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 mnt_namespace.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 mnt_namespace.count it might make a difference in following places: - put_mnt_ns(): decrement in refcount_dec_and_test() only provides RELEASE ordering and control dependency on success vs. fully ordered atomic counterpart 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/mount.h | 5 +++-- fs/namespace.c | 4 ++-- 2 files changed, 5 insertions(+), 4 deletions(-) diff --git a/fs/mount.h b/fs/mount.h index f39bc9d..e28e76d 100644 --- a/fs/mount.h +++ b/fs/mount.h @@ -4,9 +4,10 @@ #include <linux/poll.h> #include <linux/ns_common.h> #include <linux/fs_pin.h> +#include <linux/refcount.h> struct mnt_namespace { - atomic_t count; + refcount_t count; struct ns_common ns; struct mount * root; struct list_head list; @@ -112,7 +113,7 @@ static inline void detach_mounts(struct dentry *dentry) static inline void get_mnt_ns(struct mnt_namespace *ns) { - atomic_inc(&ns->count); + refcount_inc(&ns->count); } extern seqlock_t mount_lock; diff --git a/fs/namespace.c b/fs/namespace.c index 7c421e0..848e9b0 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -2895,7 +2895,7 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns) } new_ns->ns.ops = &mntns_operations; new_ns->seq = atomic64_add_return(1, &mnt_ns_seq); - atomic_set(&new_ns->count, 1); + refcount_set(&new_ns->count, 1); new_ns->root = NULL; INIT_LIST_HEAD(&new_ns->list); init_waitqueue_head(&new_ns->poll); @@ -3279,7 +3279,7 @@ void __init mnt_init(void) void put_mnt_ns(struct mnt_namespace *ns) { - if (!atomic_dec_and_test(&ns->count)) + if (!refcount_dec_and_test(&ns->count)) return; drop_collected_mounts(&ns->root->mnt); free_mnt_ns(ns); -- 2.7.4
