nsproxy: Convert nsproxy.count 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 nsproxy.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 refcount.h have different
memory ordering guarantees than their atomic counterparts. Please check
Documentation/core-api/refcount-vs-atomic.rst for more information.

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 nsproxy.count it might make a difference in following places:
 - put_nsproxy() and switch_task_namespaces(): decrement in
   refcount_dec_and_test() only provides RELEASE ordering and ACQUIRE
   ordering on success vs. fully ordered atomic counterpart

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230818041327.gonna.210-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
index fee881cded012c9b7e439697069845d2ba2decad..771cb028587242a55509e4aa99684f61b0598f03 100644 (file)
--- a/include/linux/nsproxy.h
+++ b/include/linux/nsproxy.h
@@ -29,7 +29,7 @@ struct fs_struct;
  * nsproxy is copied.
  */
 struct nsproxy {
-       atomic_t count;
+       refcount_t count;
        struct uts_namespace *uts_ns;
        struct ipc_namespace *ipc_ns;
        struct mnt_namespace *mnt_ns;
@@ -102,14 +102,13 @@ int __init nsproxy_cache_init(void);
 
 static inline void put_nsproxy(struct nsproxy *ns)
 {
-       if (atomic_dec_and_test(&ns->count)) {
+       if (refcount_dec_and_test(&ns->count))
                free_nsproxy(ns);
-       }
 }
 
 static inline void get_nsproxy(struct nsproxy *ns)
 {
-       atomic_inc(&ns->count);
+       refcount_inc(&ns->count);
 }
 
 #endif

diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index 80d9c6d77a452648045c7dfd21e375ca01b46a3c..15781acaac1ceec97fa2ae649d284c025693186e 100644 (file)
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -30,7 +30,7 @@
 static struct kmem_cache *nsproxy_cachep;
 
 struct nsproxy init_nsproxy = {
-       .count                  = ATOMIC_INIT(1),
+       .count                  = REFCOUNT_INIT(1),
        .uts_ns                 = &init_uts_ns,
 #if defined(CONFIG_POSIX_MQUEUE) || defined(CONFIG_SYSVIPC)
        .ipc_ns                 = &init_ipc_ns,
@@ -55,7 +55,7 @@ static inline struct nsproxy *create_nsproxy(void)
 
        nsproxy = kmem_cache_alloc(nsproxy_cachep, GFP_KERNEL);
        if (nsproxy)
-               atomic_set(&nsproxy->count, 1);
+               refcount_set(&nsproxy->count, 1);
        return nsproxy;
 }