nfsd: fix handling of cached open files in nfsd4_open codepath

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/45c08a752982116f3287afcd1bd9c50f4fab0c28
• https://git.kernel.org/stable/c/0b778361998d6c6356b8d2fc7ddf025fb3224654
• https://git.kernel.org/stable/c/973acfdfe90c8a4e58ade97ff0653a498531ff2e
• https://git.kernel.org/stable/c/0b3a551fa58b4da941efeb209b3770868e2eddd7