CRED: Fix BUG() upon security_cred_alloc_blank() failure

In cred_alloc_blank() since 2.6.32, abort_creds(new) is called with
new->security == NULL and new->magic == 0 when security_cred_alloc_blank()
returns an error.  As a result, BUG() will be triggered if SELinux is enabled
or CONFIG_DEBUG_CREDENTIALS=y.

If CONFIG_DEBUG_CREDENTIALS=y, BUG() is called from __invalid_creds() because
cred->magic == 0.  Failing that, BUG() is called from selinux_cred_free()
because selinux_cred_free() is not expecting cred->security == NULL.  This does
not affect smack_cred_free(), tomoyo_cred_free() or apparmor_cred_free().

Fix these bugs by

(1) Set new->magic before calling security_cred_alloc_blank().

(2) Handle null cred->security in creds_are_invalid() and selinux_cred_free().

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/kernel/cred.c b/kernel/cred.c
index 6a1aa004e376..fcf104bb5aa9 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -252,13 +252,13 @@ struct cred *cred_alloc_blank(void)
 #endif
 
         atomic_set(&new->usage, 1);
+#ifdef CONFIG_DEBUG_CREDENTIALS
+        new->magic = CRED_MAGIC;
+#endif
 
         if (security_cred_alloc_blank(new, GFP_KERNEL) < 0)
                 goto error;
 
-#ifdef CONFIG_DEBUG_CREDENTIALS
-        new->magic = CRED_MAGIC;
-#endif
         return new;
 
 error:
@@ -748,7 +748,11 @@ bool creds_are_invalid(const struct cred *cred)
         if (cred->magic != CRED_MAGIC)
                 return true;
 #ifdef CONFIG_SECURITY_SELINUX
-        if (selinux_is_enabled()) {
+        /*
+         * cred->security == NULL if security_cred_alloc_blank() or
+         * security_prepare_creds() returned an error.
+         */
+        if (selinux_is_enabled() && cred->security) {
                 if ((unsigned long) cred->security < PAGE_SIZE)
                         return true;
                 if ((*(u32 *)cred->security & 0xffffff00) ==