From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934870AbdJREWH (ORCPT ); Wed, 18 Oct 2017 00:22:07 -0400 Received: from out4-smtp.messagingengine.com ([66.111.4.28]:43197 "EHLO out4-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753499AbdJREWG (ORCPT ); Wed, 18 Oct 2017 00:22:06 -0400 X-ME-Sender: From: "Tobin C. Harding" To: kernel-hardening@lists.openwall.com Cc: "Tobin C. Harding" , Linus Torvalds , Kees Cook , Paolo Bonzini , Tycho Andersen , "Roberts, William C" , Tejun Heo , Jordan Glover , Greg KH , Petr Mladek , Joe Perches , Ian Campbell , Sergey Senozhatsky , Catalin Marinas , Will Deacon , Steven Rostedt , Chris Fries , Dave Weinstein , Daniel Micay , Djalal Harouni , linux-kernel@vger.kernel.org Subject: [PATCH v4] printk: hash addresses printed with %p Date: Wed, 18 Oct 2017 15:21:55 +1100 Message-Id: <1508300515-28824-1-git-send-email-me@tobin.cc> X-Mailer: git-send-email 2.7.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 (thanks to Joe Perches). $ git grep -E '%p[^A-Za-z0-9]' | cut -f1 -d"/" | sort | uniq -c 1084 arch 20 block 10 crypto 32 Documentation 8121 drivers 1221 fs 143 include 101 kernel 69 lib 100 mm 1510 net 40 samples 7 scripts 11 security 166 sound 152 tools 2 virt Add function ptr_to_id() to map an address to a 32 bit unique identifier. Signed-off-by: Tobin C. Harding --- 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 lib/vsprintf.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 71 insertions(+), 3 deletions(-) diff --git a/lib/vsprintf.c b/lib/vsprintf.c index 86c3385b9eb3..4609738cd2cd 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -33,6 +33,8 @@ #include #include #include +#include +#include #ifdef CONFIG_BLOCK #include #endif @@ -1591,6 +1593,70 @@ char *device_node_string(char *buf, char *end, struct device_node *dn, return widen_string(buf, buf - buf_start, end, spec); } +/* protects ptr_secret and have_key */ +DEFINE_SPINLOCK(key_lock); +static siphash_key_t ptr_secret __read_mostly; +static atomic_t have_key = ATOMIC_INIT(0); + +static int initialize_ptr_secret(void) +{ + spin_lock(&key_lock); + if (atomic_read(&have_key) == 1) + goto unlock; + + get_random_bytes(&ptr_secret, sizeof(ptr_secret)); + atomic_set(&have_key, 1); + +unlock: + spin_unlock(&key_lock); + return 0; +} + +static void schedule_async_key_init(struct random_ready_callback *rdy) +{ + initialize_ptr_secret(); +} + +/* 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 struct random_ready_callback random_ready; + unsigned int hashval; + int err; + + if (atomic_read(&have_key) == 0) { + random_ready.owner = NULL; + random_ready.func = schedule_async_key_init; + + err = add_random_ready_callback(&random_ready); + + switch (err) { + case 0: + return "(pointer value)"; + + case -EALREADY: + initialize_ptr_secret(); + break; + + default: + /* shouldn't get here */ + return "(ptr_to_id() error)"; + } + } + +#ifdef CONFIG_64BIT + hashval = (unsigned int)siphash_1u64((u64)ptr, &ptr_secret); +#else + hashval = (unsigned int)siphash_1u32((u32)ptr, &ptr_secret); +#endif + + spec.field_width = 2 + 2 * sizeof(unsigned int); /* 0x + hex */ + spec.flags = SPECIAL | SMALL | ZEROPAD; + spec.base = 16; + + return number(buf, end, hashval, spec); +} + int kptr_restrict __read_mostly; /* @@ -1703,6 +1769,9 @@ 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 +1927,13 @@ 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