From f06295b44c296c8fb08823a3118468ae343b60f2 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Wed, 7 Jan 2009 18:08:52 -0800 Subject: 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 Cc: Jakub Jelinek Cc: Andi Kleen Cc: Ulrich Drepper Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c index c41fa2a..e3ff2b9 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, diff --git a/include/linux/auxvec.h b/include/linux/auxvec.h index d7afa9d..f3b5d4e 100644 --- a/include/linux/auxvec.h +++ b/include/linux/auxvec.h @@ -23,16 +23,16 @@ #define AT_PLATFORM 15 /* string identifying CPU for optimizations */ #define AT_HWCAP 16 /* arch dependent hints at CPU capabilities */ #define AT_CLKTCK 17 /* frequency at which times() increments */ - +/* AT_* values 18 through 22 are reserved */ #define AT_SECURE 23 /* secure mode boolean */ - #define AT_BASE_PLATFORM 24 /* string identifying real platform, may * differ from AT_PLATFORM. */ +#define AT_RANDOM 25 /* address of 16 random bytes */ #define AT_EXECFN 31 /* filename of program */ #ifdef __KERNEL__ -#define AT_VECTOR_SIZE_BASE 18 /* NEW_AUX_ENT entries in auxiliary table */ +#define AT_VECTOR_SIZE_BASE 19 /* NEW_AUX_ENT entries in auxiliary table */ /* number of "#define AT_.*" above, minus {AT_NULL, AT_IGNORE, AT_NOTELF} */ #endif -- cgit v0.10.2