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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 | 25 +++++++++++++++++++++++++
include/linux/auxvec.h | 6 +++---
2 files changed, 28 insertions(+), 3 deletions(-)
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 655ed8d..479a7a4 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 16 random bytes 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 +160,7 @@ 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;
int items;
@@ -196,6 +205,18 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
return -EFAULT;
}

+ u_rand_bytes = NULL;
+ if (ELF_AUXV_RANDOM_SIZE) {
+ unsigned char k_rand_bytes[ELF_AUXV_RANDOM_SIZE];
+ get_random_bytes(k_rand_bytes, ELF_AUXV_RANDOM_SIZE);
+
+ u_rand_bytes = (elf_addr_t __user *)
+ STACK_ALLOC(p, ELF_AUXV_RANDOM_SIZE);
+ if (__copy_to_user(u_rand_bytes, k_rand_bytes,
+ ELF_AUXV_RANDOM_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 +249,10 @@ 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 (ELF_AUXV_RANDOM_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..d85b8f7 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 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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