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 nsm_handle.sm_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 nsm_handle.sm_count it might make a difference in following places: - nsm_release(): decrement in refcount_dec_and_lock() only provides RELEASE ordering, control dependency on success and holds a spin lock on success vs. fully ordered atomic counterpart. No change for the spin lock guarantees. Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx> Reviewed-by: David Windsor <dwindsor@xxxxxxxxx> Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx> --- fs/lockd/host.c | 2 +- fs/lockd/mon.c | 14 +++++++------- include/linux/lockd/lockd.h | 2 +- 3 files changed, 9 insertions(+), 9 deletions(-) diff --git a/fs/lockd/host.c b/fs/lockd/host.c index 11b6832..7d6ab72 100644 --- a/fs/lockd/host.c +++ b/fs/lockd/host.c @@ -114,7 +114,7 @@ static struct nlm_host *nlm_alloc_host(struct nlm_lookup_host_info *ni, unsigned long now = jiffies; if (nsm != NULL) - atomic_inc(&nsm->sm_count); + refcount_inc(&nsm->sm_count); else { host = NULL; nsm = nsm_get_handle(ni->net, ni->sap, ni->salen, diff --git a/fs/lockd/mon.c b/fs/lockd/mon.c index 96cfb29..654594e 100644 --- a/fs/lockd/mon.c +++ b/fs/lockd/mon.c @@ -191,7 +191,7 @@ void nsm_unmonitor(const struct nlm_host *host) struct nsm_res res; int status; - if (atomic_read(&nsm->sm_count) == 1 + if (refcount_read(&nsm->sm_count) == 1 && nsm->sm_monitored && !nsm->sm_sticky) { dprintk("lockd: nsm_unmonitor(%s)\n", nsm->sm_name); @@ -279,7 +279,7 @@ static struct nsm_handle *nsm_create_handle(const struct sockaddr *sap, if (unlikely(new == NULL)) return NULL; - atomic_set(&new->sm_count, 1); + refcount_set(&new->sm_count, 1); new->sm_name = (char *)(new + 1); memcpy(nsm_addr(new), sap, salen); new->sm_addrlen = salen; @@ -337,13 +337,13 @@ struct nsm_handle *nsm_get_handle(const struct net *net, cached = nsm_lookup_addr(&ln->nsm_handles, sap); if (cached != NULL) { - atomic_inc(&cached->sm_count); + refcount_inc(&cached->sm_count); spin_unlock(&nsm_lock); kfree(new); dprintk("lockd: found nsm_handle for %s (%s), " "cnt %d\n", cached->sm_name, cached->sm_addrbuf, - atomic_read(&cached->sm_count)); + refcount_read(&cached->sm_count)); return cached; } @@ -388,12 +388,12 @@ struct nsm_handle *nsm_reboot_lookup(const struct net *net, return cached; } - atomic_inc(&cached->sm_count); + refcount_inc(&cached->sm_count); spin_unlock(&nsm_lock); dprintk("lockd: host %s (%s) rebooted, cnt %d\n", cached->sm_name, cached->sm_addrbuf, - atomic_read(&cached->sm_count)); + refcount_read(&cached->sm_count)); return cached; } @@ -404,7 +404,7 @@ struct nsm_handle *nsm_reboot_lookup(const struct net *net, */ void nsm_release(struct nsm_handle *nsm) { - if (atomic_dec_and_lock(&nsm->sm_count, &nsm_lock)) { + if (refcount_dec_and_lock(&nsm->sm_count, &nsm_lock)) { list_del(&nsm->sm_link); spin_unlock(&nsm_lock); dprintk("lockd: destroyed nsm_handle for %s (%s)\n", diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h index 39dfeea..cded0ad 100644 --- a/include/linux/lockd/lockd.h +++ b/include/linux/lockd/lockd.h @@ -84,7 +84,7 @@ struct nlm_host { struct nsm_handle { struct list_head sm_link; - atomic_t sm_count; + refcount_t sm_count; char *sm_mon_name; char *sm_name; struct sockaddr_storage sm_addr; -- 2.7.4 -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
