CVE-2023-52751 is a use-after-free (UAF) vulnerability identified in the Linux kernel's CIFS (Common Internet File System) client implementation, specifically within the smb2_query_info_compound() function. This vulnerability arises due to a race condition between the open_cached_dir() and cached_dir_lease_break() functions. The issue manifests when the cache entry for an open directory handle receives a lease break while it is being created. Before open_cached_dir() returns, it may incorrectly release the last reference to the new cached file identifier (cfid) if the lease status (@cfid->has_lease) is not properly set, leading to a use-after-free condition. This flaw was detected during testing with KASAN (Kernel Address Sanitizer) enabled, particularly when mounting Windows Server 2022 shares with specific mount options (multichannel, max_channels=2, SMB version 3.1.1, mfsymlinks, noperm). The kernel logs show slab-use-after-free errors triggered by concurrent statfs(2) system calls, indicating that multiple threads accessing cached directory handles can cause the vulnerability to surface. The root cause is that lease breaks can be missed or processed incorrectly due to improper synchronization and reference counting in the caching mechanism. The fix involves ensuring that the lease flag (@cfid->has_lease) is set before sending compounded SMB requests, allowing the demultiplex thread to process any lease breaks while the cfid is still cached. Additionally, if the open operation fails, the code rechecks the lease status to decide whether to release the lease reference, preventing premature freeing of resources. This vulnerability affects Linux kernel versions containing the affected commit (1da177e4c3f41524e886b7f1b8a0c1fc7321cac2) and potentially other versions with similar CIFS client implementations. No known exploits are reported in the wild as of the publication date (May 21, 2024).

For European organizations, this vulnerability poses a significant risk in environments where Linux clients mount SMB shares from Windows servers or other SMB-compatible file servers, especially using CIFS with SMB 3.1.1 protocol. The use-after-free condition can lead to kernel crashes (denial of service) or potentially allow attackers to execute arbitrary code with kernel privileges if exploited, compromising system confidentiality, integrity, and availability. Organizations relying on Linux-based file servers, NAS devices, or workstations that access Windows shares are particularly at risk. The vulnerability could disrupt critical file sharing services, impacting business continuity and data access. Given the complexity of the race condition, exploitation may require specific timing and conditions, but the presence of concurrent statfs calls in typical workloads increases the likelihood of triggering the flaw unintentionally, leading to system instability. This is especially relevant for enterprises with mixed OS environments common in Europe, where SMB is widely used for file sharing. The lack of known exploits suggests that active exploitation is not yet prevalent, but the potential impact warrants prompt mitigation to prevent future attacks or accidental outages.

1. Apply the official Linux kernel patches that address CVE-2023-52751 as soon as they become available from trusted Linux distributions or the kernel mainline. 2. Temporarily avoid mounting SMB shares with the specific options that trigger the vulnerability (e.g., multichannel, max_channels=2, SMB version 3.1.1) until patches are applied. 3. Monitor kernel logs for KASAN or slab-use-after-free warnings related to CIFS to detect potential exploitation or instability. 4. Implement strict access controls and network segmentation to limit SMB traffic to trusted hosts and reduce exposure. 5. For critical systems, consider using alternative file sharing protocols or SMB versions less affected by this issue until patched. 6. Conduct thorough testing of SMB client mounts in controlled environments to identify any instability or crashes related to this vulnerability. 7. Educate system administrators about the risks and signs of exploitation or system crashes related to CIFS mounts. 8. Maintain up-to-date backups and recovery plans to mitigate potential data loss from system crashes caused by this vulnerability.