lockd: convert nlm_rqst.a_count from atomic_t to refcount_t

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 <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --git a/fs/lockd/clntproc.c b/fs/lockd/clntproc.c
index 112173dbea7605f61659b96d58fb0df2da98a255..a2c0dfc6fdc0017c2e9f41d3dc4d6dd0d6b4fce0 100644 (file)
--- 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 0d670c5c378fcf355abea12e0a811b898d88ff57..ea77c66d3cc33fa7e3bd0374761b2543d429abe8 100644 (file)
--- 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 345fced8339e62b6cd515e9022ddde52f837dc3c..94c9ff58fc91f9b0ce16206c86f95ba2c16b720b 100644 (file)
--- a/include/linux/lockd/lockd.h
+++ b/include/linux/lockd/lockd.h
@@ -136,7 +136,7 @@ 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 */