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 posix_acl.a_refcount 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 posix_acl.a_refcount it might make a difference in following places: - get_cached_acl(): increment in refcount_inc_not_zero() only guarantees control dependency on success vs. fully ordered atomic counterpart. However this operation is performed under rcu_read_lock(), so this should be fine. - posix_acl_release(): 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/f2fs/acl.c | 2 +- fs/posix_acl.c | 6 +++--- include/linux/posix_acl.h | 7 ++++--- 3 files changed, 8 insertions(+), 7 deletions(-) diff --git a/fs/f2fs/acl.c b/fs/f2fs/acl.c index 2bb7c9f..1118241 100644 --- a/fs/f2fs/acl.c +++ b/fs/f2fs/acl.c @@ -270,7 +270,7 @@ static struct posix_acl *f2fs_acl_clone(const struct posix_acl *acl, sizeof(struct posix_acl_entry); clone = kmemdup(acl, size, flags); if (clone) - atomic_set(&clone->a_refcount, 1); + refcount_set(&clone->a_refcount, 1); } return clone; } diff --git a/fs/posix_acl.c b/fs/posix_acl.c index eebf5f6..2fd0fde 100644 --- a/fs/posix_acl.c +++ b/fs/posix_acl.c @@ -43,7 +43,7 @@ struct posix_acl *get_cached_acl(struct inode *inode, int type) rcu_read_lock(); acl = rcu_dereference(*p); if (!acl || is_uncached_acl(acl) || - atomic_inc_not_zero(&acl->a_refcount)) + refcount_inc_not_zero(&acl->a_refcount)) break; rcu_read_unlock(); cpu_relax(); @@ -164,7 +164,7 @@ EXPORT_SYMBOL(get_acl); void posix_acl_init(struct posix_acl *acl, int count) { - atomic_set(&acl->a_refcount, 1); + refcount_set(&acl->a_refcount, 1); acl->a_count = count; } EXPORT_SYMBOL(posix_acl_init); @@ -197,7 +197,7 @@ posix_acl_clone(const struct posix_acl *acl, gfp_t flags) sizeof(struct posix_acl_entry); clone = kmemdup(acl, size, flags); if (clone) - atomic_set(&clone->a_refcount, 1); + refcount_set(&clone->a_refcount, 1); } return clone; } diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h index b2b7255..540595a 100644 --- a/include/linux/posix_acl.h +++ b/include/linux/posix_acl.h @@ -12,6 +12,7 @@ #include <linux/bug.h> #include <linux/slab.h> #include <linux/rcupdate.h> +#include <linux/refcount.h> #include <uapi/linux/posix_acl.h> struct posix_acl_entry { @@ -24,7 +25,7 @@ struct posix_acl_entry { }; struct posix_acl { - atomic_t a_refcount; + refcount_t a_refcount; struct rcu_head a_rcu; unsigned int a_count; struct posix_acl_entry a_entries[0]; @@ -41,7 +42,7 @@ static inline struct posix_acl * posix_acl_dup(struct posix_acl *acl) { if (acl) - atomic_inc(&acl->a_refcount); + refcount_inc(&acl->a_refcount); return acl; } @@ -51,7 +52,7 @@ posix_acl_dup(struct posix_acl *acl) static inline void posix_acl_release(struct posix_acl *acl) { - if (acl && atomic_dec_and_test(&acl->a_refcount)) + if (acl && refcount_dec_and_test(&acl->a_refcount)) kfree_rcu(acl, a_rcu); } -- 2.7.4
