[PATCH] NFS: fix mount/umount race in nlmclnt.

[PATCH] NFS: fix mount/umount race in nlmclnt.

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 1468 bytes --]


If the last NFSv3 unmount from a given host races with a mount from the
same host, we can destroy an nlm_host that is still in use.

Specifically nlmclnt_lookup_host() can increment h_count on
an nlm_host that nlmclnt_release_host() has just successfully called
refcount_dec_and_test() on.
Once nlmclnt_lookup_host() drops the mutex, nlm_destroy_host_lock()
will be called to destroy the nlmclnt which is now in use again.

The cause of the problem is that the dec_and_test happens outside the
locked region.  This is easily fixed by using
refcount_dec_and_mutex_lock().

Fixes: 8ea6ecc8b075 ("lockd: Create client-side nlm_host cache")
Cc: stable@vger.kernel.org (v2.6.38+)
Signed-off-by: NeilBrown <neilb@suse.com>
---
 fs/lockd/host.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/lockd/host.c b/fs/lockd/host.c
index 93fb7cf0b92b..f0b5c987d6ae 100644
--- a/fs/lockd/host.c
+++ b/fs/lockd/host.c
@@ -290,12 +290,11 @@ void nlmclnt_release_host(struct nlm_host *host)
 
 	WARN_ON_ONCE(host->h_server);
 
-	if (refcount_dec_and_test(&host->h_count)) {
+	if (refcount_dec_and_mutex_lock(&host->h_count, &nlm_host_mutex)) {
 		WARN_ON_ONCE(!list_empty(&host->h_lockowners));
 		WARN_ON_ONCE(!list_empty(&host->h_granted));
 		WARN_ON_ONCE(!list_empty(&host->h_reclaim));
 
-		mutex_lock(&nlm_host_mutex);
 		nlm_destroy_host_locked(host);
 		mutex_unlock(&nlm_host_mutex);
 	}
-- 
2.14.0.rc0.dirty


[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 832 bytes --]

Re: [PATCH] NFS: fix mount/umount race in nlmclnt.

[Trond Myklebust]

[-- Attachment #1: Type: text/plain, Size: 1722 bytes --]

On Tue, 2019-03-19 at 11:33 +1100, NeilBrown wrote:
> If the last NFSv3 unmount from a given host races with a mount from
> the
> same host, we can destroy an nlm_host that is still in use.
> 
> Specifically nlmclnt_lookup_host() can increment h_count on
> an nlm_host that nlmclnt_release_host() has just successfully called
> refcount_dec_and_test() on.
> Once nlmclnt_lookup_host() drops the mutex, nlm_destroy_host_lock()
> will be called to destroy the nlmclnt which is now in use again.
> 
> The cause of the problem is that the dec_and_test happens outside the
> locked region.  This is easily fixed by using
> refcount_dec_and_mutex_lock().
> 
> Fixes: 8ea6ecc8b075 ("lockd: Create client-side nlm_host cache")
> Cc: stable@vger.kernel.org (v2.6.38+)
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
>  fs/lockd/host.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/lockd/host.c b/fs/lockd/host.c
> index 93fb7cf0b92b..f0b5c987d6ae 100644
> --- a/fs/lockd/host.c
> +++ b/fs/lockd/host.c
> @@ -290,12 +290,11 @@ void nlmclnt_release_host(struct nlm_host
> *host)
>  
>  	WARN_ON_ONCE(host->h_server);
>  
> -	if (refcount_dec_and_test(&host->h_count)) {
> +	if (refcount_dec_and_mutex_lock(&host->h_count,
> &nlm_host_mutex)) {
>  		WARN_ON_ONCE(!list_empty(&host->h_lockowners));
>  		WARN_ON_ONCE(!list_empty(&host->h_granted));
>  		WARN_ON_ONCE(!list_empty(&host->h_reclaim));
>  
> -		mutex_lock(&nlm_host_mutex);
>  		nlm_destroy_host_locked(host);
>  		mutex_unlock(&nlm_host_mutex);
>  	}

Thanks Neil! Applying to linux-next.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com



[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 833 bytes --]