[PATCH 2/3] ceph: fix use-after-free in ceph_readdir

["Luís Henriques" <lhenriques@suse.de>]

After calling ceph_mdsc_put_request() on dfi->last_readdir, this field
should be set to NULL, otherwise we may end-up freeing it twince and get
the following splat:

  refcount_t: underflow; use-after-free.
  WARNING: CPU: 0 PID: 229 at lib/refcount.c:28 refcount_warn_saturate+0xa6/0xf0
  ...
  Call Trace:
    <TASK>
    ceph_readdir+0xd35/0x1460 [ceph]
    ? _raw_spin_unlock+0x12/0x30
    ? preempt_count_add+0x73/0xa0
    ? _raw_spin_unlock+0x12/0x30
    ? __mark_inode_dirty+0x27c/0x3a0
    iterate_dir+0x7d/0x190
    __x64_sys_getdents64+0x80/0x120
    ? compat_fillonedir+0x160/0x160
    do_syscall_64+0x43/0x90
    entry_SYSCALL_64_after_hwframe+0x44/0xae

Signed-off-by: Luís Henriques <lhenriques@suse.de>
---
 fs/ceph/dir.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
index 0bcb677d2199..934402f5e9e6 100644
--- a/fs/ceph/dir.c
+++ b/fs/ceph/dir.c
@@ -555,6 +555,7 @@ static int ceph_readdir(struct file *file, struct dir_context *ctx)
 			      le32_to_cpu(rde->inode.in->mode) >> 12)) {
 			dout("filldir stopping us...\n");
 			ceph_mdsc_put_request(dfi->last_readdir);
+			dfi->last_readdir = NULL;
 			err = 0;
 			goto out;
 		}