path: root/fs/binfmt_elf.c
diff options
authorKees Cook <>2009-01-07 18:08:52 -0800
committerLinus Torvalds <>2009-01-08 08:31:12 -0800
commitf06295b44c296c8fb08823a3118468ae343b60f2 (patch)
treecf50932653af81aae1dc33bbf18ba7d8a94f5223 /fs/binfmt_elf.c
parenta6684999f7c6bddd75cf9755ad7ff44435f72fff (diff)
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] 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 <> Cc: Jakub Jelinek <> Cc: Andi Kleen <> Cc: Ulrich Drepper <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'fs/binfmt_elf.c')
1 files changed, 12 insertions, 0 deletions
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index c41fa2af767..e3ff2b9e602 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) {