From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Liu Subject: [PATCH] binfmt_elf.c: Introduce a wrapper of get_random_int() to fix entropy depleting Date: Thu, 25 Oct 2012 15:53:03 +0800 Message-ID: <5088EFDF.7040505@oracle.com> Reply-To: jeff.liu@oracle.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: "viro@zeniv.linux.org.uk" , "Ted Ts'o" , James Morris , arnd@arndb.de, gregkh@linuxfoundation.org, John Sobecki To: "linux-fsdevel@vger.kernel.org" Return-path: Received: from rcsinet15.oracle.com ([148.87.113.117]:42247 "EHLO rcsinet15.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932796Ab2JYHye (ORCPT ); Thu, 25 Oct 2012 03:54:34 -0400 Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Hello, We observed an issue regarding entropy quickly depleting under any normal I/O operations like ls(1), cat(1),etc... for instance: $ date; cat /proc/sys/kernel/random/entropy_avail Thu Oct 25 15:24:37 CST 2012 3264 $ date; cat /proc/sys/kernel/random/entropy_avail Thu Oct 25 15:24:40 CST 2012 2791 $ date; cat /proc/sys/kernel/random/entropy_avail Thu Oct 25 15:24:42 CST 2012 2581 $ date; cat /proc/sys/kernel/random/entropy_avail Thu Oct 25 15:24:43 CST 2012 2122 According to John's analysis, it started to happen with 2.6.30 with: fs/binfmt_elf.c: create_elf_tables()->get_random_bytes() was introduced: /* * Generate 16 random bytes for userspace PRNG seeding. */ get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes)); Here is proposal patch to replace get_random_bytes() with a wrapper function get_random_int() which has low overhead to generate randoms, it looks stupid but works: $ date;cat /proc/sys/kernel/random/entropy_avail Thu Oct 25 15:31:32 CST 2012 2546 $ date;cat /proc/sys/kernel/random/entropy_avail Thu Oct 25 15:31:33 CST 2012 2558 $ date;cat /proc/sys/kernel/random/entropy_avail Thu Oct 25 15:31:34 CST 2012 2572 $ date;cat /proc/sys/kernel/random/entropy_avail Thu Oct 25 15:31:36 CST 2012 2614 Also, I have a question about whether stack randomization tunable parameter could be considered in this point or not, i.e, If the user disabled the stack randomization via "kernel.randomize_va_space=0" or /proc/... Does it sounds make sense if just copying the k_rand_bytes[] back to user space with current uninitialized stack stuff rather than filling it with really strong random bytes, something like: /* * Generate 16 random bytes for userspace PRNG seeding if randomize is required. */ if (current->flags & PF_RANDOMIZE) get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes)); Above fix also works although Glibc->elf_loader need a random bytes array for stack guarding, which means that the user want to take the risk by disabling stack randomize. Any comments are appreciated! -Jeff Signed-off-by: Jie Liu Analyzed-by: John Sobecki CC: Al Viro CC: Arnd Bergmann CC: James Morris CC: Ted Ts'o CC: Greg Kroah-Hartman --- fs/binfmt_elf.c | 16 +++++++++++++++- 1 files changed, 15 insertions(+), 1 deletions(-) diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c index fbd9f60..4fc92d5 100644 --- a/fs/binfmt_elf.c +++ b/fs/binfmt_elf.c @@ -48,6 +48,7 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs); static int load_elf_library(struct file *); static unsigned long elf_map(struct file *, unsigned long, struct elf_phdr *, int, int, unsigned long); +static void randomize_stack_user(unsigned char *random_bytes, size_t nr); /* * If we don't support core dumping, then supply a NULL so we @@ -200,7 +201,7 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec, /* * Generate 16 random bytes for userspace PRNG seeding. */ - get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes)); + randomize_stack_user(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))) @@ -558,6 +559,19 @@ static unsigned long randomize_stack_top(unsigned long stack_top) #endif } +static void randomize_stack_user(unsigned char *random_bytes, size_t nr) +{ + unsigned int random_variable; + size_t i; + + for (i = 0; i < nr; i += sizeof(random_variable)) { + random_variable = get_random_int() & STACK_RND_MASK; + random_variable <= PAGE_SHIFT; + memcpy(&random_bytes[i], &random_variable, + sizeof(random_variable)); + } +} + static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs) { struct file *interpreter = NULL; /* to shut gcc up */ -- 1.7.4.1