CVE-2021-38199 Overview
CVE-2021-38199 is a Denial of Service vulnerability in the Linux kernel's NFSv4 client implementation. The flaw exists in fs/nfs/nfs4client.c where incorrect connection-setup ordering allows operators of remote NFSv4 servers to cause a denial of service condition by arranging for those servers to be unreachable during trunking detection, resulting in hanging NFS mounts.
Critical Impact
Remote NFSv4 server operators can exploit incorrect connection-setup ordering to cause NFS mount operations to hang indefinitely, leading to system unavailability and potential service disruption for dependent applications.
Affected Products
- Linux Kernel (versions before 5.13.4)
- NetApp HCI Bootstrap OS
- NetApp HCI Compute Node
- NetApp HCI Management Node
- NetApp SolidFire
- NetApp Element Software
- NetApp HCI Storage Node
- Debian Linux 9.0
- Debian Linux 11.0
Discovery Timeline
- August 8, 2021 - CVE-2021-38199 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2021-38199
Vulnerability Analysis
The vulnerability resides in the nfs4_alloc_client() function within fs/nfs/nfs4client.c. The issue stems from improper initialization ordering when establishing connections to NFSv4 servers. During the trunking detection phase, the NFS client attempts to identify if multiple server addresses belong to the same server. When this detection occurs while a target server is unreachable, the mount operation hangs indefinitely rather than failing gracefully with an appropriate timeout or error.
This race condition in the connection establishment logic creates a window where an attacker controlling or influencing NFSv4 server availability can trigger the vulnerability. The attack requires adjacent network access, meaning the attacker must have some level of network proximity to the affected systems or control over the NFSv4 server infrastructure.
Root Cause
The root cause is incorrect connection-setup ordering in the NFSv4 client code. The nfs4_alloc_client() function did not properly initialize the server connection before proceeding with trunking detection operations. This sequencing flaw means that if the server becomes unreachable at a critical point during client initialization, the mount operation enters a hung state rather than handling the failure appropriately.
Attack Vector
The attack vector requires adjacent network access. An attacker operating or compromising NFSv4 servers can exploit this vulnerability by making servers unreachable during the specific window when trunking detection occurs. This can be achieved through:
- Controlling the NFSv4 server and timing network disruptions
- Network-level attacks that make the server temporarily unreachable
- Manipulating server responses during the trunking detection phase
The attack results in denial of service through indefinitely hanging mount operations, which can cascade to affect applications and services dependent on the NFS mounts.
// Security patch from Linux Kernel commit dd99e9f
// Source: https://github.com/torvalds/linux/commit/dd99e9f98fbf423ff6d365b37a98e8879170f17c
struct nfs_client *nfs4_alloc_client(const struct nfs_client_initdata *cl_init)
{
- int err;
+ char buf[INET6_ADDRSTRLEN + 1];
+ const char *ip_addr = cl_init->ip_addr;
struct nfs_client *clp = nfs_alloc_client(cl_init);
+ int err;
+
if (IS_ERR(clp))
return clp;
Source: GitHub Commit dd99e9f
Detection Methods for CVE-2021-38199
Indicators of Compromise
- NFS mount operations hanging indefinitely without timeout or error messages
- Increased number of stuck mount.nfs4 processes visible in process listings
- System logs showing incomplete NFSv4 client initialization sequences
- Applications experiencing timeouts when accessing NFS-mounted filesystems
Detection Strategies
- Monitor for abnormal NFS mount operation durations exceeding expected timeouts
- Track kernel logs for NFSv4 client initialization anomalies and trunking detection failures
- Implement process monitoring to detect hung mount.nfs4 or related kernel threads
- Use network monitoring to identify patterns of NFSv4 server unreachability during mount operations
Monitoring Recommendations
- Configure NFS mount timeouts and alerting for operations exceeding normal duration thresholds
- Implement system health checks that verify NFS mount responsiveness
- Monitor NFSv4 server availability and network connectivity metrics
- Enable detailed NFS client logging to capture connection establishment sequences
How to Mitigate CVE-2021-38199
Immediate Actions Required
- Update Linux kernel to version 5.13.4 or later which contains the fix
- Review and audit NFSv4 mount configurations in production environments
- Implement network-level controls to ensure NFSv4 server reachability
- Consider temporarily switching to NFSv3 if patching cannot be immediately applied
Patch Information
The vulnerability has been addressed in Linux kernel version 5.13.4. The fix, implemented in commit dd99e9f98fbf423ff6d365b37a98e8879170f17c, corrects the connection-setup ordering in the nfs4_alloc_client() function to properly initialize server connections before trunking detection.
Patch information is available through:
Distribution-specific patches are available from:
Workarounds
- Ensure high availability and reliability of NFSv4 servers to reduce exposure
- Implement network redundancy for critical NFSv4 infrastructure
- Configure appropriate mount timeouts using NFS mount options
- Use monitoring to detect and automatically recover from hung mount operations
# Configuration example - NFS mount with timeout options
# Add timeout parameters to reduce impact of hanging mounts
mount -t nfs4 -o timeo=600,retrans=2,soft server:/export /mnt/nfs
# Verify current kernel version
uname -r
# Check if kernel version is patched (5.13.4 or later)
# If not, update using your distribution's package manager
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
