CVE-2023-52909

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

nfsd: fix handling of cached open files in nfsd4_open codepath

Commit fb70bf124b05 ("NFSD: Instantiate a struct file when creating a

regular NFSv4 file") added the ability to cache an open fd over a

compound. There are a couple of problems with the way this currently

works:

It's racy, as a newly-created nfsd_file can end up with its PENDING bit

cleared while the nf is hashed, and the nf_file pointer is still zeroed

out. Other tasks can find it in this state and they expect to see a

valid nf_file, and can oops if nf_file is NULL.

Also, there is no guarantee that we'll end up creating a new nfsd_file

if one is already in the hash. If an extant entry is in the hash with a

valid nf_file, nfs4_get_vfs_file will clobber its nf_file pointer with

the value of op_file and the old nf_file will leak.

Fix both issues by making a new nfsd_file_acquirei_opened variant that

takes an optional file pointer. If one is present when this is called,

we'll take a new reference to it instead of trying to open the file. If

the nfsd_file already has a valid nf_file, we'll just ignore the

optional file and pass the nfsd_file back as-is.

Also rework the tracepoints a bit to allow for an "opened" variant and

don't try to avoid counting acquisitions in the case where we already

have a cached open file.