CVE-2026-74664 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
net: openvswitch: reallocate update replies for mismatched IDs
ovs_flow_cmd_new() preallocates the optional reply skb before it takes ovs_mutex and before it knows which existing flow will be updated.
That is normally fine because the skb is sized from the request flow identifier. That identifier also becomes the inserted flow's identifier. For updates, however, a request with a UFID may miss the UFID lookup and then fall back to the flow key lookup. That lookup can legitimately find an existing key-identified flow. UFIDs are optional and the flow key is the primary identifier.
For echoed replies, ovs_flow_cmd_fill_info() writes the matched flow's identifier, not the request identifier used for the preallocation. A short request UFID can therefore leave too little room for the key identifier. The fill can then fail with -EMSGSIZE and hit the BUG_ON(error < 0) in the update path.
Once the update target has been resolved, reallocate the reply skb if the matched flow needs a larger reply than the request identifier allowed. Do this before replacing the actions so the request can still fail cleanly if the rare extra allocation fails.
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Analysis
by VulDB Data Team • 08/22/2026
The vulnerability in question resides within the Open vSwitch implementation of the Linux kernel, specifically affecting the netlink command handler for flow updates known as ovs_flow_cmd_new. This function is responsible for processing requests to create or update network flows managed by the virtual switch infrastructure. The core technical flaw stems from a preallocation strategy that fails to account for discrepancies between different types of flow identifiers used during the lookup and reply generation phases. Specifically, the code allocates a socket buffer (skb) sized based on the user-flow identifier provided in the incoming request before acquiring the necessary mutex locks or determining which specific existing flow will be updated. This approach assumes that the size required to serialize the response is determined solely by the input identifier, an assumption that breaks down under certain lookup conditions.
The operational context involves two distinct methods for identifying flows within Open vSwitch: user-flow identifiers and flow keys. While UFIDs are optional metadata attached to flows, the flow key serves as the primary and definitive identifier used for packet matching and forwarding decisions. When a request includes a UFID, the system attempts to locate an existing flow using that ID first. However if this lookup fails or is bypassed due to specific configuration states, the system falls back to performing a lookup based on the flow key itself. In such scenarios, it is entirely possible for the system to match against an existing flow identified by its full flow key rather than the shorter UFID provided in the request. This divergence creates a critical size mismatch because the preallocated buffer was sized according to the potentially smaller UFID length, whereas the subsequent reply generation process must serialize the complete and often larger flow key identifier.
This discrepancy leads directly to a kernel panic condition triggered by an unhandled error state. When ovs_flow_cmd_fill_info attempts to populate the netlink message with details of the matched flow, it writes out the full flow key identifier into the preallocated buffer. If this identifier is significantly longer than the UFID used for allocation, the serialization process exceeds the allocated memory bounds. Instead of gracefully handling this overflow or size mismatch by reallocating a larger buffer, the function returns an -EMSGSIZE error code indicating that the message was too large to fit in the socket buffer. The calling logic then encounters this negative return value and passes it into a BUG_ON macro check which asserts that errors should not be negative in this specific update path. Consequently, the assertion fails, causing an immediate kernel panic or denial of service rather than returning a proper error code to the user space application initiating the flow modification request.
From a classification perspective, this issue aligns with CWE-1342, which covers off-by-one errors and buffer size miscalculations in network protocols, as well as CWE-755 related to improper handling of unexpected input data structures that lead to resource exhaustion or crashes. In the context of the MITRE ATT&CK framework for enterprise security, this vulnerability facilitates Denial of Service via kernel panic, allowing a local user with appropriate permissions to disrupt system stability by crafting specific netlink messages that trigger the identifier mismatch scenario. The impact is severe as it compromises the availability of the virtual networking infrastructure and potentially the entire host operating system depending on how critical the Open vSwitch service is to the environment's operation.
To mitigate this vulnerability, the kernel developers implemented a fix that introduces dynamic reallocation logic within the update path. Once the target flow has been successfully resolved through either UFID or key lookup, the code now checks whether the matched flow requires a larger reply buffer than what was originally allocated based on the request identifier. If such a requirement is detected, the system reallocates the socket buffer to accommodate the full size of the response data before proceeding with any modifications to the flow actions. This ensures that the serialization process completes successfully without triggering error conditions or assertion failures. Additionally, this reallocation occurs prior to replacing actions, preserving the ability for the request to fail cleanly if memory allocation resources are temporarily exhausted, thereby maintaining system stability and preventing kernel panics under high-load or specific configuration scenarios involving mixed identifier types.