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, 5 insertions, 1 deletions

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 27b4c552735..c3bb31ecc5a 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -1531,6 +1531,8 @@ static int inode_has_perm(const struct cred *cred,
 	struct common_audit_data ad;
 	u32 sid;
 
+	validate_creds(cred);
+
 	if (unlikely(IS_PRIVATE(inode)))
 		return 0;
 
@@ -3236,7 +3238,9 @@ static int selinux_task_create(unsigned long clone_flags)
 static void selinux_cred_free(struct cred *cred)
 {
 	struct task_security_struct *tsec = cred->security;
-	cred->security = NULL;
+
+	BUG_ON((unsigned long) cred->security < PAGE_SIZE);
+	cred->security = (void *) 0x7UL;
 	kfree(tsec);
 }