From: Rik van Riel <riel@xxxxxxxxxx>
Subject: random,stackprotect: introduce get_random_canary function
Patch series "stackprotector: ascii armor the stack canary", v2.
Zero out the first byte of the stack canary value on 64 bit systems,
in order to mitigate unterminated C string overflows.
The null byte both prevents C string functions from reading the
canary, and from writing it if the canary value were guessed or
obtained through some other means.
Reducing the entropy by 8 bits is acceptable on 64-bit systems,
which will still have 56 bits of entropy left, but not on 32
bit systems, so the "ascii armor" canary is only implemented on
64-bit systems.
Inspired by the "ascii armor" code in execshield and Daniel Micay's
linux-hardened tree.
Also see https://github.com/thestinger/linux-hardened/
This patch (of 5):
Introduce get_random_canary(), 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.
The null byte both prevents C string functions from reading the canary,
and from writing it if the canary value were guessed or obtained through
some other means.
Reducing the entropy by 8 bits is acceptable on 64-bit systems, which will
still have 56 bits of entropy left, but not on 32 bit systems, so the
"ascii armor" canary is only implemented on 64-bit systems.
Inspired by the "ascii armor" code in the old execshield patches, and
Daniel Micay's linux-hardened tree.
Link: http://lkml.kernel.org/r/20170524155751.424-2-riel@xxxxxxxxxx
Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Daniel Micay <danielmicay@xxxxxxxxx>
Cc: "Theodore Ts'o" <tytso@xxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Yoshinori Sato <ysato@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/random.h | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff -puN include/linux/random.h~randomstackprotect-introduce-get_random_canary-function include/linux/random.h
--- a/include/linux/random.h~randomstackprotect-introduce-get_random_canary-function
+++ a/include/linux/random.h
@@ -57,6 +57,27 @@ static inline unsigned long get_random_l
#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);
_
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