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Subject[PATCH] userfaultfd: convert userfaultfd_ctx::refcount to refcount_t
Date
From: Eric Biggers <ebiggers@google.com>

Reference counters should use refcount_t rather than atomic_t, since the
refcount_t implementation can prevent overflows, reducing the
exploitability of reference leak bugs. userfaultfd_ctx::refcount is a
reference counter with the usual semantics, so convert it to refcount_t.

Note: I replaced the BUG() on incrementing a 0 refcount with just
refcount_inc(), since part of the semantics of refcount_t is that that
incrementing a 0 refcount is not allowed; with CONFIG_REFCOUNT_FULL,
refcount_inc() already checks for it and warns.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
fs/userfaultfd.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 356d2b8568c14..8375faac2790d 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -53,7 +53,7 @@ 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 */
@@ -140,8 +140,7 @@ 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))
- BUG();
+ refcount_inc(&ctx->refcount);
}

/**
@@ -154,7 +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));
@@ -686,7 +685,7 @@ 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;
@@ -1911,7 +1910,7 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
if (!ctx)
return -ENOMEM;

- atomic_set(&ctx->refcount, 1);
+ refcount_set(&ctx->refcount, 1);
ctx->flags = flags;
ctx->features = 0;
ctx->state = UFFD_STATE_WAIT_API;
--
2.19.1.930.g4563a0d9d0-goog
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