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


> -----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?

> /* 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

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