ELF: implement AT_RANDOM for glibc PRNG seeding

While discussing[1] the need for glibc to have access to random bytes
during program load, it seems that an earlier attempt to implement
AT_RANDOM got stalled.  This implements a random 16 byte string, available
to every ELF program via a new auxv AT_RANDOM vector.

[1] http://sourceware.org/ml/libc-alpha/2008-10/msg00006.html

Ulrich said:

glibc needs right after startup a bit of random data for internal
protections (stack canary etc).  What is now in upstream glibc is that we
always unconditionally open /dev/urandom, read some data, and use it.  For
every process startup.  That's slow.

...

The solution is to provide a limited amount of random data to the
starting process in the aux vector.  I suggested 16 bytes and this is
what the patch implements.  If we need only 16 bytes or less we use the
data directly.  If we need more we'll use the 16 bytes to see a PRNG.
This avoids the costly /dev/urandom use and it allows the kernel to use
the most adequate source of random data for this purpose.  It might not
be the same pool as that for /dev/urandom.

Concerns were expressed about the depletion of the randomness pool.  But
this patch doesn't make the situation worse, it doesn't deplete entropy
more than happens now.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index c41fa2af7677..e3ff2b9e602f 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -152,8 +152,10 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
         elf_addr_t __user *sp;
         elf_addr_t __user *u_platform;
         elf_addr_t __user *u_base_platform;
+        elf_addr_t __user *u_rand_bytes;
         const char *k_platform = ELF_PLATFORM;
         const char *k_base_platform = ELF_BASE_PLATFORM;
+        unsigned char k_rand_bytes[16];
         int items;
         elf_addr_t *elf_info;
         int ei_index = 0;
@@ -196,6 +198,15 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
                         return -EFAULT;
         }
 
+        /*
+         * Generate 16 random bytes for userspace PRNG seeding.
+         */
+        get_random_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)))
+                return -EFAULT;
+
         /* Create the ELF interpreter info */
         elf_info = (elf_addr_t *)current->mm->saved_auxv;
         /* update AT_VECTOR_SIZE_BASE if the number of NEW_AUX_ENT() changes */
@@ -228,6 +239,7 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
         NEW_AUX_ENT(AT_GID, cred->gid);
         NEW_AUX_ENT(AT_EGID, cred->egid);
         NEW_AUX_ENT(AT_SECURE, security_bprm_secureexec(bprm));
+        NEW_AUX_ENT(AT_RANDOM, (elf_addr_t)(unsigned long)u_rand_bytes);
         NEW_AUX_ENT(AT_EXECFN, bprm->exec);
         if (k_platform) {
                 NEW_AUX_ENT(AT_PLATFORM,