On Wed, Nov 14, 2018 at 04:39:16PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebigg...@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 <ebigg...@google.com>
Reviewed-by: Mike Rapoport <r...@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.
