[PATCH v2] creds: Convert cred.usage to refcount_t

[Kees Cook]

From: Elena Reshetova <elena.reshetova@xxxxxxxxx>

atomic_t variables are currently used to implement reference counters
with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows and
underflows. This is important since overflows and underflows can lead
to use-after-free situation and be exploitable.

The variable cred.usage is used as pure reference counter. Convert it
to refcount_t and fix up the operations.

**Important note for maintainers:

Some functions from refcount_t API defined in refcount.h have different
memory ordering guarantees than their atomic counterparts. Please check
Documentation/core-api/refcount-vs-atomic.rst for more information.

Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in some
rare cases it might matter.  Please double check that you don't have
some undocumented memory guarantees for this variable usage.

For the cred.usage it might make a difference in following places:
 - get_task_cred(): increment in refcount_inc_not_zero() only
   guarantees control dependency on success vs. fully ordered atomic
   counterpart
 - put_cred(): decrement in refcount_dec_and_test() only
   provides RELEASE ordering and ACQUIRE ordering on success vs. fully
   ordered atomic counterpart

Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v2: rebase
v1: https://lore.kernel.org/lkml/20200612183450.4189588-4-keescook@xxxxxxxxxxxx/
---
 include/linux/cred.h |  8 ++++----
 kernel/cred.c        | 42 +++++++++++++++++++++---------------------
 net/sunrpc/auth.c    |  2 +-
 3 files changed, 26 insertions(+), 26 deletions(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 8661f6294ad4..bf1c142afcec 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -109,7 +109,7 @@ static inline int groups_search(const struct group_info *group_info, kgid_t grp)
  * same context as task->real_cred.
  */
 struct cred {
-	atomic_t	usage;
+	refcount_t	usage;
 #ifdef CONFIG_DEBUG_CREDENTIALS
 	atomic_t	subscribers;	/* number of processes subscribed */
 	void		*put_addr;
@@ -229,7 +229,7 @@ static inline bool cap_ambient_invariant_ok(const struct cred *cred)
  */
 static inline struct cred *get_new_cred(struct cred *cred)
 {
-	atomic_inc(&cred->usage);
+	refcount_inc(&cred->usage);
 	return cred;
 }
 
@@ -261,7 +261,7 @@ static inline const struct cred *get_cred_rcu(const struct cred *cred)
 	struct cred *nonconst_cred = (struct cred *) cred;
 	if (!cred)
 		return NULL;
-	if (!atomic_inc_not_zero(&nonconst_cred->usage))
+	if (!refcount_inc_not_zero(&nonconst_cred->usage))
 		return NULL;
 	validate_creds(cred);
 	nonconst_cred->non_rcu = 0;
@@ -285,7 +285,7 @@ static inline void put_cred(const struct cred *_cred)
 
 	if (cred) {
 		validate_creds(cred);
-		if (atomic_dec_and_test(&(cred)->usage))
+		if (refcount_dec_and_test(&(cred)->usage))
 			__put_cred(cred);
 	}
 }
diff --git a/kernel/cred.c b/kernel/cred.c
index bed458cfb812..33090c43bcac 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -39,7 +39,7 @@ static struct group_info init_groups = { .usage = REFCOUNT_INIT(2) };
  * The initial credentials for the initial task
  */
 struct cred init_cred = {
-	.usage			= ATOMIC_INIT(4),
+	.usage			= REFCOUNT_INIT(4),
 #ifdef CONFIG_DEBUG_CREDENTIALS
 	.subscribers		= ATOMIC_INIT(2),
 	.magic			= CRED_MAGIC,
@@ -99,17 +99,17 @@ static void put_cred_rcu(struct rcu_head *rcu)
 
 #ifdef CONFIG_DEBUG_CREDENTIALS
 	if (cred->magic != CRED_MAGIC_DEAD ||
-	    atomic_read(&cred->usage) != 0 ||
+	    refcount_read(&cred->usage) != 0 ||
 	    read_cred_subscribers(cred) != 0)
 		panic("CRED: put_cred_rcu() sees %p with"
 		      " mag %x, put %p, usage %d, subscr %d\n",
 		      cred, cred->magic, cred->put_addr,
-		      atomic_read(&cred->usage),
+		      refcount_read(&cred->usage),
 		      read_cred_subscribers(cred));
 #else
-	if (atomic_read(&cred->usage) != 0)
+	if (refcount_read(&cred->usage) != 0)
 		panic("CRED: put_cred_rcu() sees %p with usage %d\n",
-		      cred, atomic_read(&cred->usage));
+		      cred, refcount_read(&cred->usage));
 #endif
 
 	security_cred_free(cred);
@@ -135,10 +135,10 @@ static void put_cred_rcu(struct rcu_head *rcu)
 void __put_cred(struct cred *cred)
 {
 	kdebug("__put_cred(%p{%d,%d})", cred,
-	       atomic_read(&cred->usage),
+	       refcount_read(&cred->usage),
 	       read_cred_subscribers(cred));
 
-	BUG_ON(atomic_read(&cred->usage) != 0);
+	BUG_ON(refcount_read(&cred->usage) != 0);
 #ifdef CONFIG_DEBUG_CREDENTIALS
 	BUG_ON(read_cred_subscribers(cred) != 0);
 	cred->magic = CRED_MAGIC_DEAD;
@@ -162,7 +162,7 @@ void exit_creds(struct task_struct *tsk)
 	struct cred *cred;
 
 	kdebug("exit_creds(%u,%p,%p,{%d,%d})", tsk->pid, tsk->real_cred, tsk->cred,
-	       atomic_read(&tsk->cred->usage),
+	       refcount_read(&tsk->cred->usage),
 	       read_cred_subscribers(tsk->cred));
 
 	cred = (struct cred *) tsk->real_cred;
@@ -221,7 +221,7 @@ struct cred *cred_alloc_blank(void)
 	if (!new)
 		return NULL;
 
-	atomic_set(&new->usage, 1);
+	refcount_set(&new->usage, 1);
 #ifdef CONFIG_DEBUG_CREDENTIALS
 	new->magic = CRED_MAGIC;
 #endif
@@ -267,7 +267,7 @@ struct cred *prepare_creds(void)
 	memcpy(new, old, sizeof(struct cred));
 
 	new->non_rcu = 0;
-	atomic_set(&new->usage, 1);
+	refcount_set(&new->usage, 1);
 	set_cred_subscribers(new, 0);
 	get_group_info(new->group_info);
 	get_uid(new->user);
@@ -356,7 +356,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 		get_cred(p->cred);
 		alter_cred_subscribers(p->cred, 2);
 		kdebug("share_creds(%p{%d,%d})",
-		       p->cred, atomic_read(&p->cred->usage),
+		       p->cred, refcount_read(&p->cred->usage),
 		       read_cred_subscribers(p->cred));
 		inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
 		return 0;
@@ -450,7 +450,7 @@ int commit_creds(struct cred *new)
 	const struct cred *old = task->real_cred;
 
 	kdebug("commit_creds(%p{%d,%d})", new,
-	       atomic_read(&new->usage),
+	       refcount_read(&new->usage),
 	       read_cred_subscribers(new));
 
 	BUG_ON(task->cred != old);
@@ -459,7 +459,7 @@ int commit_creds(struct cred *new)
 	validate_creds(old);
 	validate_creds(new);
 #endif
-	BUG_ON(atomic_read(&new->usage) < 1);
+	BUG_ON(refcount_read(&new->usage) < 1);
 
 	get_cred(new); /* we will require a ref for the subj creds too */
 
@@ -533,13 +533,13 @@ EXPORT_SYMBOL(commit_creds);
 void abort_creds(struct cred *new)
 {
 	kdebug("abort_creds(%p{%d,%d})", new,
-	       atomic_read(&new->usage),
+	       refcount_read(&new->usage),
 	       read_cred_subscribers(new));
 
 #ifdef CONFIG_DEBUG_CREDENTIALS
 	BUG_ON(read_cred_subscribers(new) != 0);
 #endif
-	BUG_ON(atomic_read(&new->usage) < 1);
+	BUG_ON(refcount_read(&new->usage) < 1);
 	put_cred(new);
 }
 EXPORT_SYMBOL(abort_creds);
@@ -556,7 +556,7 @@ const struct cred *override_creds(const struct cred *new)
 	const struct cred *old = current->cred;
 
 	kdebug("override_creds(%p{%d,%d})", new,
-	       atomic_read(&new->usage),
+	       refcount_read(&new->usage),
 	       read_cred_subscribers(new));
 
 	validate_creds(old);
@@ -579,7 +579,7 @@ const struct cred *override_creds(const struct cred *new)
 	alter_cred_subscribers(old, -1);
 
 	kdebug("override_creds() = %p{%d,%d}", old,
-	       atomic_read(&old->usage),
+	       refcount_read(&old->usage),
 	       read_cred_subscribers(old));
 	return old;
 }
@@ -597,7 +597,7 @@ void revert_creds(const struct cred *old)
 	const struct cred *override = current->cred;
 
 	kdebug("revert_creds(%p{%d,%d})", old,
-	       atomic_read(&old->usage),
+	       refcount_read(&old->usage),
 	       read_cred_subscribers(old));
 
 	validate_creds(old);
@@ -728,7 +728,7 @@ struct cred *prepare_kernel_cred(struct task_struct *daemon)
 
 	*new = *old;
 	new->non_rcu = 0;
-	atomic_set(&new->usage, 1);
+	refcount_set(&new->usage, 1);
 	set_cred_subscribers(new, 0);
 	get_uid(new->user);
 	get_user_ns(new->user_ns);
@@ -843,7 +843,7 @@ static void dump_invalid_creds(const struct cred *cred, const char *label,
 	printk(KERN_ERR "CRED: ->magic=%x, put_addr=%p\n",
 	       cred->magic, cred->put_addr);
 	printk(KERN_ERR "CRED: ->usage=%d, subscr=%d\n",
-	       atomic_read(&cred->usage),
+	       refcount_read(&cred->usage),
 	       read_cred_subscribers(cred));
 	printk(KERN_ERR "CRED: ->*uid = { %d,%d,%d,%d }\n",
 		from_kuid_munged(&init_user_ns, cred->uid),
@@ -917,7 +917,7 @@ void validate_creds_for_do_exit(struct task_struct *tsk)
 {
 	kdebug("validate_creds_for_do_exit(%p,%p{%d,%d})",
 	       tsk->real_cred, tsk->cred,
-	       atomic_read(&tsk->cred->usage),
+	       refcount_read(&tsk->cred->usage),
 	       read_cred_subscribers(tsk->cred));
 
 	__validate_process_creds(tsk, __FILE__, __LINE__);
diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
index 2f16f9d17966..f9f406249e7d 100644
--- a/net/sunrpc/auth.c
+++ b/net/sunrpc/auth.c
@@ -39,7 +39,7 @@ static LIST_HEAD(cred_unused);
 static unsigned long number_cred_unused;
 
 static struct cred machine_cred = {
-	.usage = ATOMIC_INIT(1),
+	.usage = REFCOUNT_INIT(1),
 #ifdef CONFIG_DEBUG_CREDENTIALS
 	.magic = CRED_MAGIC,
 #endif
-- 
2.34.1