| Message ID | 20170524155751.424-2-riel@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed, May 24, 2017 at 8:57 AM, <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. > > 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 <riel@redhat.com> Since the other recent canary fix went via -mm, perhaps these should go that way too? If not, I can take them via my KSPP tree. Acked-by: Kees Cook <keescook@chromium.org> -Kees > --- > 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 >
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);