On Wed, Nov 29, 2017 at 01:15:43PM +0200, Elena Reshetova wrote: > 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.) >Whoops, I forgot that this doesn't apply to h_count. >Well, it's confusing, because h_count is actually used in two different >ways: depending on whether a nlm_host represents a client or server, it >may have the above properties or not. So, what happens when it is not having the above properties? Is the object being reused or? I am just trying to understand if there is a way to fix this patch to work for the case or is the drop is the only correct way to go. Best Regards, Elena. >Inclined to drop this patch for now. --b. > > 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
