linux-kernel - [PATCH V8 0/2] printk: hash addresses printed with %p

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Message-Id: <1508986436-31966-1-git-send-email-me@tobin.cc>
Date:   Thu, 26 Oct 2017 13:53:54 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     kernel-hardening@...ts.openwall.com
Cc:     "Tobin C. Harding" <me@...in.cc>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Theodore Ts'o <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tycho Andersen <tycho@...ker.com>,
        "Roberts, William C" <william.c.roberts@...el.com>,
        Tejun Heo <tj@...nel.org>,
        Jordan Glover <Golden_Miller83@...tonmail.ch>,
        Greg KH <gregkh@...uxfoundation.org>,
        Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>,
        Ian Campbell <ijc@...lion.org.uk>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <wilal.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Chris Fries <cfries@...gle.com>,
        Dave Weinstein <olorin@...gle.com>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>, linux-kernel@...r.kernel.org
Subject: [PATCH V8 0/2] printk: hash addresses printed with %p

Currently there are many places in the kernel where addresses are being
printed using an unadorned %p. Kernel pointers should be printed using
%pK allowing some control via the kptr_restrict sysctl. Exposing
addresses gives attackers sensitive information about the kernel layout
in memory.

We can reduce the attack surface by hashing all addresses printed with
%p. This will of course break some users, forcing code printing needed
addresses to be updated.

With this version we include hashing of malformed specifiers also.
Malformed specifiers include incomplete (e.g %pi) and also non-existent
specifiers. checkpatch should warn for non-existent specifiers but
AFAICT won't warn for incomplete specifiers.

Here is the behaviour that this set implements.

For kpt_restrict==0

Randomness not ready:
  printed with %p: 		(pointer)          # NOTE: with padding
Valid pointer:
  printed with %pK: 		deadbeefdeadbeef
  printed with %p: 		0xdeadbeef
  malformed specifier (eg %i):  0xdeadbeef
NULL pointer:
  printed with %pK: 		0000000000000000
  printed with %p: 		(null)               # NOTE: no padding
  malformed specifier (eg %i):  (null)

For kpt_restrict==2

Valid pointer:
  printed with %pK: 		0000000000000000

All other output as for kptr_restrict==0

V8:
 - Add second patch cleaning up null pointer printing in pointer()
 - Move %pK handling to separate function, further cleaning up pointer()
 - Move ptr_to_id() call outside of switch statement making hashing
   the default behaviour (including malformed specifiers).
 - Remove use of static_key, replace with simple boolean.

V7:
 - Use tabs instead of spaces (ouch!).

V6:
 - Use __early_initcall() to fill the SipHash key.
 - Use static keys to guard hashing before the key is available.

V5:
 - Remove spin lock.
 - Add Jason A. Donenfeld to CC list by request.
 - Add Theodore Ts'o to CC list due to comment on previous version.

V4:
 - Remove changes to siphash.{ch}
 - Do word size check, and return value cast, directly in ptr_to_id().
 - Use add_ready_random_callback() to guard call to get_random_bytes()

V3:
 - Use atomic_xchg() to guard setting [random] key.
 - Remove erroneous white space change.

V2:
 - Use SipHash to do the hashing.

The discussion related to this patch has been fragmented. There are
three threads associated with this patch. Email threads by subject:

[PATCH] printk: hash addresses printed with %p
[PATCH 0/3] add %pX specifier
[kernel-hardening] [RFC V2 0/6] add more kernel pointer filter options

Tobin C. Harding (2):
  printk: remove tabular output for NULL pointer
  printk: hash addresses printed with %p

 lib/vsprintf.c | 166 +++++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 108 insertions(+), 58 deletions(-)

-- 
2.7.4