From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932201AbdJVX2m (ORCPT ); Sun, 22 Oct 2017 19:28:42 -0400 Received: from out4-smtp.messagingengine.com ([66.111.4.28]:34173 "EHLO out4-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932120AbdJVX2k (ORCPT ); Sun, 22 Oct 2017 19:28:40 -0400 X-ME-Sender: From: "Tobin C. Harding" To: kernel-hardening@lists.openwall.com Cc: "Tobin C. Harding" , "Jason A. Donenfeld" , "Theodore Ts'o" , 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 v6] printk: hash addresses printed with %p Date: Mon, 23 Oct 2017 10:28:32 +1100 Message-Id: <1508714912-2266-1-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 --- 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 lib/vsprintf.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 57 insertions(+) diff --git a/lib/vsprintf.c b/lib/vsprintf.c index 86c3385b9eb3..e2f712666882 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -33,6 +33,7 @@ #include #include #include +#include #ifdef CONFIG_BLOCK #include #endif @@ -1591,6 +1592,55 @@ char *device_node_string(char *buf, char *end, struct device_node *dn, return widen_string(buf, buf - buf_start, end, spec); } +static siphash_key_t ptr_secret __read_mostly; +static DEFINE_STATIC_KEY_TRUE(no_ptr_secret); + +static void fill_random_ptr_key(struct random_ready_callback *rdy) +{ + get_random_bytes(&ptr_secret, sizeof(ptr_secret)); + static_branch_disable(&no_ptr_secret); +} + +static struct random_ready_callback random_ready = { + .func = fill_random_ptr_key +}; + +static int __init initialize_ptr_random(void) +{ + int ret = add_random_ready_callback(&random_ready); + + if (!ret) + return 0; + else if (ret == -EALREADY) { + fill_random_ptr_key(&random_ready); + return 0; + } + + return ret; +} +early_initcall(initialize_ptr_random); + +/* Maps a pointer to a 32 bit unique identifier. */ +static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec) +{ + unsigned int hashval; + + if (static_branch_unlikely(&no_ptr_secret)) + return "(pointer value)"; + +#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 * sizeof(unsigned int); + spec.flags = SMALL; + spec.base = 16; + + return number(buf, end, hashval, spec); +} + int kptr_restrict __read_mostly; /* @@ -1703,6 +1753,9 @@ 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. + * + * Note: The 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, @@ -1857,7 +1910,11 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr, case 'F': return device_node_string(buf, end, ptr, spec, fmt + 1); } + default: /* default is to _not_ leak addresses, hash before printing */ + return ptr_to_id(buf, end, ptr, spec); } + + /* OK, let's print the address */ spec.flags |= SMALL; if (spec.field_width == -1) { spec.field_width = default_width; -- 2.7.4