[kernel-hardening] Re: [PATCH v2] printk: hash addresses printed with %p

["Tobin C. Harding" <me@tobin.cc>]

On Tue, Oct 17, 2017 at 05:27:15PM +0000, Roberts, William C wrote:
> 
> 
> > -----Original Message-----
> > From: Tobin C. Harding [mailto:me@tobin.cc]
> > Sent: Monday, October 16, 2017 9:53 PM
> > To: kernel-hardening@lists.openwall.com
> > Cc: Tobin C. Harding <me@tobin.cc>; Linus Torvalds <torvalds@linux-
> > foundation.org>; Kees Cook <keescook@chromium.org>; Paolo Bonzini
> > <pbonzini@redhat.com>; Tycho Andersen <tycho@docker.com>; Roberts,
> > William C <william.c.roberts@intel.com>; Tejun Heo <tj@kernel.org>; Jordan
> > Glover <Golden_Miller83@protonmail.ch>; Greg KH
> > <gregkh@linuxfoundation.org>; Petr Mladek <pmladek@suse.com>; Joe
> > Perches <joe@perches.com>; Ian Campbell <ijc@hellion.org.uk>; Sergey
> > Senozhatsky <sergey.senozhatsky@gmail.com>; Catalin Marinas
> > <catalin.marinas@arm.com>; Will Deacon <will.deacon@arm.com>; Steven
> > Rostedt <rostedt@goodmis.org>; Chris Fries <cfries@google.com>; Dave
> > Weinstein <olorin@google.com>; Daniel Micay <danielmicay@gmail.com>; Djalal
> > Harouni <tixxdz@gmail.com>; linux-kernel@vger.kernel.org
> > Subject: [PATCH v2] 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.
> > 
> > For what it's worth, usage of unadorned %p can be broken down as follows
> > 
> >     git grep '%p[^KFfSsBRrbMmIiEUVKNhdDgCGO]' | wc -l
> > 
> > arch: 2512
> > block: 20
> > crypto: 12
> > fs: 1221
> > include: 147
> > kernel: 109
> > lib: 77
> > mm: 120
> > net: 1516
> > security: 11
> > sound: 168
> > virt: 2
> > drivers: 8420
> > 
> > Add helper function siphash_1ulong(). Add function ptr_to_id() to map an
> > address to a 32 bit unique identifier.
> > 
> > Signed-off-by: Tobin C. Harding <me@tobin.cc>
> > ---
> > 
> > V2:
> >  - Use SipHash to do the hashing
> > 
> > The discussion related to this patch has been fragmented. There are three other
> > 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
> > 
> >  include/linux/siphash.h |  2 ++
> >  lib/siphash.c           | 13 +++++++++++++
> >  lib/vsprintf.c          | 32 +++++++++++++++++++++++++++++---
> >  3 files changed, 44 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/siphash.h b/include/linux/siphash.h index
> > fa7a6b9cedbf..a9392568c8b8 100644
> > --- a/include/linux/siphash.h
> > +++ b/include/linux/siphash.h
> > @@ -26,6 +26,8 @@ u64 __siphash_aligned(const void *data, size_t len, const
> > siphash_key_t *key);
> >  u64 __siphash_unaligned(const void *data, size_t len, const siphash_key_t
> > *key);  #endif
> > 
> > +unsigned long siphash_1ulong(const unsigned long a, const siphash_key_t
> > +*key);
> > +
> >  u64 siphash_1u64(const u64 a, const siphash_key_t *key);
> >  u64 siphash_2u64(const u64 a, const u64 b, const siphash_key_t *key);
> >  u64 siphash_3u64(const u64 a, const u64 b, const u64 c, diff --git a/lib/siphash.c
> > b/lib/siphash.c index 3ae58b4edad6..63f4ff57c9ce 100644
> > --- a/lib/siphash.c
> > +++ b/lib/siphash.c
> > @@ -116,6 +116,19 @@ EXPORT_SYMBOL(__siphash_unaligned);
> >  #endif
> > 
> >  /**
> > + * siphash_1ulong - computes siphash PRF value
> > + * @first: value to hash
> > + * @key: the siphash key
> > + */
> > +unsigned long siphash_1ulong(const unsigned long first, const
> > +siphash_key_t *key) { #ifdef CONFIG_64BIT
> > +	return (unsigned long)siphash_1u64((u64)first, key); #endif
> > +	return (unsigned long)siphash_1u32((u32)first, key); }
> > +
> > +/**
> >   * siphash_1u64 - compute 64-bit siphash PRF value of a u64
> >   * @first: first u64
> >   * @key: the siphash key
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c index 86c3385b9eb3..afd1c835b0f6 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -33,6 +33,7 @@
> >  #include <linux/uuid.h>
> >  #include <linux/of.h>
> >  #include <net/addrconf.h>
> > +#include <linux/siphash.h>
> >  #ifdef CONFIG_BLOCK
> >  #include <linux/blkdev.h>
> >  #endif
> > @@ -503,6 +504,7 @@ char *number(char *buf, char *end, unsigned long long
> > num,
> >  			*buf = '0';
> >  		++buf;
> >  	}
> > +
> 
> Unneeded whitespace change?

:) thanks

> 
> >  	/* actual digits of result */
> >  	while (--i >= 0) {
> >  		if (buf < end)
> > @@ -1591,6 +1593,28 @@ char *device_node_string(char *buf, char *end, struct
> > device_node *dn,
> >  	return widen_string(buf, buf - buf_start, end, spec);  }
> > 
> > +/* Maps a pointer to a 32 bit unique identifier. */ static char
> > +*ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec) {
> > +	static siphash_key_t ptr_secret __read_mostly;
> > +	static bool have_key = false;
> > +	unsigned long hashval;
> > +
> > +	/* Kernel doesn't boot if we use get_random_once() */
> > +	if (!have_key) {
> > +		get_random_bytes(&ptr_secret, sizeof(ptr_secret));
> > +		have_key = true;
> 
> Wouldn't one want to use an atomic test and swap for this
> block?

Great, thanks for the pointer.

Thanks for the review William.

Tobin.