CVE-2024-50121

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

nfsd: cancel nfsd_shrinker_work using sync mode in nfs4_state_shutdown_net

In the normal case, when we excute echo 0 > /proc/fs/nfsd/threads, the function nfs4_state_destroy_net in nfs4_state_shutdown_net will release all resources related to the hashed nfs4_client. If the nfsd_client_shrinker is running concurrently, the expire_client function will first unhash this client and then destroy it. This can lead to the following warning. Additionally, numerous use-after-free errors may occur as well.

nfsd_client_shrinker echo 0 > /proc/fs/nfsd/threads

expire_client nfsd_shutdown_net unhash_client ... nfs4_state_shutdown_net /* won't wait shrinker exit / / cancel_work(&nn->nfsd_shrinker_work)

• nfsd_file for this /* won't destroy unhashed client1 */

• client1 still alive nfs4_state_destroy_net */

                        nfsd_file_cache_shutdown
                          /* trigger warning */
                          kmem_cache_destroy(nfsd_file_slab)
                          kmem_cache_destroy(nfsd_file_mark_slab)
  

/* release nfsd_file and mark */ __destroy_client

==================================================================== BUG nfsd_file (Not tainted): Objects remaining in nfsd_file on __kmem_cache_shutdown()

CPU: 4 UID: 0 PID: 764 Comm: sh Not tainted 6.12.0-rc3+ #1

dump_stack_lvl+0x53/0x70 slab_err+0xb0/0xf0 __kmem_cache_shutdown+0x15c/0x310 kmem_cache_destroy+0x66/0x160 nfsd_file_cache_shutdown+0xac/0x210 [nfsd] nfsd_destroy_serv+0x251/0x2a0 [nfsd] nfsd_svc+0x125/0x1e0 [nfsd] write_threads+0x16a/0x2a0 [nfsd] nfsctl_transaction_write+0x74/0xa0 [nfsd] vfs_write+0x1a5/0x6d0 ksys_write+0xc1/0x160 do_syscall_64+0x5f/0x170 entry_SYSCALL_64_after_hwframe+0x76/0x7e

==================================================================== BUG nfsd_file_mark (Tainted: G B W ): Objects remaining nfsd_file_mark on __kmem_cache_shutdown()

dump_stack_lvl+0x53/0x70 slab_err+0xb0/0xf0 __kmem_cache_shutdown+0x15c/0x310 kmem_cache_destroy+0x66/0x160 nfsd_file_cache_shutdown+0xc8/0x210 [nfsd] nfsd_destroy_serv+0x251/0x2a0 [nfsd] nfsd_svc+0x125/0x1e0 [nfsd] write_threads+0x16a/0x2a0 [nfsd] nfsctl_transaction_write+0x74/0xa0 [nfsd] vfs_write+0x1a5/0x6d0 ksys_write+0xc1/0x160 do_syscall_64+0x5f/0x170 entry_SYSCALL_64_after_hwframe+0x76/0x7e

To resolve this issue, cancel nfsd_shrinker_work using synchronous mode in nfs4_state_shutdown_net.

This vulnerability is rated as Moderate severity because exploiting the race condition requires administrative privileges to initiate NFS server shutdown alongside concurrent kernel memory pressure, restricting potential impact to local users with elevated permissions. In typical Red Hat Enterprise Linux environments, the kernel NFS server is not enabled by default and is only exposed when explicitly configured. Furthermore, successful triggering requires precise timing during service termination rather than interactions through standard remote NFS client traffic.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this issue, prevent module nfsd from being loaded. Please see https://access.redhat.com/solutions/41278 for how to blacklist a kernel module to prevent it from loading automatically.