CVE-2024-56558

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

nfsd: make sure exp active before svc_export_show

The function e_show was called with protection from RCU. This only ensures that exp will not be freed. Therefore, the reference count for exp can drop to zero, which will trigger a refcount use-after-free warning when exp_get is called. To resolve this issue, use cache_get_rcu to ensure that exp remains active.

------------[ cut here ]------------ refcount_t: addition on 0; use-after-free. WARNING: CPU: 3 PID: 819 at lib/refcount.c:25 refcount_warn_saturate+0xb1/0x120 CPU: 3 UID: 0 PID: 819 Comm: cat Not tainted 6.12.0-rc3+ #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014 RIP: 0010:refcount_warn_saturate+0xb1/0x120 ... Call Trace: <TASK> e_show+0x20b/0x230 [nfsd] seq_read_iter+0x589/0x770 seq_read+0x1e5/0x270 vfs_read+0x125/0x530 ksys_read+0xc1/0x160 do_syscall_64+0x5f/0x170 entry_SYSCALL_64_after_hwframe+0x76/0x7e

This vulnerability is rated as Moderate impact because exploitation requires local access and applies exclusively to hosts actively configured as NFS servers. In standard Red Hat Enterprise Linux deployments, the kernel NFS server daemon is not enabled by default, restricting exposure to systems intentionally hosting NFS export shares. An authenticated local user must execute specific read operations against export pseudo-files while cache entries expire to trigger the reference count mismatch, primarily risking kernel instability.

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.