nsfs: mark dentry with DCACHE_RCUACCESS

Andrey reported a use-after-free in __ns_get_path():

  spin_lock include/linux/spinlock.h:299 [inline]
  lockref_get_not_dead+0x19/0x80 lib/lockref.c:179
  __ns_get_path+0x197/0x860 fs/nsfs.c:66
  open_related_ns+0xda/0x200 fs/nsfs.c:143
  sock_ioctl+0x39d/0x440 net/socket.c:1001
  vfs_ioctl fs/ioctl.c:45 [inline]
  do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:685
  SYSC_ioctl fs/ioctl.c:700 [inline]
  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691

We are under rcu read lock protection at that point:

        rcu_read_lock();
        d = atomic_long_read(&ns->stashed);
        if (!d)
                goto slow;
        dentry = (struct dentry *)d;
        if (!lockref_get_not_dead(&dentry->d_lockref))
                goto slow;
        rcu_read_unlock();

but don't use a proper RCU API on the free path, therefore a parallel
__d_free() could free it at the same time.  We need to mark the stashed
dentry with DCACHE_RCUACCESS so that __d_free() will be called after all
readers leave RCU.

Fixes: e149ed2b805f ("take the targets of /proc/*/ns/* symlinks to separate fs")
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 1656843e87d2bef47b69d65b23c23946e8936f33..323f492e0822dd3286365d5cdbe56c59ec2d5463 100644 (file)
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -91,6 +91,7 @@ slow:
                return ERR_PTR(-ENOMEM);
        }
        d_instantiate(dentry, inode);
+       dentry->d_flags |= DCACHE_RCUACCESS;
        dentry->d_fsdata = (void *)ns->ops;
        d = atomic_long_cmpxchg(&ns->stashed, 0, (unsigned long)dentry);
        if (d) {