CVE-2026-72338 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: act_pedit: fix TOCTOU heap OOB write in tc offload
There is a TOCTOU race condition in flower lockless approach between sizing a flow_rule buffer and filling it. [email protected] reports: The cls_flower classifier operates with TCF_PROTO_OPS_DOIT_UNLOCKED (fl_change runs without RTNL), while RTM_NEWACTION holds RTNL, so the independent locking domains make the race reachable in practice. KASAN confirms: BUG: KASAN: slab-out-of-bounds in tcf_pedit_offload_act_setup+0x81b/0x930 Write of size 4 at addr ffff888001f27520 by task poc-toctou/312 The buggy address is located 0 bytes to the right of allocated 288-byte region [ffff888001f27400, ffff888001f27520)
(cache kmalloc-512)
Note: The result is a heap OOB write attacker-controlled content into the adjacent slab object (requires CAP_NET_ADMIN).
The fix introduces reading tcfp_nkeys under act->tcfa_lock in all places using a new tcf_pedit_nkeys_locked() which replaces the old tcf_pedit_nkeys(). Additionally we close the remaining TOCTOU window between the sizing read and the fill reads by more careful accounting. Rather than silently truncating the key count, which leads to incorrect action semantics offloaded to hardware and secondary OOB writes if the remaining capacity is zero or consumed by prior actions, we enforce remaining capacity checks and return -ENOSPC if the required space exceeds the remaining capacity.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability exists within the linux kernel's traffic control subsystem specifically in the cls_flower classifier implementation where a time-of-check to time-of-use race condition has been identified leading to a heap out-of-bounds write. This flaw manifests in the net/sched directory under act_pedit functionality when handling tc offload operations. The issue arises from the flower classifier operating with TCF_PROTO_OPS_DOIT_UNLOCKED flag which allows fl_change operations to run without holding the RTNL lock, while RTM_NEWACTION operations hold this lock creating independent locking domains that make the race condition exploitable in practice. The vulnerability has been confirmed through KASAN reporting a slab-out-of-bounds error in tcf_pedit_offload_act_setup function where a 4-byte write occurs at address ffff888001f27520, which is positioned exactly at the boundary of a 288-byte allocated region. The memory access pattern indicates that attacker-controlled content is written into adjacent slab objects, requiring CAP_NET_ADMIN capability for exploitation. This vulnerability directly maps to CWE-367 Time-of-Check to Time-of-Use race condition and aligns with ATT&CK technique T1059.001 Command and Scripting Interpreter: Python where malicious actors could leverage such kernel vulnerabilities for privilege escalation.
The technical flaw stems from improper synchronization between buffer sizing operations and actual data filling in the flow_rule buffer management. The original implementation used tcf_pedit_nkeys() function without proper locking mechanisms, allowing a race condition between when the buffer size is determined and when the buffer is actually filled with data. This creates an opportunity where the number of keys can be modified between the sizing check and the actual fill operation, leading to memory corruption when the kernel attempts to write beyond allocated boundaries. The implementation uses a flower lockless approach that was designed for performance but inadvertently introduced this security flaw due to insufficient atomicity in the flow rule buffer management process. The race condition specifically occurs during hardware offloading operations where the kernel must determine how much space is needed for key specifications before actually populating those specifications, creating a temporal gap that can be exploited by malicious actors.
The operational impact of this vulnerability is significant as it allows for heap memory corruption that could potentially lead to privilege escalation or system instability when exploited. The requirement for CAP_NET_ADMIN capability limits direct exploitation to users with network administration privileges, but such capabilities are often available in containerized environments or when administrators delegate network management tasks. The out-of-bounds write occurs within kernel memory space, making it particularly dangerous as it can corrupt adjacent data structures that may contain sensitive information or control data. The vulnerability affects systems using tc (traffic control) with flower classifier and hardware offloading capabilities, which are common in high-performance networking environments including data centers, network switches, and virtualized infrastructure. This issue could be leveraged to cause denial of service through kernel memory corruption or potentially enable more sophisticated attacks if combined with other vulnerabilities.
The fix addresses this vulnerability by implementing proper locking mechanisms around the key count access operations. The solution introduces a new tcf_pedit_nkeys_locked() function that reads tcfp_nkeys under act->tcfa_lock in all locations that previously used the unsafe tcf_pedit_nkeys() function. This ensures atomic access to the key count information preventing the race condition between sizing and filling operations. Additionally, the implementation closes the remaining TOCTOU window by implementing more careful accounting of buffer space requirements and enforcing strict capacity checks rather than silently truncating key counts. When the required space exceeds remaining capacity, the system now returns -ENOSPC error instead of allowing potentially corrupt memory operations to proceed. This approach prevents incorrect action semantics in hardware offloading scenarios and eliminates secondary out-of-bounds writes that could occur when prior actions consume available buffer space. The fix aligns with security best practices for concurrent programming and follows established patterns for kernel memory management to prevent similar race conditions from occurring in other parts of the networking subsystem.