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 <m...@tobin.cc>
---
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 <linux/uuid.h>
#include <linux/of.h>
#include <net/addrconf.h>
+#include <linux/siphash.h>
+#include <linux/spinlock.h>
#ifdef CONFIG_BLOCK
#include <linux/blkdev.h>
#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
