RE: [PATCH v2] printk: hash addresses printed with %p

[Roberts, William C]

> -----Original Message-----
> From: Tobin C. Harding [mailto:me@xxxxxxxx]
> Sent: Monday, October 16, 2017 9:53 PM
> To: kernel-hardening@xxxxxxxxxxxxxxxxxx
> Cc: Tobin C. Harding <me@xxxxxxxx>; Linus Torvalds <torvalds@linux-
> foundation.org>; Kees Cook <keescook@xxxxxxxxxxxx>; Paolo Bonzini
> <pbonzini@xxxxxxxxxx>; Tycho Andersen <tycho@xxxxxxxxxx>; Roberts,
> William C <william.c.roberts@xxxxxxxxx>; Tejun Heo <tj@xxxxxxxxxx>; Jordan
> Glover <Golden_Miller83@xxxxxxxxxxxxx>; Greg KH
> <gregkh@xxxxxxxxxxxxxxxxxxx>; Petr Mladek <pmladek@xxxxxxxx>; Joe
> Perches <joe@xxxxxxxxxxx>; Ian Campbell <ijc@xxxxxxxxxxxxxx>; Sergey
> Senozhatsky <sergey.senozhatsky@xxxxxxxxx>; Catalin Marinas
> <catalin.marinas@xxxxxxx>; Will Deacon <will.deacon@xxxxxxx>; Steven
> Rostedt <rostedt@xxxxxxxxxxx>; Chris Fries <cfries@xxxxxxxxxx>; Dave
> Weinstein <olorin@xxxxxxxxxx>; Daniel Micay <danielmicay@xxxxxxxxx>; Djalal
> Harouni <tixxdz@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> 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@xxxxxxxx>
> ---
> 
> 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?

>  	/* 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?

> +	}
> +
> +	hashval = siphash_1ulong((unsigned long)ptr, &ptr_secret);
> +
> +	spec.field_width = 2 + 2 * sizeof(unsigned int); /* 0x + hex */
> +	spec.flags = SPECIAL | SMALL | ZEROPAD;
> +	spec.base = 16;
> +
> +	return number(buf, end, (u32)hashval, spec); }
> +
>  int kptr_restrict __read_mostly;
> 
>  /*
> @@ -1703,6 +1727,9 @@ int kptr_restrict __read_mostly;
>   * Note: The difference between 'S' and 'F' is that on ia64 and ppc64
>   * function pointers are really function descriptors, which contain a
>   * pointer to the real address.
> + *
> + * Default behaviour (unadorned %p) is to hash the address, rendering
> + it useful
> + * as a unique identifier.
>   */
>  static noinline_for_stack
>  char *pointer(const char *fmt, char *buf, char *end, void *ptr, @@ -1858,14
> +1885,13 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>  			return device_node_string(buf, end, ptr, spec, fmt + 1);
>  		}
>  	}
> -	spec.flags |= SMALL;
> +
>  	if (spec.field_width == -1) {
>  		spec.field_width = default_width;
>  		spec.flags |= ZEROPAD;
>  	}
> -	spec.base = 16;
> 
> -	return number(buf, end, (unsigned long) ptr, spec);
> +	return ptr_to_id(buf, end, ptr, spec);
>  }
> 
>  /*
> --
> 2.7.4