Re: [PATCH 1/4] lockd: convert nlm_host.h_count from atomic_t to refcount_t

["J. Bruce Fields" <bfields@fieldses.org>]

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.

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@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>
> ---
>  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