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Subject[PATCH 1/1] userfaultfd: non-cooperative: fix fork use after free
Date
When reading the event from the uffd, we put it on a temporary
fork_event list to detect if we can still access it after releasing
and retaking the event_wqh.lock.

If fork aborts and removes the event from the fork_event all is fine
as long as we're still in the userfault read context and fork_event
head is still alive.

We've to put the event allocated in the fork kernel stack, back from
fork_event list-head to the event_wqh head, before returning from
userfaultfd_ctx_read, because the fork_event head lifetime is limited
to the userfaultfd_ctx_read stack lifetime.

Forgetting to move the event back to its event_wqh place then results
in __remove_wait_queue(&ctx->event_wqh, &ewq->wq); in
userfaultfd_event_wait_completion to remove it from a head that has
been already freed from the reader stack.

This could only happen if resolve_userfault_fork failed (for example
if there are no file descriptors available to allocate the fork
uffd). If it succeeded it was put back correctly.

Furthermore, after find_userfault_evt receives a fork event, the
forked userfault context in fork_nctx and
uwq->msg.arg.reserved.reserved1 can be released by the fork thread as
soon as the event_wqh.lock is released. Taking a reference on the
fork_nctx before dropping the lock prevents an use after free in
resolve_userfault_fork().

If the fork side aborted and it already released everything, we still
try to succeed resolve_userfault_fork(), if possible.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
fs/userfaultfd.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 56 insertions(+), 10 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 06d6cfda1e8e..16366587e579 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -599,6 +599,12 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
break;
if (ACCESS_ONCE(ctx->released) ||
fatal_signal_pending(current)) {
+ /*
+ * &ewq->wq may be queued in fork_event, but
+ * __remove_wait_queue ignores the head
+ * parameter. It would be a problem if it
+ * didn't.
+ */
__remove_wait_queue(&ctx->event_wqh, &ewq->wq);
if (ewq->msg.event == UFFD_EVENT_FORK) {
struct userfaultfd_ctx *new;
@@ -1072,6 +1078,12 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
(unsigned long)
uwq->msg.arg.reserved.reserved1;
list_move(&uwq->wq.entry, &fork_event);
+ /*
+ * fork_nctx can be freed as soon as
+ * we drop the lock, unless we take a
+ * reference on it.
+ */
+ userfaultfd_ctx_get(fork_nctx);
spin_unlock(&ctx->event_wqh.lock);
ret = 0;
break;
@@ -1102,19 +1114,53 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,

if (!ret && msg->event == UFFD_EVENT_FORK) {
ret = resolve_userfault_fork(ctx, fork_nctx, msg);
+ spin_lock(&ctx->event_wqh.lock);
+ if (!list_empty(&fork_event)) {
+ /*
+ * The fork thread didn't abort, so we can
+ * drop the temporary refcount.
+ */
+ userfaultfd_ctx_put(fork_nctx);
+
+ uwq = list_first_entry(&fork_event,
+ typeof(*uwq),
+ wq.entry);
+ /*
+ * If fork_event list wasn't empty and in turn
+ * the event wasn't already released by fork
+ * (the event is allocated on fork kernel
+ * stack), put the event back to its place in
+ * the event_wq. fork_event head will be freed
+ * as soon as we return so the event cannot
+ * stay queued there no matter the current
+ * "ret" value.
+ */
+ list_del(&uwq->wq.entry);
+ __add_wait_queue(&ctx->event_wqh, &uwq->wq);

- if (!ret) {
- spin_lock(&ctx->event_wqh.lock);
- if (!list_empty(&fork_event)) {
- uwq = list_first_entry(&fork_event,
- typeof(*uwq),
- wq.entry);
- list_del(&uwq->wq.entry);
- __add_wait_queue(&ctx->event_wqh, &uwq->wq);
+ /*
+ * Leave the event in the waitqueue and report
+ * error to userland if we failed to resolve
+ * the userfault fork.
+ */
+ if (likely(!ret))
userfaultfd_event_complete(ctx, uwq);
- }
- spin_unlock(&ctx->event_wqh.lock);
+ } else {
+ /*
+ * Here the fork thread aborted and the
+ * refcount from the fork thread on fork_nctx
+ * has already been released. We still hold
+ * the reference we took before releasing the
+ * lock above. If resolve_userfault_fork
+ * failed we've to drop it because the
+ * fork_nctx has to be freed in such case. If
+ * it succeeded we'll hold it because the new
+ * uffd references it.
+ */
+ if (ret)
+ userfaultfd_ctx_put(fork_nctx);
}
+ spin_unlock(&ctx->event_wqh.lock);
}

return ret;
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