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    Subject[PATCH] 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

    Signed-off-by: Kees Cook <kees.cook@canonical.com>
    ---
    fs/binfmt_elf.c | 27 +++++++++++++++++++++++++++
    include/linux/auxvec.h | 7 ++++---
    2 files changed, 31 insertions(+), 3 deletions(-)

    diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
    index 655ed8d..11931f8 100644
    --- a/fs/binfmt_elf.c
    +++ b/fs/binfmt_elf.c
    @@ -140,6 +140,14 @@ static int padzero(unsigned long elf_bss)
    #define ELF_BASE_PLATFORM NULL
    #endif

    +#ifndef ELF_AUXV_RANDOM_SIZE
    +/*
    + * AT_RANDOM provides a random byte stream which can be used to seed
    + * userspace PRNGs at program load time.
    + */
    +#define ELF_AUXV_RANDOM_SIZE 16
    +#endif
    +
    static int
    create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
    unsigned long load_addr, unsigned long interp_load_addr)
    @@ -152,6 +160,8 @@ 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;
    + unsigned int rand_size;
    const char *k_platform = ELF_PLATFORM;
    const char *k_base_platform = ELF_BASE_PLATFORM;
    int items;
    @@ -196,6 +206,17 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
    return -EFAULT;
    }

    + rand_size = ELF_AUXV_RANDOM_SIZE;
    + u_rand_bytes = NULL;
    + if (rand_size) {
    + unsigned char k_rand_bytes[ELF_AUXV_RANDOM_SIZE];
    + get_random_bytes(k_rand_bytes, rand_size);
    +
    + u_rand_bytes = (elf_addr_t __user *)STACK_ALLOC(p, rand_size);
    + if (__copy_to_user(u_rand_bytes, k_rand_bytes, rand_size))
    + 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 +249,12 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
    NEW_AUX_ENT(AT_GID, tsk->gid);
    NEW_AUX_ENT(AT_EGID, tsk->egid);
    NEW_AUX_ENT(AT_SECURE, security_bprm_secureexec(bprm));
    + if (rand_size) {
    + NEW_AUX_ENT(AT_RANDOM_SIZE,
    + (elf_addr_t)(unsigned long)rand_size);
    + 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..dac7fc2 100644
    --- a/include/linux/auxvec.h
    +++ b/include/linux/auxvec.h
    @@ -23,16 +23,17 @@
    #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_SIZE 25 /* number of random bytes at AT_RANDOM */
    +#define AT_RANDOM 26 /* address of 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 20 /* NEW_AUX_ENT entries in auxiliary table */
    /* number of "#define AT_.*" above, minus {AT_NULL, AT_IGNORE, AT_NOTELF} */
    #endif

    --
    1.5.6.3
    --
    Kees Cook
    Ubuntu Security Team


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