lockd: fix TEST handling when not all permissions are available.

The F_GETLK fcntl can work with either read access or write access or
both.  It can query F_RDLCK and F_WRLCK locks in either case.

However lockd currently treats F_GETLK similar to F_SETLK in that read
access is required to query an F_RDLCK lock and write access is required
to query a F_WRLCK lock.

This is wrong and can cause problems - e.g.  when qemu accesses a
read-only (e.g. iso) filesystem image over NFS (though why it queries
if it can get a write lock - I don't know.  But it does, and this works
with local filesystems).

So we need TEST requests to be handled differently.  To do this:

- change nlm_do_fopen() to accept O_RDWR as a mode and in that case
  succeed if either a O_RDONLY or O_WRONLY file can be opened.
- change nlm_lookup_file() to accept a mode argument from caller,
  instead of deducing base on lock time, and pass that on to nlm_do_fopen()
- change nlm4svc_retrieve_args() and nlmsvc_retrieve_args() to detect
  TEST requests and pass O_RDWR as a mode to nlm_lookup_file, passing
  the same mode as before for other requests.  Also set
   lock->fl.c.flc_file to whichever file is available for TEST requests.
- change nlmsvc_testlock() to also not calculate the mode, but to use
  whatever was stored in lock->fl.c.flc_file.

This behaviour of lockd - requesting O_WRONLY access to TEST for
exclusive locks - has been present at least since git history began.
However it was hidden until recently because knfsd ignored the access
requested by lockd and required only READ access for all locking
requests (unless the underlying filesystem provided an f_op->open
function which checked access permissions).

The commit mentioned in Fixes: below changed nfsd_permission() to NOT
override the access request for LOCK requests and this exposed the bug
that we are now fixing.

Note that there is another issue that this patch does not address.
The flock(.., LOCK_EX) call is permitted on a read-only file descriptor.
Linux NFS maps this to NLM locking as whole-file byte-range locks.
nfsd will see this as though it were fcntl( F_SETLK (F_WRLCK)) and will
now require write access, which it might not be able to get.
It is not clear if this is a problem in practice, or what the best
solution might be.  So no attempt is made to address it.

Reported-by: Tj <tj.iam.tj@proton.me>
Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1128861
Fixes: 4cc9b9f2bf4d ("nfsd: refine and rename NFSD_MAY_LOCK")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

0 files changed, 0 insertions, 0 deletions