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Date: Wed, 18 Oct 2017 11:54:48 +1100
From: "Tobin C. Harding" <me@...in.cc>
To: kernel-hardening@...ts.openwall.com
Cc: "Tobin C. Harding" <me@...in.cc>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Kees Cook <keescook@...omium.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Tycho Andersen <tycho@...ker.com>,
"Roberts, William C" <william.c.roberts@...el.com>,
Tejun Heo <tj@...nel.org>,
Jordan Glover <Golden_Miller83@...tonmail.ch>,
Greg KH <gregkh@...uxfoundation.org>,
Petr Mladek <pmladek@...e.com>,
Joe Perches <joe@...ches.com>,
Ian Campbell <ijc@...lion.org.uk>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Chris Fries <cfries@...gle.com>,
Dave Weinstein <olorin@...gle.com>,
Daniel Micay <danielmicay@...il.com>,
Djalal Harouni <tixxdz@...il.com>,
linux-kernel@...r.kernel.org
Subject: [PATCH v3] 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 (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 <me@...in.cc>
---
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 <linux/uuid.h>
#include <linux/of.h>
#include <net/addrconf.h>
+#include <linux/siphash.h>
#ifdef CONFIG_BLOCK
#include <linux/blkdev.h>
#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);
}
/*
--
2.7.4
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