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

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
f06295b44c296c8f ("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 <jeff.liu@oracle.com>
Cc: John Sobecki <john.sobecki@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andreas Dilger <aedilger@gmail.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnn@arndb.de>
Cc: James Morris <james.l.morris@oracle.com>
Cc: Ted Ts'o <tytso@mit.edu>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 0c42cdbabecff0c4c8d9d0e597273ff4fce79507..172a5b17800841a766d386397774c83ade7cb7d4 100644 (file)
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -48,6 +48,7 @@ static int load_elf_binary(struct linux_binprm *bprm);
 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;
+       }
+}
+
 static int load_elf_binary(struct linux_binprm *bprm)
 {
        struct file *interpreter = NULL; /* to shut gcc up */