apparmor: enforce nullbyte at end of tag string

A packed AppArmor policy contains null-terminated tag strings that are read
by unpack_nameX(). However, unpack_nameX() uses string functions on them
without ensuring that they are actually null-terminated, potentially
leading to out-of-bounds accesses.

Make sure that the tag string is null-terminated before passing it to
strcmp().

Cc: stable@vger.kernel.org
Fixes: 736ec752 ("AppArmor: policy routines for loading and unpacking policy")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/policy_unpack.c

@@ -272,7 +272,7 @@ static bool unpack_nameX(struct aa_ext *e, enum aa_code code, const char *name)
 		char *tag = NULL;
 		size_t size = unpack_u16_chunk(e, &tag);
 		/* if a name is specified it must match. otherwise skip tag */
-		if (name && (!size || strcmp(name, tag)))
+		if (name && (!size || tag[size-1] != '\0' || strcmp(name, tag)))
 			goto fail;
 	} else if (name) {
 		/* if a name is specified and there is no name tag fail */