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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 Date: Wed, 18 Oct 2017 11:54:48 +1100 Message-Id: <1508288088-31867-1-git-send-email-me@tobin.cc> X-Mailer: git-send-email 2.7.4 Subject: [kernel-hardening] [PATCH v3] printk: hash addresses printed with %p X-Virus-Scanned: ClamAV using ClamSMTP 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 --- 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 include/linux/siphash.h | 2 ++ lib/siphash.c | 13 +++++++++++++ lib/vsprintf.c | 28 +++++++++++++++++++++++++--- 3 files changed, 40 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..b3b680357a85 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -33,6 +33,7 @@ #include #include #include +#include #ifdef CONFIG_BLOCK #include #endif @@ -1591,6 +1592,25 @@ 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 atomic_t have_key = ATOMIC_INIT(0); + unsigned long hashval; + + if (atomic_xchg(&have_key, 1) == 0) + get_random_bytes(&ptr_secret, sizeof(ptr_secret)); + + 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 +1723,9 @@ 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 +1881,13 @@ 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); } /*