Description of problem:

We received a report of a remotely exploitable crash vulnerability in OpenStack (http://www.openstack.org).  The root cause turned out to be in glibc, when a password of excessive length made it down to sha512 crypt and resulted in an unbounded alloca() that exhausted the stack.

Here is an abbreviated demonstration of the bug in Python as it occurred in OpenStack (with python-passlib installed):

    import passlib.hash

    pw_utf8 = ('a' * 9999999).encode('utf-8')
    pw_hash = passlib.hash.sha512_crypt.encrypt(pw_utf8)
    passlib.hash.sha512_crypt.verify(pw_utf8, pw_hash)

It can also be reproduced using the crypt Python module directly:

    import crypt

    pw_utf8 = ('a' * 9999999).encode('utf-8')
    crypt.crypt(pw_utf8, '$6$' + '.'*16)

Both of these examples will result in a seg fault that resembles the following:

Program received signal SIGSEGV, Segmentation fault.
__sha512_crypt_r (key=0x7fffeb1db034 'a' <repeats 200 times>...,
salt=0x7f9bec "dWs9ZDUtzP4oF/03$",
   buffer=0x7a8a40
"$6$rounds=1000$test$2M/Lx6MtobqjLjobw0Wmo4Q5OFx5nVLJvmgseatA6oMnyWeBdRDx4DU.1H3eGmse6pgsOgDisWBGI5c7TZauS0")
at sha512-crypt.c:153
153           key = copied_key =

(gdb) list
148       key_len = strlen (key);
149
150       if ((key - (char *) 0) % __alignof__ (uint64_t) != 0)
151         {
152           char *tmp = (char *) alloca (key_len + __alignof__ (uint64_t));
153           key = copied_key =
154             memcpy (tmp + __alignof__ (uint64_t)
155                     - (tmp - (char *) 0) % __alignof__ (uint64_t),
156                     key, key_len);
157           assert ((key - (char *) 0) % __alignof__ (uint64_t) == 0);


Version-Release number of selected component (if applicable):

glibc-2.14.90-24.fc16.6.x86_64