nfsd: fix nfsd_file reference leak in nfsd4_add_rdaccess_to_wrdeleg()

In the Linux kernel, the following vulnerability has been resolved:

nfsd: fix nfsd_file reference leak in nfsd4_add_rdaccess_to_wrdeleg()

nfsd4_add_rdaccess_to_wrdeleg() unconditionally overwrites

fp->fi_fds[O_RDONLY] with a newly acquired nfsd_file. However, if

the client already has a SHARE_ACCESS_READ open from a previous OPEN

operation, this action overwrites the existing pointer without

releasing its reference, orphaning the previous reference.

Additionally, the function originally stored the same nfsd_file

pointer in both fp->fi_fds[O_RDONLY] and fp->fi_rdeleg_file with

only a single reference. When put_deleg_file() runs, it clears

fi_rdeleg_file and calls nfs4_file_put_access() to release the file.

However, nfs4_file_put_access() only releases fi_fds[O_RDONLY] when

the fi_access[O_RDONLY] counter drops to zero. If another READ open

exists on the file, the counter remains elevated and the nfsd_file

reference from the delegation is never released. This potentially

causes open conflicts on that file.

Then, on server shutdown, these leaks cause __nfsd_file_cache_purge()

to encounter files with an elevated reference count that cannot be

cleaned up, ultimately triggering a BUG() in kmem_cache_destroy()

because there are still nfsd_file objects allocated in that cache.