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

[Jeff Layton]

On Thu, 2023-08-17 at 21:17 -0700, Kees Cook wrote:
> 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

I wonder what sort of bugs this will uncover.

Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>