From: Rik van Riel <r...@redhat.com>
Introduce the get_random_canary function, which provides a random
unsigned long canary value with the first byte zeroed out on 64
bit architectures, in order to mitigate non-terminated C string
overflows.
Inspired by the "ascii armor" code in the old execshield patches,
and the current PaX/grsecurity code base.
Signed-off-by: Rik van Riel <r...@redhat.com>
---
include/linux/random.h | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
diff --git a/include/linux/random.h b/include/linux/random.h
index ed5c3838780d..765a992c6774 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -57,6 +57,26 @@ static inline unsigned long get_random_long(void)
#endif
}
+/*
+ * On 64 bit architectures, protect against non-terminated C string overflows
+ * by zeroing out the first byte of the canary; this leaves 56 bits of entropy.
+ */
+#ifdef CONFIG_64BIT
+#ifdef __LITTLE_ENDIAN
+#define CANARY_MASK 0xffffffffffffff00UL
+#else /* big endian 64 bits */
+#define CANARY_MASK 0x00ffffffffffffffUL
+#endif
+#else /* 32 bits */
+#define CANARY_MASK 0xffffffffUL
+#endif
+static inline unsigned long get_random_canary(void)
+{
+ unsigned long val = get_random_long();
+
+ return val & CANARY_MASK;
+}
+
unsigned long randomize_page(unsigned long start, unsigned long range);
u32 prandom_u32(void);
--
2.9.3
