fuse: fix illegal access to inode with reused nodeid

commit 15db1683

 upstream.

Server responds to LOOKUP and other ops (READDIRPLUS/CREATE/MKNOD/...)
with ourarg containing nodeid and generation.

If a fuse inode is found in inode cache with the same nodeid but different
generation, the existing fuse inode should be unhashed and marked "bad" and
a new inode with the new generation should be hashed instead.

This can happen, for example, with passhrough fuse filesystem that returns
the real filesystem ino/generation on lookup and where real inode numbers
can get recycled due to real files being unlinked not via the fuse
passthrough filesystem.

With current code, this situation will not be detected and an old fuse
dentry that used to point to an older generation real inode, can be used to
access a completely new inode, which should be accessed only via the new
dentry.

Note that because the FORGET message carries the nodeid w/o generation, the
server should wait to get FORGET counts for the nlookup counts of the old
and reused inodes combined, before it can free the resources associated to
that nodeid.

Stable backport notes:
* This is not a regression. The bug has been in fuse forever, but only
  a certain class of low level fuse filesystems can trigger this bug
* Because there is no way to check if this fix is applied in runtime,
  libfuse test_examples.py tests this fix with hardcoded check for
  kernel version >= 5.14
* After backport to stable kernel(s), the libfuse test can be updated
  to also check minimal stable kernel version(s)
* Depends on "fuse: fix bad inode" which is already applied to stable
  kernels v5.4.y and v5.10.y
* Required backporting helper inode_wrong_type()

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxi8DymG=JO_sAU+wS8akFdzh+PuXwW3Ebgahd2Nwnh7zA@mail.gmail.com/


Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>