cred: get rid of CONFIG_DEBUG_CREDENTIALS

This code is rarely (never?) enabled by distros, and it hasn't caught
anything in decades. Let's kill off this legacy debug code.

Suggested-by: Linus Torvalds <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/powerpc/configs/skiroot_defconfig

···
 CONFIG_DEBUG_SG=y
 CONFIG_DEBUG_NOTIFIERS=y
 CONFIG_BUG_ON_DATA_CORRUPTION=y
-CONFIG_DEBUG_CREDENTIALS=y
 # CONFIG_FTRACE is not set
 CONFIG_XMON=y
 # CONFIG_RUNTIME_TESTING_MENU is not set

arch/s390/configs/debug_defconfig

···
 CONFIG_DEBUG_LIST=y
 CONFIG_DEBUG_SG=y
 CONFIG_DEBUG_NOTIFIERS=y
-CONFIG_DEBUG_CREDENTIALS=y
 CONFIG_RCU_TORTURE_TEST=m
 CONFIG_RCU_REF_SCALE_TEST=m
 CONFIG_RCU_CPU_STALL_TIMEOUT=300

fs/nfsd/auth.c

···
 	int i;
 	int flags = nfsexp_flags(rqstp, exp);
 
-	validate_process_creds();
-
 	/* discard any old override before preparing the new set */
 	revert_creds(get_cred(current_real_cred()));
 	new = prepare_creds();
···
 	else
 		new->cap_effective = cap_raise_nfsd_set(new->cap_effective,
 							new->cap_permitted);
-	validate_process_creds();
 	put_cred(override_creds(new));
 	put_cred(new);
-	validate_process_creds();
 	return 0;
 
 oom:

fs/nfsd/nfssvc.c

···
 		rqstp->rq_server->sv_maxconn = nn->max_connections;
 
 		svc_recv(rqstp);
-		validate_process_creds();
 	}
 
 	atomic_dec(&nfsdstats.th_cnt);

fs/nfsd/vfs.c

···
 	int host_err;
 	bool retried = false;
 
-	validate_process_creds();
 	/*
 	 * If we get here, then the client has already done an "open",
 	 * and (hopefully) checked permission - so allow OWNER_OVERRIDE
···
 		}
 		err = nfserrno(host_err);
 	}
-	validate_process_creds();
 	return err;
 }
 
···
 nfsd_open_verified(struct svc_rqst *rqstp, struct svc_fh *fhp, int may_flags,
 		   struct file **filp)
 {
-	int err;
-
-	validate_process_creds();
-	err = __nfsd_open(rqstp, fhp, S_IFREG, may_flags, filp);
-	validate_process_creds();
-	return err;
+	return __nfsd_open(rqstp, fhp, S_IFREG, may_flags, filp);
 }
 
 /*

fs/open.c

···
 	int error;
 	struct file *f;
 
-	validate_creds(cred);
-
 	/* We must always pass in a valid mount pointer. */
 	BUG_ON(!path->mnt);
 
···
 	struct file *f;
 	int error;
 
-	validate_creds(cred);
 	f = alloc_empty_file(flags, cred);
 	if (IS_ERR(f))
 		return f;

include/linux/cred.h

···
  */
 struct cred {
 	atomic_long_t	usage;
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	atomic_t	subscribers;	/* number of processes subscribed */
-	void		*put_addr;
-	unsigned	magic;
-#define CRED_MAGIC	0x43736564
-#define CRED_MAGIC_DEAD	0x44656144
-#endif
 	kuid_t		uid;		/* real UID of the task */
 	kgid_t		gid;		/* real GID of the task */
 	kuid_t		suid;		/* saved UID of the task */
···
 extern void __init cred_init(void);
 extern int set_cred_ucounts(struct cred *);
 
-/*
- * check for validity of credentials
- */
-#ifdef CONFIG_DEBUG_CREDENTIALS
-extern void __noreturn __invalid_creds(const struct cred *, const char *, unsigned);
-extern void __validate_process_creds(struct task_struct *,
-				     const char *, unsigned);
-
-extern bool creds_are_invalid(const struct cred *cred);
-
-static inline void __validate_creds(const struct cred *cred,
-				    const char *file, unsigned line)
-{
-	if (unlikely(creds_are_invalid(cred)))
-		__invalid_creds(cred, file, line);
-}
-
-#define validate_creds(cred)				\
-do {							\
-	__validate_creds((cred), __FILE__, __LINE__);	\
-} while(0)
-
-#define validate_process_creds()				\
-do {								\
-	__validate_process_creds(current, __FILE__, __LINE__);	\
-} while(0)
-
-extern void validate_creds_for_do_exit(struct task_struct *);
-#else
-static inline void validate_creds(const struct cred *cred)
-{
-}
-static inline void validate_creds_for_do_exit(struct task_struct *tsk)
-{
-}
-static inline void validate_process_creds(void)
-{
-}
-#endif
-
 static inline bool cap_ambient_invariant_ok(const struct cred *cred)
 {
 	return cap_issubset(cred->cap_ambient,
···
 	struct cred *nonconst_cred = (struct cred *) cred;
 	if (!cred)
 		return cred;
-	validate_creds(cred);
 	nonconst_cred->non_rcu = 0;
 	return get_new_cred_many(nonconst_cred, nr);
 }
···
 		return NULL;
 	if (!atomic_long_inc_not_zero(&nonconst_cred->usage))
 		return NULL;
-	validate_creds(cred);
 	nonconst_cred->non_rcu = 0;
 	return cred;
 }
···
 	struct cred *cred = (struct cred *) _cred;
 
 	if (cred) {
-		validate_creds(cred);
 		if (atomic_long_sub_and_test(nr, &cred->usage))
 			__put_cred(cred);
 	}

kernel/cred.c

···
  */
 struct cred init_cred = {
 	.usage			= ATOMIC_INIT(4),
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	.subscribers		= ATOMIC_INIT(2),
-	.magic			= CRED_MAGIC,
-#endif
 	.uid			= GLOBAL_ROOT_UID,
 	.gid			= GLOBAL_ROOT_GID,
 	.suid			= GLOBAL_ROOT_UID,
···
 	.ucounts		= &init_ucounts,
 };
 
-static inline void set_cred_subscribers(struct cred *cred, int n)
-{
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	atomic_set(&cred->subscribers, n);
-#endif
-}
-
-static inline int read_cred_subscribers(const struct cred *cred)
-{
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	return atomic_read(&cred->subscribers);
-#else
-	return 0;
-#endif
-}
-
-static inline void alter_cred_subscribers(const struct cred *_cred, int n)
-{
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	struct cred *cred = (struct cred *) _cred;
-
-	atomic_add(n, &cred->subscribers);
-#endif
-}
-
 /*
  * The RCU callback to actually dispose of a set of credentials
  */
···
 
 	kdebug("put_cred_rcu(%p)", cred);
 
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	if (cred->magic != CRED_MAGIC_DEAD ||
-	    atomic_long_read(&cred->usage) != 0 ||
-	    read_cred_subscribers(cred) != 0)
-		panic("CRED: put_cred_rcu() sees %p with"
-		      " mag %x, put %p, usage %ld, subscr %d\n",
-		      cred, cred->magic, cred->put_addr,
-		      atomic_long_read(&cred->usage),
-		      read_cred_subscribers(cred));
-#else
 	if (atomic_long_read(&cred->usage) != 0)
 		panic("CRED: put_cred_rcu() sees %p with usage %ld\n",
 		      cred, atomic_long_read(&cred->usage));
-#endif
 
 	security_cred_free(cred);
 	key_put(cred->session_keyring);
···
  */
 void __put_cred(struct cred *cred)
 {
-	kdebug("__put_cred(%p{%ld,%d})", cred,
-	       atomic_long_read(&cred->usage),
-	       read_cred_subscribers(cred));
+	kdebug("__put_cred(%p{%ld})", cred,
+	       atomic_long_read(&cred->usage));
 
 	BUG_ON(atomic_long_read(&cred->usage) != 0);
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	BUG_ON(read_cred_subscribers(cred) != 0);
-	cred->magic = CRED_MAGIC_DEAD;
-	cred->put_addr = __builtin_return_address(0);
-#endif
 	BUG_ON(cred == current->cred);
 	BUG_ON(cred == current->real_cred);
 
···
 {
 	struct cred *real_cred, *cred;
 
-	kdebug("exit_creds(%u,%p,%p,{%ld,%d})", tsk->pid, tsk->real_cred, tsk->cred,
-	       atomic_long_read(&tsk->cred->usage),
-	       read_cred_subscribers(tsk->cred));
+	kdebug("exit_creds(%u,%p,%p,{%ld})", tsk->pid, tsk->real_cred, tsk->cred,
+	       atomic_long_read(&tsk->cred->usage));
 
 	real_cred = (struct cred *) tsk->real_cred;
 	tsk->real_cred = NULL;
···
 	cred = (struct cred *) tsk->cred;
 	tsk->cred = NULL;
 
-	validate_creds(cred);
 	if (real_cred == cred) {
-		alter_cred_subscribers(cred, -2);
 		put_cred_many(cred, 2);
 	} else {
-		validate_creds(real_cred);
-		alter_cred_subscribers(real_cred, -1);
 		put_cred(real_cred);
-		alter_cred_subscribers(cred, -1);
 		put_cred(cred);
 	}
 
···
 		return NULL;
 
 	atomic_long_set(&new->usage, 1);
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	new->magic = CRED_MAGIC;
-#endif
 	if (security_cred_alloc_blank(new, GFP_KERNEL_ACCOUNT) < 0)
 		goto error;
 
···
 	const struct cred *old;
 	struct cred *new;
 
-	validate_process_creds();
-
 	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
 	if (!new)
 		return NULL;
···
 
 	new->non_rcu = 0;
 	atomic_long_set(&new->usage, 1);
-	set_cred_subscribers(new, 0);
 	get_group_info(new->group_info);
 	get_uid(new->user);
 	get_user_ns(new->user_ns);
···
 	if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
 		goto error;
 
-	validate_creds(new);
 	return new;
 
 error:
···
 		clone_flags & CLONE_THREAD
 	    ) {
 		p->real_cred = get_cred_many(p->cred, 2);
-		alter_cred_subscribers(p->cred, 2);
-		kdebug("share_creds(%p{%ld,%d})",
-		       p->cred, atomic_long_read(&p->cred->usage),
-		       read_cred_subscribers(p->cred));
+		kdebug("share_creds(%p{%ld})",
+		       p->cred, atomic_long_read(&p->cred->usage));
 		inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
 		return 0;
 	}
···
 
 	p->cred = p->real_cred = get_cred(new);
 	inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
-	alter_cred_subscribers(new, 2);
-	validate_creds(new);
 	return 0;
 
 error_put:
···
 	struct task_struct *task = current;
 	const struct cred *old = task->real_cred;
 
-	kdebug("commit_creds(%p{%ld,%d})", new,
-	       atomic_long_read(&new->usage),
-	       read_cred_subscribers(new));
+	kdebug("commit_creds(%p{%ld})", new,
+	       atomic_long_read(&new->usage));
 
 	BUG_ON(task->cred != old);
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	BUG_ON(read_cred_subscribers(old) < 2);
-	validate_creds(old);
-	validate_creds(new);
-#endif
 	BUG_ON(atomic_long_read(&new->usage) < 1);
 
 	get_cred(new); /* we will require a ref for the subj creds too */
···
 	 * RLIMIT_NPROC limits on user->processes have already been checked
 	 * in set_user().
 	 */
-	alter_cred_subscribers(new, 2);
 	if (new->user != old->user || new->user_ns != old->user_ns)
 		inc_rlimit_ucounts(new->ucounts, UCOUNT_RLIMIT_NPROC, 1);
 	rcu_assign_pointer(task->real_cred, new);
 	rcu_assign_pointer(task->cred, new);
 	if (new->user != old->user || new->user_ns != old->user_ns)
 		dec_rlimit_ucounts(old->ucounts, UCOUNT_RLIMIT_NPROC, 1);
-	alter_cred_subscribers(old, -2);
 
 	/* send notifications */
 	if (!uid_eq(new->uid,   old->uid)  ||
···
  */
 void abort_creds(struct cred *new)
 {
-	kdebug("abort_creds(%p{%ld,%d})", new,
-	       atomic_long_read(&new->usage),
-	       read_cred_subscribers(new));
+	kdebug("abort_creds(%p{%ld})", new,
+	       atomic_long_read(&new->usage));
 
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	BUG_ON(read_cred_subscribers(new) != 0);
-#endif
 	BUG_ON(atomic_long_read(&new->usage) < 1);
 	put_cred(new);
 }
···
 {
 	const struct cred *old = current->cred;
 
-	kdebug("override_creds(%p{%ld,%d})", new,
-	       atomic_long_read(&new->usage),
-	       read_cred_subscribers(new));
-
-	validate_creds(old);
-	validate_creds(new);
+	kdebug("override_creds(%p{%ld})", new,
+	       atomic_long_read(&new->usage));
 
 	/*
 	 * NOTE! This uses 'get_new_cred()' rather than 'get_cred()'.
···
 	 * we are only installing the cred into the thread-synchronous
 	 * '->cred' pointer, not the '->real_cred' pointer that is
 	 * visible to other threads under RCU.
-	 *
-	 * Also note that we did validate_creds() manually, not depending
-	 * on the validation in 'get_cred()'.
 	 */
 	get_new_cred((struct cred *)new);
-	alter_cred_subscribers(new, 1);
 	rcu_assign_pointer(current->cred, new);
-	alter_cred_subscribers(old, -1);
 
-	kdebug("override_creds() = %p{%ld,%d}", old,
-	       atomic_long_read(&old->usage),
-	       read_cred_subscribers(old));
+	kdebug("override_creds() = %p{%ld}", old,
+	       atomic_long_read(&old->usage));
 	return old;
 }
 EXPORT_SYMBOL(override_creds);
···
 {
 	const struct cred *override = current->cred;
 
-	kdebug("revert_creds(%p{%ld,%d})", old,
-	       atomic_long_read(&old->usage),
-	       read_cred_subscribers(old));
+	kdebug("revert_creds(%p{%ld})", old,
+	       atomic_long_read(&old->usage));
 
-	validate_creds(old);
-	validate_creds(override);
-	alter_cred_subscribers(old, 1);
 	rcu_assign_pointer(current->cred, old);
-	alter_cred_subscribers(override, -1);
 	put_cred(override);
 }
 EXPORT_SYMBOL(revert_creds);
···
 	kdebug("prepare_kernel_cred() alloc %p", new);
 
 	old = get_task_cred(daemon);
-	validate_creds(old);
 
 	*new = *old;
 	new->non_rcu = 0;
 	atomic_long_set(&new->usage, 1);
-	set_cred_subscribers(new, 0);
 	get_uid(new->user);
 	get_user_ns(new->user_ns);
 	get_group_info(new->group_info);
···
 		goto error;
 
 	put_cred(old);
-	validate_creds(new);
 	return new;
 
 error:
···
 	return security_kernel_create_files_as(new, inode);
 }
 EXPORT_SYMBOL(set_create_files_as);
-
-#ifdef CONFIG_DEBUG_CREDENTIALS
-
-bool creds_are_invalid(const struct cred *cred)
-{
-	if (cred->magic != CRED_MAGIC)
-		return true;
-	return false;
-}
-EXPORT_SYMBOL(creds_are_invalid);
-
-/*
- * dump invalid credentials
- */
-static void dump_invalid_creds(const struct cred *cred, const char *label,
-			       const struct task_struct *tsk)
-{
-	pr_err("%s credentials: %p %s%s%s\n",
-	       label, cred,
-	       cred == &init_cred ? "[init]" : "",
-	       cred == tsk->real_cred ? "[real]" : "",
-	       cred == tsk->cred ? "[eff]" : "");
-	pr_err("->magic=%x, put_addr=%p\n",
-	       cred->magic, cred->put_addr);
-	pr_err("->usage=%ld, subscr=%d\n",
-	       atomic_long_read(&cred->usage),
-	       read_cred_subscribers(cred));
-	pr_err("->*uid = { %d,%d,%d,%d }\n",
-		from_kuid_munged(&init_user_ns, cred->uid),
-		from_kuid_munged(&init_user_ns, cred->euid),
-		from_kuid_munged(&init_user_ns, cred->suid),
-		from_kuid_munged(&init_user_ns, cred->fsuid));
-	pr_err("->*gid = { %d,%d,%d,%d }\n",
-		from_kgid_munged(&init_user_ns, cred->gid),
-		from_kgid_munged(&init_user_ns, cred->egid),
-		from_kgid_munged(&init_user_ns, cred->sgid),
-		from_kgid_munged(&init_user_ns, cred->fsgid));
-#ifdef CONFIG_SECURITY
-	pr_err("->security is %p\n", cred->security);
-	if ((unsigned long) cred->security >= PAGE_SIZE &&
-	    (((unsigned long) cred->security & 0xffffff00) !=
-	     (POISON_FREE << 24 | POISON_FREE << 16 | POISON_FREE << 8)))
-		pr_err("->security {%x, %x}\n",
-		       ((u32*)cred->security)[0],
-		       ((u32*)cred->security)[1]);
-#endif
-}
-
-/*
- * report use of invalid credentials
- */
-void __noreturn __invalid_creds(const struct cred *cred, const char *file, unsigned line)
-{
-	pr_err("Invalid credentials\n");
-	pr_err("At %s:%u\n", file, line);
-	dump_invalid_creds(cred, "Specified", current);
-	BUG();
-}
-EXPORT_SYMBOL(__invalid_creds);
-
-/*
- * check the credentials on a process
- */
-void __validate_process_creds(struct task_struct *tsk,
-			      const char *file, unsigned line)
-{
-	if (tsk->cred == tsk->real_cred) {
-		if (unlikely(read_cred_subscribers(tsk->cred) < 2 ||
-			     creds_are_invalid(tsk->cred)))
-			goto invalid_creds;
-	} else {
-		if (unlikely(read_cred_subscribers(tsk->real_cred) < 1 ||
-			     read_cred_subscribers(tsk->cred) < 1 ||
-			     creds_are_invalid(tsk->real_cred) ||
-			     creds_are_invalid(tsk->cred)))
-			goto invalid_creds;
-	}
-	return;
-
-invalid_creds:
-	pr_err("Invalid process credentials\n");
-	pr_err("At %s:%u\n", file, line);
-
-	dump_invalid_creds(tsk->real_cred, "Real", tsk);
-	if (tsk->cred != tsk->real_cred)
-		dump_invalid_creds(tsk->cred, "Effective", tsk);
-	else
-		pr_err("Effective creds == Real creds\n");
-	BUG();
-}
-EXPORT_SYMBOL(__validate_process_creds);
-
-/*
- * check creds for do_exit()
- */
-void validate_creds_for_do_exit(struct task_struct *tsk)
-{
-	kdebug("validate_creds_for_do_exit(%p,%p{%ld,%d})",
-	       tsk->real_cred, tsk->cred,
-	       atomic_long_read(&tsk->cred->usage),
-	       read_cred_subscribers(tsk->cred));
-
-	__validate_process_creds(tsk, __FILE__, __LINE__);
-}
-
-#endif /* CONFIG_DEBUG_CREDENTIALS */

kernel/exit.c

···
 	ptrace_event(PTRACE_EVENT_EXIT, code);
 	user_events_exit(tsk);
 
-	validate_creds_for_do_exit(tsk);
-
 	io_uring_files_cancel();
 	exit_signals(tsk);  /* sets PF_EXITING */
 
···
 	if (tsk->task_frag.page)
 		put_page(tsk->task_frag.page);
 
-	validate_creds_for_do_exit(tsk);
 	exit_task_stack_account(tsk);
 
 	check_stack_usage();

lib/Kconfig.debug

···
 
 endmenu
 
-config DEBUG_CREDENTIALS
-	bool "Debug credential management"
-	depends on DEBUG_KERNEL
-	help
-	  Enable this 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.
-
-	  Furthermore, if SELinux is enabled, this also checks that the
-	  security pointer in the cred struct is never seen to be invalid.
-
-	  If unsure, say N.
-
 source "kernel/rcu/Kconfig.debug"
 
 config DEBUG_WQ_FORCE_RR_CPU

net/sunrpc/auth.c

···
 
 static struct cred machine_cred = {
 	.usage = ATOMIC_INIT(1),
-#ifdef CONFIG_DEBUG_CREDENTIALS
-	.magic = CRED_MAGIC,
-#endif
 };
 
 /*

security/selinux/hooks.c

···
 	struct inode_security_struct *isec;
 	u32 sid;
 
-	validate_creds(cred);
-
 	if (unlikely(IS_PRIVATE(inode)))
 		return 0;
 
···
 	struct inode_security_struct *isec;
 	u32 sid;
 
-	validate_creds(cred);
-
 	ad.type = LSM_AUDIT_DATA_DENTRY;
 	ad.u.dentry = dentry;
 	sid = cred_sid(cred);
···
 	/* No permission to check.  Existence test. */
 	if (!mask)
 		return 0;
-
-	validate_creds(cred);
 
 	if (unlikely(IS_PRIVATE(inode)))
 		return 0;

tools/objtool/noreturns.h

···
  *
  * Yes, this is unfortunate.  A better solution is in the works.
  */
-NORETURN(__invalid_creds)
 NORETURN(__kunit_abort)
 NORETURN(__module_put_and_kthread_exit)
 NORETURN(__reiserfs_panic)

tools/testing/selftests/bpf/config.x86_64

···
 CONFIG_CRYPTO_XXHASH=y
 CONFIG_DCB=y
 CONFIG_DEBUG_ATOMIC_SLEEP=y
-CONFIG_DEBUG_CREDENTIALS=y
 CONFIG_DEBUG_INFO_BTF=y
 CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
 CONFIG_DEBUG_MEMORY_INIT=y

tools/testing/selftests/hid/config.common

···
 CONFIG_CRYPTO_XXHASH=y
 CONFIG_DCB=y
 CONFIG_DEBUG_ATOMIC_SLEEP=y
-CONFIG_DEBUG_CREDENTIALS=y
 CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
 CONFIG_DEBUG_MEMORY_INIT=y
 CONFIG_DEFAULT_FQ_CODEL=y