as %p".
>
> Now, I feel that we should probably get rid of 'restricted_pointer()'
> entirely, since now the regular '%p' is arguably safer than '%pK' is,
> but I also didn't want to mess with the case that I have never used
> and that most distros don't seem to set.

kptr_restrict=0 is now much safer, yes (i.e. %pK becomes hashed %p).
Strictly speaking, kptr_restrict > 0 is "better" than hashed %p, in
that it only says "0".

> Alternatively, we might make the 'K' behavior of clearing the pointer
> be in addition to the other flags, so that you could do '%pxK' and get
> the old %pK behavior. But since I am not a huge fan of %pK to begin
> with, I can't find it in myself to care too much.
>
> So I'll leave that for Kees & co to decide on. Comments?

I'm not hugely attached to %pK, but retaining its ability to zero out
the result would be nice.

(If we in the future we have a toggle for %p that switches it from
hashing to zeroing, then we could entirely drop %pK.)

-Kees

-- 
Kees Cook
Pixel Security