lkml.org 
[lkml]   [2017]   [Nov]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [GIT PULL] hash addresses printed with %p
On Wed, Nov 29, 2017 at 11:39 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Wed, Nov 29, 2017 at 11:22 AM, Linus Torvalds
> <torvalds@linux-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

\
 
 \ /
  Last update: 2017-11-29 22:31    [W:0.116 / U:0.740 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site