CVE-2024-26689 in Linux
Summary
by MITRE • 04/03/2024
In the Linux kernel, the following vulnerability has been resolved:
ceph: prevent use-after-free in encode_cap_msg()
In fs/ceph/caps.c, in encode_cap_msg(), "use after free" error was caught by KASAN at this line - 'ceph_buffer_get(arg->xattr_buf);'. This implies before the refcount could be increment here, it was freed.
In same file, in "handle_cap_grant()" refcount is decremented by this line - 'ceph_buffer_put(ci->i_xattrs.blob);'. It appears that a race occurred and resource was freed by the latter line before the former line could increment it.
encode_cap_msg() is called by __send_cap() and __send_cap() is called by ceph_check_caps() after calling __prep_cap(). __prep_cap() is where arg->xattr_buf is assigned to ci->i_xattrs.blob. This is the spot where the refcount must be increased to prevent "use after free" error.
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Analysis
by VulDB Data Team • 08/03/2025
The vulnerability identified as CVE-2024-26689 represents a critical use-after-free condition within the Linux kernel's Ceph file system implementation. This flaw exists in the fs/ceph/caps.c file where the encode_cap_msg() function attempts to increment a reference count on a buffer object that has already been freed by concurrent operations. The issue manifests through KASAN (Kernel Address Sanitizer) detection at the specific line ceph_buffer_get(arg->xattr_buf), indicating that the buffer reference count increment operation occurs after the buffer has already been released, creating a dangerous memory access pattern that could lead to system instability or potential exploitation.
The root cause of this vulnerability stems from a race condition between two distinct functions within the Ceph kernel module. The handle_cap_grant() function decrements the reference count of a buffer through ceph_buffer_put(ci->i_xattrs.blob), while the encode_cap_msg() function attempts to increment the same reference count via ceph_buffer_get(arg->xattr_buf). This race condition occurs because the buffer object becomes available for deallocation before the reference increment operation can complete, creating a temporal window where the memory location may be reused or freed. The sequence of operations begins with __prep_cap() assigning arg->xattr_buf to ci->i_xattrs.blob, followed by __send_cap() calling encode_cap_msg(), which then attempts to access the buffer that may have already been freed by handle_cap_grant().
The operational impact of this vulnerability extends beyond simple memory corruption, potentially enabling privilege escalation and system compromise through memory corruption attacks. When the kernel attempts to access freed memory through the use-after-free condition, it may result in unpredictable behavior including kernel crashes, data corruption, or more critically, the possibility of executing arbitrary code with kernel privileges. This vulnerability specifically affects systems running the Linux kernel with Ceph file system support, making it particularly relevant for storage infrastructure, cloud computing environments, and distributed systems that rely on Ceph for data management. The timing and complexity of the race condition make this vulnerability challenging to exploit reliably, but the potential consequences remain severe enough to warrant immediate attention.
Mitigation strategies for CVE-2024-26689 should prioritize applying the kernel patch that resolves the race condition by ensuring proper reference counting mechanisms are implemented before buffer operations. The fix typically involves modifying the code path to increment reference counts before assignment operations, ensuring that the buffer object remains valid throughout the entire sequence of operations. System administrators should also implement monitoring for kernel memory corruption patterns and consider implementing additional security controls such as kernel module signing and runtime integrity checking. This vulnerability aligns with CWE-416, which specifically addresses use-after-free errors, and may be categorized under ATT&CK technique T1068, which involves exploiting privileges through kernel vulnerabilities. Organizations should also consider implementing automated patch management systems to ensure timely deployment of kernel security updates and maintain detailed inventory of systems running Ceph file systems to facilitate rapid remediation efforts.