nfs: use nfsi->rwsem to protect traversal of the file lock list

Overview

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

nfs: use nfsi->rwsem to protect traversal of the file lock list

Lingfeng identified a bug and suggested two solutions, but both appear to have issues.

Generally, we cannot release flc_lock while iterating over the file lock list to avoid use-after-free (UAF) problems with file locks. However, functions like nfs_delegation_claim_locks and nfs4_reclaim_locks cannot adhere to this rule because recover_lock or nfs4_lock_delegation_recall may take a long time. To resolve this, NFS switches to using nfsi->rwsem for the same protection, and nfs_reclaim_locks follows this approach. Although nfs_delegation_claim_locks uses so_delegreturn_mutex instead, this is inadequate since a single inode can have multiple nfs4_state instances. Therefore, the fix is to also use nfsi->rwsem in this case.

Furthermore, after commit c69899a17ca4 ("NFSv4: Update of VFS byte range lock must be atomic with the stateid update"), the functions nfs4_locku_done and nfs4_lock_done also break this rule because they call locks_lock_inode_wait without holding nfsi->rwsem. Simply adding this protection could cause many deadlocks, so instead, the call to locks_lock_inode_wait is moved into _nfs4_proc_setlk. Regarding the bug fixed by commit c69899a17ca4 ("NFSv4: Update of VFS byte range lock must be atomic with the stateid update"), it has been resolved after commit 0460253913e5 ("NFSv4: nfs4_do_open() is incorrectly triggering state recovery") because all slots are drained before calling nfs4_do_reclaim, which prevents concurrent stateid changes along this path. Also, nfs_delegation_claim_locks does not cause this concurrency either since when _nfs4_proc_setlk is called with NFS_DELEGATED_STATE, no RPC is sent, so nfs4_lock_done is not called. Therefore, nfs4_lock_delegation_recall from nfs_delegation_claim_locks is the first time the stateid is set.

Affected

• Linux >= c69899a17ca4836230720e65493942d9582a0424 < 1cda95bf2e9c0e6b63545b7565fe4a1e474322f4
• Linux >= c69899a17ca4836230720e65493942d9582a0424 < f161ef7b0dd2f51fdb002ba4a4e9bf0ee7409218
• Linux >= c69899a17ca4836230720e65493942d9582a0424 < e68035178e65e2b3aa386ce61202ff33724ae418
• Linux >= c69899a17ca4836230720e65493942d9582a0424 < 4837fb36219e6c08b666bc31a86841bad8526358
• Linux 4.0

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.