CRED: Add some configurable debugging [try #6]

Add a config option (CONFIG_DEBUG_CREDENTIALS) to turn on some debug checking
for credential management.  The additional code keeps track of the number of
pointers from task_structs to any given cred struct, and checks to see that
this number never exceeds the usage count of the cred struct (which includes
all references, not just those from task_structs).

Furthermore, if SELinux is enabled, the code also checks that the security
pointer in the cred struct is never seen to be invalid.

This attempts to catch the bug whereby inode_has_perm() faults in an nfsd
kernel thread on seeing cred->security be a NULL pointer (it appears that the
credential struct has been previously released):

	http://www.kerneloops.org/oops.php?number=252883

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 5a7ae57f983f..4e8cae2e9148 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -466,6 +466,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info,
         int retval = 0;
 
         BUG_ON(atomic_read(&sub_info->cred->usage) != 1);
+        validate_creds(sub_info->cred);
 
         helper_lock();
         if (sub_info->path[0] == '\0')