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 userfaultfd_ctx.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 userfaultfd_ctx.refcount it might make a difference
in following places:
- userfaultfd_ctx_get(): increment in refcount_inc_not_zero() only
guarantees control dependency on success vs. fully ordered
atomic counterpart
- userfaultfd_ctx_put(): decrement in refcount_dec_and_test() only
provides RELEASE ordering and control dependency on success
vs. fully ordered atomic counterpart
Suggested-by: Kees Cook <[email protected]>
Reviewed-by: David Windsor <[email protected]>
Reviewed-by: Hans Liljestrand <[email protected]>
Signed-off-by: Elena Reshetova <[email protected]>
---
fs/userfaultfd.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ac9a4e6..b63ebcac 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -29,6 +29,7 @@
#include <linux/ioctl.h>
#include <linux/security.h>
#include <linux/hugetlb.h>
+#include <linux/refcount.h>
static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
@@ -53,7 +54,7 @@ struct userfaultfd_ctx {
/* a refile sequence protected by fault_pending_wqh lock */
struct seqcount refile_seq;
/* pseudo fd refcounting */
- atomic_t refcount;
+ refcount_t refcount;
/* userfaultfd syscall flags */
unsigned int flags;
/* features requested from the userspace */
@@ -138,7 +139,7 @@ static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
*/
static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
{
- if (!atomic_inc_not_zero(&ctx->refcount))
+ if (!refcount_inc_not_zero(&ctx->refcount))
BUG();
}
@@ -152,7 +153,7 @@ static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
*/
static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
{
- if (atomic_dec_and_test(&ctx->refcount)) {
+ if (refcount_dec_and_test(&ctx->refcount)) {
VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
VM_BUG_ON(waitqueue_active(&ctx->fault_pending_wqh));
VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
@@ -662,7 +663,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
return -ENOMEM;
}
- atomic_set(&ctx->refcount, 1);
+ refcount_set(&ctx->refcount, 1);
ctx->flags = octx->flags;
ctx->state = UFFD_STATE_RUNNING;
ctx->features = octx->features;
@@ -1902,7 +1903,7 @@ static struct file *userfaultfd_file_create(int flags)
if (!ctx)
goto out;
- atomic_set(&ctx->refcount, 1);
+ refcount_set(&ctx->refcount, 1);
ctx->flags = flags;
ctx->features = 0;
ctx->state = UFFD_STATE_WAIT_API;
--
2.7.4
From 1589701425171756377@xxx Mon Jan 15 23:03:39 +0000 2018
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