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_host.h_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_host.h_count it might make a difference in following places: - nlmsvc_release_host(): decrement in refcount_dec() provides RELEASE ordering, while original atomic_dec() was fully unordered. Since the change is for better, it should not matter. - nlmclnt_release_host(): decrement in refcount_dec_and_test() only provides RELEASE ordering and control dependency on success vs. fully ordered atomic counterpart. It doesn't seem to matter in this case since object freeing happens under mutex lock anyway. 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 | 14 +++++++------- include/linux/lockd/lockd.h | 3 ++- 2 files changed, 9 insertions(+), 8 deletions(-) diff --git a/fs/lockd/host.c b/fs/lockd/host.c index 826a891..11b6832 100644 --- a/fs/lockd/host.c +++ b/fs/lockd/host.c @@ -151,7 +151,7 @@ static struct nlm_host *nlm_alloc_host(struct nlm_lookup_host_info *ni, host->h_state = 0; host->h_nsmstate = 0; host->h_pidcount = 0; - atomic_set(&host->h_count, 1); + refcount_set(&host->h_count, 1); mutex_init(&host->h_mutex); host->h_nextrebind = now + NLM_HOST_REBIND; host->h_expires = now + NLM_HOST_EXPIRE; @@ -290,7 +290,7 @@ void nlmclnt_release_host(struct nlm_host *host) WARN_ON_ONCE(host->h_server); - if (atomic_dec_and_test(&host->h_count)) { + if (refcount_dec_and_test(&host->h_count)) { WARN_ON_ONCE(!list_empty(&host->h_lockowners)); WARN_ON_ONCE(!list_empty(&host->h_granted)); WARN_ON_ONCE(!list_empty(&host->h_reclaim)); @@ -410,7 +410,7 @@ void nlmsvc_release_host(struct nlm_host *host) dprintk("lockd: release server host %s\n", host->h_name); WARN_ON_ONCE(!host->h_server); - atomic_dec(&host->h_count); + refcount_dec(&host->h_count); } /* @@ -504,7 +504,7 @@ struct nlm_host * nlm_get_host(struct nlm_host *host) { if (host) { dprintk("lockd: get host %s\n", host->h_name); - atomic_inc(&host->h_count); + refcount_inc(&host->h_count); host->h_expires = jiffies + NLM_HOST_EXPIRE; } return host; @@ -593,7 +593,7 @@ static void nlm_complain_hosts(struct net *net) if (net && host->net != net) continue; dprintk(" %s (cnt %d use %d exp %ld net %x)\n", - host->h_name, atomic_read(&host->h_count), + host->h_name, refcount_read(&host->h_count), host->h_inuse, host->h_expires, host->net->ns.inum); } } @@ -662,11 +662,11 @@ nlm_gc_hosts(struct net *net) for_each_host_safe(host, next, chain, nlm_server_hosts) { if (net && host->net != net) continue; - if (atomic_read(&host->h_count) || host->h_inuse + if (refcount_read(&host->h_count) || host->h_inuse || time_before(jiffies, host->h_expires)) { dprintk("nlm_gc_hosts skipping %s " "(cnt %d use %d exp %ld net %x)\n", - host->h_name, atomic_read(&host->h_count), + host->h_name, refcount_read(&host->h_count), host->h_inuse, host->h_expires, host->net->ns.inum); continue; diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h index d7d313f..39dfeea 100644 --- a/include/linux/lockd/lockd.h +++ b/include/linux/lockd/lockd.h @@ -17,6 +17,7 @@ #include <net/ipv6.h> #include <linux/fs.h> #include <linux/kref.h> +#include <linux/refcount.h> #include <linux/utsname.h> #include <linux/lockd/bind.h> #include <linux/lockd/xdr.h> @@ -58,7 +59,7 @@ struct nlm_host { u32 h_state; /* pseudo-state counter */ u32 h_nsmstate; /* true remote NSM state */ u32 h_pidcount; /* Pseudopids */ - atomic_t h_count; /* reference count */ + refcount_t h_count; /* reference count */ struct mutex h_mutex; /* mutex for pmap binding */ unsigned long h_nextrebind; /* next portmap call */ unsigned long h_expires; /* eligible for GC */ -- 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
