On Thu, 01 Nov 2012 15:22:51 +0800
Jeff Liu <jeff.liu@xxxxxxxxxx> wrote:
> Hello,
>
> Entropy quickly depleting under normal I/O operations like ls(1), cat(1), etc...
> between 2.6.30 to current mainline,
Well that's bad. Let's cc Kees, who broke it ;)
> for instance:
>
> $ cat /proc/sys/kernel/random/entropy_avail
> 3428
> $ cat /proc/sys/kernel/random/entropy_avail
> 2911
> $cat /proc/sys/kernel/random/entropy_avail
> 2620
>
> We observed this problem has been occurred with fs/binfmt_elf.c: create_elf_tables()->get_random_bytes()
> was introduced began at 2.6.30.
> /*
> * Generate 16 random bytes for userspace PRNG seeding.
> */
> get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes));
>
> This proposal patch is trying to introduce a wrapper of get_random_int() which has lower overhead
> than calling get_random_bytes() directly.
>
> With this patch applied:
> $ cat /proc/sys/kernel/random/entropy_avail
> 2731
> $ cat /proc/sys/kernel/random/entropy_avail
> 2802
> $ cat /proc/sys/kernel/random/entropy_avail
> 2878
>
> ...
>
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -48,6 +48,7 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs);
> static int load_elf_library(struct file *);
> static unsigned long elf_map(struct file *, unsigned long, struct elf_phdr *,
> int, int, unsigned long);
> +static void randomize_stack_user(unsigned char *buf, size_t nbytes);
>
> /*
> * If we don't support core dumping, then supply a NULL so we
> @@ -200,7 +201,7 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
> /*
> * Generate 16 random bytes for userspace PRNG seeding.
> */
> - get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes));
> + randomize_stack_user(k_rand_bytes, sizeof(k_rand_bytes));
> u_rand_bytes = (elf_addr_t __user *)
> STACK_ALLOC(p, sizeof(k_rand_bytes));
> if (__copy_to_user(u_rand_bytes, k_rand_bytes, sizeof(k_rand_bytes)))
> @@ -558,6 +559,29 @@ static unsigned long randomize_stack_top(unsigned long stack_top)
> #endif
> }
>
> +/*
> + * A wrapper of get_random_int() to generate random bytes which has lower
> + * overhead than call get_random_bytes() directly.
> + * create_elf_tables() call this function to generate 16 random bytes for
> + * userspace PRNG seeding.
> + */
> +static void randomize_stack_user(unsigned char *buf, size_t nbytes)
> +{
> + unsigned char *p = buf;
> +
> + while (nbytes) {
> + unsigned int random_variable;
> + size_t chunk = min(nbytes, sizeof(unsigned int));
> +
> + random_variable = get_random_int() & STACK_RND_MASK;
> + random_variable <<= PAGE_SHIFT;
> +
> + memcpy(p, &random_variable, chunk);
> + p += chunk;
> + nbytes -= chunk;
> + }
> +}
Prior to f06295b44c296 ("ELF: implement AT_RANDOM for glibc PRNG
seeding"), glibc was opening and using /dev/urandom for this. So
presumably the urandom level of security was sufficient.
Or perhaps it wasn't and the stronger get_random_bytes() works better -
I don't know?
>From my reading of the source, get_random_int() is weaker even than
/dev/urandom?
So my bottom line is: I don't know! Kees? Ted? Ulrich?
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