[PATCH] ELF: implement AT_RANDOM for glibc PRNG seeding

[Kees Cook <kees.cook@canonical.com>]

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