kNFSD Signed Filehandles

The following series enables the linux NFS server to add a Message Authentication Code (MAC) to the filehandles it gives to clients. This provides additional protection to the exported filesystem against filehandle guessing attacks.

Filesystems generate their own filehandles through the export_operation "encode_fh" and a filehandle provides sufficient access to open a file without needing to perform a lookup. A trusted NFS client holding a valid filehandle can remotely access the corresponding file without reference to access-path restrictions that might be imposed by the ancestor directories or the server exports.

In order to acquire a filehandle, you must perform lookup operations on the parent directory(ies), and the permissions on those directories may prohibit you from walking into them to find the files within. This would normally be considered sufficient protection on a local filesystem to prohibit users from accessing those files, however when the filesystem is exported via NFS an exported file can be accessed whenever the NFS server is presented with the correct filehandle, which can be guessed or acquired by means other than LOOKUP.

Filehandles are easy to guess because they are well-formed. The open_by_handle_at(2) man page contains an example C program (t_name_to_handle_at.c) that can display a filehandle given a path. Here's an example filehandle from a fairly modern XFS:

# ./t_name_to_handle_at /exports/foo 
57
12 129    99 00 00 00 00 00 00 00 b4 10 0b 8c

          ^---------  filehandle  ----------^
          ^------- inode -------^ ^-- gen --^

This filehandle consists of a 64-bit inode number and 32-bit generation number. Because the handle is well-formed, its easy to fabricate filehandles that match other files within the same filesystem. You can simply insert inode numbers and iterate on the generation number. Eventually you'll be able to access the file using open_by_handle_at(2). For a local system, open_by_handle_at(2) requires CAP_DAC_READ_SEARCH, which protects against guessing attacks by unprivileged users.

Simple testing confirms that the correct generation number can be found within ~1200 minutes using open_by_handle_at() over NFS on a local system and it is estimated that adding network delay with genuine NFS calls may only increase this to around 24 hours.

In contrast to a local user using open_by_handle(2), the NFS server must permissively allow remote clients to open by filehandle without being able to check or trust the remote caller's access. Therefore additional protection against this attack is needed for NFS case. We propose to sign filehandles by appending an 8-byte MAC which is the siphash of the filehandle from a key set from the nfs-utilities. NFS server can then ensure that guessing a valid filehandle+MAC is practically impossible without knowledge of the MAC's key. The NFS server performs optional signing by possessing a key set from userspace and having the "sign_fh" export option.

Because filehandles are long-lived, and there's no method for expiring them, the server's key should be set once and not changed. It also should be persisted across restarts. The methods to set the key allow only setting it once, afterward it cannot be changed. A separate patchset for nfs-utils contains the userspace changes required to set the server's key.

I had planned on adding additional work to enable the server to check whether the 8-byte MAC will overflow maximum filehandle length for the protocol at export time. There could be some filesystems with 40-byte fileid and 24-byte fsid which would break NFSv3's 64-byte filehandle maximum with an 8-byte MAC appended. The server should refuse to export those filesystems when "sign_fh" is requested. However, the way the export caches work (the server may not even be running when a user sets up the export) its impossible to do this check at export time. Instead, the server will refuse to give out filehandles at mount time and emit a pr_warn().

Thanks for any comments and critique.

Changes from encrypt_fh posting:
https://lore.kernel.org/linux-nfs/[email protected]
- sign filehandles instead of encrypt them (Eric Biggers)
- fix the NFSEXP_ macros, specifically NFSEXP_ALLFLAGS (NeilBrown)
- rebase onto cel/nfsd-next (Chuck Lever)
- condensed/clarified problem explantion (thanks Chuck Lever)
- add nfsctl file "fh_key" for rpc.nfsd to also set the key

Changes from v1 posting:
https://lore.kernel.org/linux-nfs/[email protected]
- remove fh_fileid_offset() (Chuck Lever)
- fix pr_warns, fix memcmp (Chuck Lever)
- remove incorrect rootfh comment (NeilBrown)
- make fh_key setting an optional attr to threads verb (Jeff Layton)
- drop BIT() EXP_ flag conversion
- cover-letter tune-ups (NeilBrown, Chuck Lever)
- fix NFSEXP_ALLFLAGS on 2/3
- cast fh->fh_size + sizeof(hash) result to int (avoid x86_64 WARNING)
- move MAC signing into __fh_update() (Chuck Lever)

Changes from v2 posting:
https://lore.kernel.org/linux-nfs/[email protected]
- more cover-letter detail (NeilBrown)
- Documentation/filesystems/nfs/exporting.rst section (Jeff Layton)
- fix key copy (Eric Biggers)
- use NFSD_A_SERVER_MAX (NeilBrown)
- remove procfs fh_key interface (Chuck Lever)
- remove FH_AT_MAC (Chuck Lever)
- allow fh_key change when server is not running (Chuck/Jeff)
- accept fh_key as netlink attribute instead of command (Jeff Layton)

Changes from v3 posting:
https://lore.kernel.org/linux-nfs/[email protected]
- /actually/ fix up endianness problems (Eric Biggers)
- comment typo
- fix Documentation underline warnings
- fix possible uninitialized fh_key var

Changes from v4 posting:
https://lore.kernel.org/linux-nfs/[email protected]
- again (!!) fix endian copy from userspace (Chuck Lever)
- fixup protocol return error for MAC verification failure (Chuck Lever)
- fix filehandle size after MAC verification (Chuck Lever)
- fix two sparse errors (LKP)

Changes from v5 posting:
https://lore.kernel.org/linux-nfs/[email protected]
- fixup 3/3 commit message to match code return _STALE (Chuck Lever)
- convert fh sign functions to return bool (Chuck Lever)
- comment for FILEID_ROOT always unsigned (Chuck Lever)
- tracepoint error value match return -ESTALE (Chuck Lever)
- fix a fh data_left bug (Chuck Lever)
- symbolize size of signing value in words (Chuck Lever)
- 3/3 add simple rational for choice of hash (Chuck Lever)
- fix an incorrect error return leak introduced on v5
- remove a duplicate include (Chuck Lever)
- inform callers of nfsd_nl_fh_key_set of shutdown req (Chuck Lever)
- hash key in tracepoint output (Chuck Lever)

Changes from v6 posting:
https://lore.kernel.org/linux-nfs/[email protected]
- rebase onto current cel/nfsd-testing, take maintainer changes
- move Kconfig "select CRYPTO" from NFSD_V4 to NFSD for crypto_memneq()

Benjamin Coddington (3):
NFSD: Add a key for signing filehandles
NFSD/export: Add sign_fh export option
NFSD: Sign filehandles

 Documentation/filesystems/nfs/exporting.rst | 85 +++++++++++++++++++++
 Documentation/netlink/specs/nfsd.yaml       |  6 ++
 fs/nfsd/Kconfig                             |  2 +-
 fs/nfsd/export.c                            |  5 +-
 fs/nfsd/netlink.c                           |  5 +-
 fs/nfsd/netns.h                             |  1 +
 fs/nfsd/nfsctl.c                            | 38 ++++++++-
 fs/nfsd/nfsfh.c                             | 74 +++++++++++++++++-
 fs/nfsd/trace.h                             | 23 ++++++
 include/uapi/linux/nfsd/export.h            |  4 +-
 include/uapi/linux/nfsd_netlink.h           |  1 +
 11 files changed, 232 insertions(+), 12 deletions(-)

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