fuse: Fix use-after-free in fuse_dev_do_read()

We may pick freed req in this way:

[cpu0]                                  [cpu1]
fuse_dev_do_read()                      fuse_dev_do_write()
   list_move_tail(&req->list, ...);     ...
   spin_unlock(&fpq->lock);             ...
   ...                                  request_end(fc, req);
   ...                                    fuse_put_request(fc, req);
   if (test_bit(FR_INTERRUPTED, ...))
         queue_interrupt(fiq, req);

Fix that by keeping req alive until we finish all manipulations.

Reported-by: syzbot+4e975615ca01f2277bdd@syzkaller.appspotmail.com
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 46c34a348b0a ("fuse: no fc->lock for pqueue parts")
Cc: <stable@vger.kernel.org> # v4.2

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 11ea2c4a38ab..675caed3e655 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1311,12 +1311,14 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
                 goto out_end;
         }
         list_move_tail(&req->list, &fpq->processing);
+        __fuse_get_request(req);
         spin_unlock(&fpq->lock);
         set_bit(FR_SENT, &req->flags);
         /* matches barrier in request_wait_answer() */
         smp_mb__after_atomic();
         if (test_bit(FR_INTERRUPTED, &req->flags))
                 queue_interrupt(fiq, req);
+        fuse_put_request(fc, req);
 
         return reqsize;