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 nlm_rqst.a_count 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 lib/refcount.c have different memory ordering guarantees than their atomic counterparts. The full comparison can be seen in https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon in state to be merged to the documentation tree. 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 nlm_rqst.a_count it might make a difference in following places: - nlmclnt_release_call() and nlmsvc_release_call(): decrement in refcount_dec_and_test() only provides RELEASE ordering and control dependency on success vs. fully ordered atomic counterpart Suggested-by: Kees Cook <keesc...@chromium.org> Reviewed-by: David Windsor <dwind...@gmail.com> Reviewed-by: Hans Liljestrand <ishkam...@gmail.com> Signed-off-by: Elena Reshetova <elena.reshet...@intel.com> --- fs/lockd/clntproc.c | 8 ++++---- fs/lockd/svcproc.c | 2 +- include/linux/lockd/lockd.h | 2 +- 3 files changed, 6 insertions(+), 6 deletions(-) diff --git a/fs/lockd/clntproc.c b/fs/lockd/clntproc.c index 112173d..a2c0dfc 100644 --- a/fs/lockd/clntproc.c +++ b/fs/lockd/clntproc.c @@ -204,7 +204,7 @@ struct nlm_rqst *nlm_alloc_call(struct nlm_host *host) for(;;) { call = kzalloc(sizeof(*call), GFP_KERNEL); if (call != NULL) { - atomic_set(&call->a_count, 1); + refcount_set(&call->a_count, 1); locks_init_lock(&call->a_args.lock.fl); locks_init_lock(&call->a_res.lock.fl); call->a_host = nlm_get_host(host); @@ -222,7 +222,7 @@ void nlmclnt_release_call(struct nlm_rqst *call) { const struct nlmclnt_operations *nlmclnt_ops = call->a_host->h_nlmclnt_ops; - if (!atomic_dec_and_test(&call->a_count)) + if (!refcount_dec_and_test(&call->a_count)) return; if (nlmclnt_ops && nlmclnt_ops->nlmclnt_release_call) nlmclnt_ops->nlmclnt_release_call(call->a_callback_data); @@ -678,7 +678,7 @@ nlmclnt_unlock(struct nlm_rqst *req, struct file_lock *fl) goto out; } - atomic_inc(&req->a_count); + refcount_inc(&req->a_count); status = nlmclnt_async_call(nfs_file_cred(fl->fl_file), req, NLMPROC_UNLOCK, &nlmclnt_unlock_ops); if (status < 0) @@ -769,7 +769,7 @@ static int nlmclnt_cancel(struct nlm_host *host, int block, struct file_lock *fl nlmclnt_setlockargs(req, fl); req->a_args.block = block; - atomic_inc(&req->a_count); + refcount_inc(&req->a_count); status = nlmclnt_async_call(nfs_file_cred(fl->fl_file), req, NLMPROC_CANCEL, &nlmclnt_cancel_ops); if (status == 0 && req->a_res.status == nlm_lck_denied) diff --git a/fs/lockd/svcproc.c b/fs/lockd/svcproc.c index 0d670c5..ea77c66 100644 --- a/fs/lockd/svcproc.c +++ b/fs/lockd/svcproc.c @@ -295,7 +295,7 @@ static void nlmsvc_callback_exit(struct rpc_task *task, void *data) void nlmsvc_release_call(struct nlm_rqst *call) { - if (!atomic_dec_and_test(&call->a_count)) + if (!refcount_dec_and_test(&call->a_count)) return; nlmsvc_release_host(call->a_host); kfree(call); diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h index 86d012a..4fd95db 100644 --- a/include/linux/lockd/lockd.h +++ b/include/linux/lockd/lockd.h @@ -137,7 +137,7 @@ struct nlm_wait; */ #define NLMCLNT_OHSIZE ((__NEW_UTS_LEN) + 10u) struct nlm_rqst { - atomic_t a_count; + refcount_t a_count; unsigned int a_flags; /* initial RPC task flags */ struct nlm_host * a_host; /* host handle */ struct nlm_args a_args; /* arguments */ -- 2.7.4
