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[PATCH 1/5] random,stackprotect: introduce get_random_canary function

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  [PATCH 1/5] random,stackprotect: introduce get_random_canary function riel@redhat.com - 2017-05-19 23:40 +0200
    Re: [PATCH 1/5] random,stackprotect: introduce get_random_canary  function Ingo Molnar <mingo@kernel.org> - 2017-05-24 10:40 +0200

#1645968 — [PATCH 1/5] random,stackprotect: introduce get_random_canary function

From: Rik van Riel <riel@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 <riel@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

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#1649270 — Re: [PATCH 1/5] random,stackprotect: introduce get_random_canary function

* riel@redhat.com <riel@redhat.com> wrote:

> From: Rik van Riel <riel@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 <riel@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

s/64 bit/64-bit

> + * 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;
> +}

Please separate function definitions from macros by a separate empty line.

Also, a bit of structure and organization would make the macro easier to read:

#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 0x00000000ffffffffUL
#endif

Thanks,

	Ingo

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