lkml.org 
[lkml]   [2017]   [Oct]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH 4.13 102/160] userfaultfd: non-cooperative: fix fork use after free
Date
4.13-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andrea Arcangeli <aarcange@redhat.com>

commit 384632e67e0829deb8015ee6ad916b180049d252 upstream.

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.

Fixes: 893e26e61d04eac9 ("userfaultfd: non-cooperative: Add fork() event")
Link: http://lkml.kernel.org/r/20170920180413.26713-1-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/userfaultfd.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 56 insertions(+), 10 deletions(-)

--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -566,6 +566,12 @@ static void userfaultfd_event_wait_compl
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;
@@ -1039,6 +1045,12 @@ static ssize_t userfaultfd_ctx_read(stru
(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;
@@ -1069,19 +1081,53 @@ static ssize_t userfaultfd_ctx_read(stru

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;

\
 
 \ /
  Last update: 2017-10-10 22:23    [W:0.879 / U:0.176 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site