Re: [PATCH] userfaultfd: convert userfaultfd_ctx::refcount to refcount_t

[Mike Rapoport <rppt@linux.ibm.com>]

On Wed, Nov 14, 2018 at 04:39:16PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Reference counters should use refcount_t rather than atomic_t, since the
> refcount_t implementation can prevent overflows, reducing the
> exploitability of reference leak bugs.  userfaultfd_ctx::refcount is a
> reference counter with the usual semantics, so convert it to refcount_t.
> 
> Note: I replaced the BUG() on incrementing a 0 refcount with just
> refcount_inc(), since part of the semantics of refcount_t is that that
> incrementing a 0 refcount is not allowed; with CONFIG_REFCOUNT_FULL,
> refcount_inc() already checks for it and warns.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>

> ---
>  fs/userfaultfd.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 356d2b8568c14..8375faac2790d 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -53,7 +53,7 @@ struct userfaultfd_ctx {
>  	/* a refile sequence protected by fault_pending_wqh lock */
>  	struct seqcount refile_seq;
>  	/* pseudo fd refcounting */
> -	atomic_t refcount;
> +	refcount_t refcount;
>  	/* userfaultfd syscall flags */
>  	unsigned int flags;
>  	/* features requested from the userspace */
> @@ -140,8 +140,7 @@ static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
>   */
>  static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
>  {
> -	if (!atomic_inc_not_zero(&ctx->refcount))
> -		BUG();
> +	refcount_inc(&ctx->refcount);
>  }
> 
>  /**
> @@ -154,7 +153,7 @@ static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
>   */
>  static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
>  {
> -	if (atomic_dec_and_test(&ctx->refcount)) {
> +	if (refcount_dec_and_test(&ctx->refcount)) {
>  		VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
>  		VM_BUG_ON(waitqueue_active(&ctx->fault_pending_wqh));
>  		VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
> @@ -686,7 +685,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
>  			return -ENOMEM;
>  		}
> 
> -		atomic_set(&ctx->refcount, 1);
> +		refcount_set(&ctx->refcount, 1);
>  		ctx->flags = octx->flags;
>  		ctx->state = UFFD_STATE_RUNNING;
>  		ctx->features = octx->features;
> @@ -1911,7 +1910,7 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
>  	if (!ctx)
>  		return -ENOMEM;
> 
> -	atomic_set(&ctx->refcount, 1);
> +	refcount_set(&ctx->refcount, 1);
>  	ctx->flags = flags;
>  	ctx->features = 0;
>  	ctx->state = UFFD_STATE_WAIT_API;
> -- 
> 2.19.1.930.g4563a0d9d0-goog
> 

-- 
Sincerely yours,
Mike.