From mboxrd@z Thu Jan 1 00:00:00 1970 From: riel@redhat.com (riel at redhat.com) Date: Wed, 24 May 2017 11:57:47 -0400 Subject: [PATCH 1/5] random, stackprotect: introduce get_random_canary function In-Reply-To: <20170524155751.424-1-riel@redhat.com> References: <20170524155751.424-1-riel@redhat.com> Message-ID: <20170524155751.424-2-riel@redhat.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org From: Rik van Riel 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. 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. Signed-off-by: Rik van Riel --- include/linux/random.h | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) diff --git a/include/linux/random.h b/include/linux/random.h index ed5c3838780d..1fa0dc880bd7 100644 --- a/include/linux/random.h +++ b/include/linux/random.h @@ -57,6 +57,27 @@ 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