binfmt_elf.c: use get_random_int() to fix entropy depleting

Changes:
--------
v4->v3:
- s/random_stack_user()/get_atrandom_bytes()/
- Move this function to ahead of its use to avoid the predeclaration.

v3->v2:
- Tweak code comments of random_stack_user().
- Remove redundant bits mask and shift upon the random variable.

v2->v1:
- Fix random copy to check up buffer length that are not 4-byte multiples.

v3 can be found at:
http://www.spinics.net/lists/linux-fsdevel/msg59597.html
v2 can be found at:
http://www.spinics.net/lists/linux-fsdevel/msg59418.html
v1 can be found at:
http://www.spinics.net/lists/linux-fsdevel/msg59128.html

Thanks,
-Jeff

Entropy is quickly depleted under normal operations like ls(1), cat(1),
etc...  between 2.6.30 to current mainline, 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 occurring since 2.6.30 with
fs/binfmt_elf.c: create_elf_tables()->get_random_bytes(), introduced by
f06295b ("ELF: implement AT_RANDOM for glibc PRNG seeding").

/*
 * Generate 16 random bytes for userspace PRNG seeding.
 */
get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes));

The patch introduces a wrapper around 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

Analyzed by John Sobecki.

Signed-off-by: Jie Liu <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andreas Dilger <[email protected]>
Cc: Alan Cox <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: John Sobecki <[email protected]>
Cc: James Morris <[email protected]>
Cc: Jakub Jelinek <[email protected]>
Cc: Ted Ts'o <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Ulrich Drepper <[email protected]>
Signed-off-by: Alex Naidis <[email protected]>
Signed-off-by: Adam W. Willis <[email protected]>
Signed-off-by: Oktapra Amtono <[email protected]>
Signed-off-by: Anush02198 <[email protected]>
Signed-off-by: clarencelol <[email protected]>
Signed-off-by: Anush02198 <[email protected]>

fs/binfmt_elf.c

@@ -161,6 +161,25 @@ static int padzero(unsigned long elf_bss)
 #define ELF_BASE_PLATFORM NULL
 #endif
 
+/*
+ * Use get_random_int() to implement AT_RANDOM while avoiding depletion
+ * of the entropy pool.
+ */
+static void get_atrandom_bytes(unsigned char *buf, size_t nbytes)
+{
+	unsigned char *p = buf;
+
+	while (nbytes) {
+		unsigned int random_variable;
+		size_t chunk = min(nbytes, sizeof(random_variable));
+
+		random_variable = get_random_int();
+		memcpy(p, &random_variable, chunk);
+		p += chunk;
+		nbytes -= chunk;
+	}
+}
+
 static int
 create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
 		unsigned long load_addr, unsigned long interp_load_addr)
@@ -220,7 +239,7 @@ 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));
+	get_atrandom_bytes(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)))