CVE-2023-1611: Linux Kernel Btrfs Use-After-Free Flaw

CVE-2023-1611 Overview

A use-after-free vulnerability was discovered in the Linux Kernel's Btrfs file system implementation, specifically in the btrfs_search_slot function located in fs/btrfs/ctree.c. This flaw exists due to a race condition between the quota disable and quota assign ioctls, which can lead to accessing a freed quota root structure. An attacker with local access can exploit this vulnerability to crash the system and potentially leak sensitive kernel memory information.

Critical Impact

Local attackers can trigger a kernel crash leading to denial of service, with potential for kernel memory information disclosure affecting system confidentiality.

Affected Products

• Linux Kernel versions 2.6.12 through 6.3 (including rc releases up to rc5)
• Fedora 36
• Fedora 37

Discovery Timeline

• 2023-04-03 - CVE CVE-2023-1611 published to NVD
• 2025-02-13 - Last updated in NVD database

Technical Details for CVE-2023-1611

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) occurs in the Btrfs quota management subsystem of the Linux Kernel. The vulnerability stems from a race condition between two ioctl operations: quota disable and quota assign. When these operations execute concurrently without proper synchronization, the btrfs_run_qgroups function can access the quota root structure after it has been freed by the quota disable operation.

The vulnerability requires local access to the system and the ability to invoke Btrfs ioctl operations. Successful exploitation requires winning a race condition, which adds complexity to the attack. However, once exploited, the impact includes both system availability (kernel crash/denial of service) and potential confidentiality breaches through kernel memory information leakage.

Root Cause

The root cause is insufficient synchronization (missing mutex lock) when calling btrfs_run_qgroups from the qgroup assign ioctl path. While the transaction commit path had proper protections, the ioctl call path lacked the necessary qgroup_ioctl_lock mutex protection, creating a window where the quota root could be freed by a concurrent quota disable operation while still being accessed.

Attack Vector

This vulnerability requires local access to the system with the ability to mount and manipulate Btrfs filesystems. An attacker would need to:

1. Have local access with privileges to perform Btrfs ioctl operations
2. Trigger concurrent quota disable and quota assign ioctl calls
3. Win the race condition to access the freed quota root structure

The attack complexity is considered high due to the timing-dependent nature of the race condition.

// Security patch in fs/btrfs/ioctl.c - btrfs: fix race between quota disable and quota assign ioctls
 	}
 
 	/* update qgroup status and info */
+	mutex_lock(&fs_info->qgroup_ioctl_lock);
 	err = btrfs_run_qgroups(trans);
+	mutex_unlock(&fs_info->qgroup_ioctl_lock);
 	if (err < 0)
 		btrfs_handle_fs_error(fs_info, err,
 				      "failed to update qgroup status and info");

Source: GitHub Linux Commit

// Security patch in fs/btrfs/qgroup.c - btrfs: fix race between quota disable and quota assign ioctls
 }
 
 /*
- * called from commit_transaction. Writes all changed qgroups to disk.
+ * Writes all changed qgroups to disk.
+ * Called by the transaction commit path and the qgroup assign ioctl.
  */
 int btrfs_run_qgroups(struct btrfs_trans_handle *trans)
 {
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 	int ret = 0;
 
+	/*
+	 * In case we are called from the qgroup assign ioctl, assert that we
+	 * are holding the qgroup_ioctl_lock, otherwise we can race with a quota
+	 * disable operation (ioctl) and access a freed quota root.
+	 */
+	if (trans->transaction->state != TRANS_STATE_COMMIT_DOING)
+		lockdep_assert_held(&fs_info->qgroup_ioctl_lock);
+
 	if (!fs_info->quota_root)
 		return ret;
 

Source: GitHub Linux Commit

Detection Methods for CVE-2023-1611

Indicators of Compromise

• Unexpected kernel panics or crashes with stack traces referencing btrfs_search_slot, btrfs_run_qgroups, or related Btrfs quota functions
• Kernel oops messages indicating use-after-free conditions in Btrfs subsystem
• System instability when Btrfs quota operations are being performed
• Memory corruption errors in kernel logs related to Btrfs operations

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for Btrfs-related crashes or memory corruption messages
• Deploy kernel debugging tools like KASAN (Kernel Address Sanitizer) to detect use-after-free conditions
• Use SentinelOne's Linux agent to monitor for suspicious Btrfs ioctl system calls and kernel crash patterns
• Implement audit rules to track Btrfs quota-related ioctl invocations

Monitoring Recommendations

• Enable kernel crash dumps to capture detailed information during exploitation attempts
• Configure alerting for repeated Btrfs-related kernel crashes indicating potential exploitation attempts
• Monitor for processes making rapid successive Btrfs ioctl calls that could indicate race condition exploitation
• Review system stability metrics for Btrfs-mounted filesystems

How to Mitigate CVE-2023-1611

Immediate Actions Required

• Update the Linux Kernel to a patched version that includes commit 2f1a6be12ab6c8470d5776e68644726c94257c54
• Apply vendor-specific patches from Fedora, Debian, or your Linux distribution
• Consider temporarily disabling Btrfs quota functionality on production systems until patches can be applied
• Restrict local access to systems with Btrfs filesystems to trusted users only

Patch Information

The vulnerability has been addressed in the upstream Linux Kernel through commit 2f1a6be12ab6c8470d5776e68644726c94257c54. The fix adds proper mutex locking around the btrfs_run_qgroups call in the ioctl path and includes a lockdep assertion to catch future regressions.

Distribution-specific patches are available:

• Red Hat Bug Report
• Debian LTS Security Announcement
• Fedora Package Announcement

Workarounds

• Disable Btrfs quota functionality using btrfs quota disable <mount-point> if not required for operations
• Limit access to Btrfs ioctl operations through AppArmor or SELinux policies
• Restrict local system access to only essential users until the kernel is patched
• Consider using alternative filesystems (ext4, XFS) for sensitive workloads until patches are applied

# Disable Btrfs quota on affected filesystems as a temporary workaround
btrfs quota disable /path/to/btrfs/mount

# Verify quota is disabled
btrfs quota status /path/to/btrfs/mount

# Update kernel package (example for Fedora)
sudo dnf update kernel

# Reboot to apply new kernel
sudo systemctl reboot