From fee21fb587f57748c3c971e3432c4a28d74146fc Mon Sep 17 00:00:00 2001 From: Elena Reshetova Date: Wed, 29 Nov 2017 13:15:43 +0200 Subject: lockd: convert nlm_host.h_count from atomic_t to refcount_t 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 Reviewed-by: David Windsor Reviewed-by: Hans Liljestrand Signed-off-by: Elena Reshetova Signed-off-by: Trond Myklebust --- include/linux/lockd/lockd.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'include/linux/lockd') diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h index d7d313fb9cd4..39dfeea20963 100644 --- a/include/linux/lockd/lockd.h +++ b/include/linux/lockd/lockd.h @@ -17,6 +17,7 @@ #include #include #include +#include #include #include #include @@ -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 */ -- cgit v1.2.3