posix_acl: convert posix_acl.a_refcount 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 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@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>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --git a/fs/f2fs/acl.c b/fs/f2fs/acl.c
index 2bb7c9fc5144aba162b01ba72295994f9fd23f00..111824199a886c11f3a543463aeba0385a8a50ac 100644 (file)
--- 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 eebf5f6cf6d5645a128b9b3560ac9ce853a4bdc7..2fd0fde16fe17f56bec417090dcd6e10ce162259 100644 (file)
--- 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 b2b7255ec7f56b650a74da54fab184f2060a69d8..540595a321a762d305bea8bb1ffa6586a6df0585 100644 (file)
--- 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);
 }