CVE-2025-40254
Unknown Unknown - Not Provided
BaseFortify

Publication date: 2025-12-04

Last updated on: 2025-12-06

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: net: openvswitch: remove never-working support for setting nsh fields The validation of the set(nsh(...)) action is completely wrong. It runs through the nsh_key_put_from_nlattr() function that is the same function that validates NSH keys for the flow match and the push_nsh() action. However, the set(nsh(...)) has a very different memory layout. Nested attributes in there are doubled in size in case of the masked set(). That makes proper validation impossible. There is also confusion in the code between the 'masked' flag, that says that the nested attributes are doubled in size containing both the value and the mask, and the 'is_mask' that says that the value we're parsing is the mask. This is causing kernel crash on trying to write into mask part of the match with SW_FLOW_KEY_PUT() during validation, while validate_nsh() doesn't allocate any memory for it: BUG: kernel NULL pointer dereference, address: 0000000000000018 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 1c2383067 P4D 1c2383067 PUD 20b703067 PMD 0 Oops: Oops: 0000 [#1] SMP NOPTI CPU: 8 UID: 0 Kdump: loaded Not tainted 6.17.0-rc4+ #107 PREEMPT(voluntary) RIP: 0010:nsh_key_put_from_nlattr+0x19d/0x610 [openvswitch] Call Trace: <TASK> validate_nsh+0x60/0x90 [openvswitch] validate_set.constprop.0+0x270/0x3c0 [openvswitch] __ovs_nla_copy_actions+0x477/0x860 [openvswitch] ovs_nla_copy_actions+0x8d/0x100 [openvswitch] ovs_packet_cmd_execute+0x1cc/0x310 [openvswitch] genl_family_rcv_msg_doit+0xdb/0x130 genl_family_rcv_msg+0x14b/0x220 genl_rcv_msg+0x47/0xa0 netlink_rcv_skb+0x53/0x100 genl_rcv+0x24/0x40 netlink_unicast+0x280/0x3b0 netlink_sendmsg+0x1f7/0x430 ____sys_sendmsg+0x36b/0x3a0 ___sys_sendmsg+0x87/0xd0 __sys_sendmsg+0x6d/0xd0 do_syscall_64+0x7b/0x2c0 entry_SYSCALL_64_after_hwframe+0x76/0x7e The third issue with this process is that while trying to convert the non-masked set into masked one, validate_set() copies and doubles the size of the OVS_KEY_ATTR_NSH as if it didn't have any nested attributes. It should be copying each nested attribute and doubling them in size independently. And the process must be properly reversed during the conversion back from masked to a non-masked variant during the flow dump. In the end, the only two outcomes of trying to use this action are either validation failure or a kernel crash. And if somehow someone manages to install a flow with such an action, it will most definitely not do what it is supposed to, since all the keys and the masks are mixed up. Fixing all the issues is a complex task as it requires re-writing most of the validation code. Given that and the fact that this functionality never worked since introduction, let's just remove it altogether. It's better to re-introduce it later with a proper implementation instead of trying to fix it in stable releases.
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Meta Information
Published
2025-12-04
Last Modified
2025-12-06
Generated
2026-05-07
AI Q&A
2025-12-04
EPSS Evaluated
2026-05-05
NVD
CVE-2025-40254
Affected Vendors & Products
Showing 1 associated CPE
Vendor Product Version / Range
openvswitch openvswitch *
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-UNKNOWN
Attack-Flow Graph
AI Powered Q&A
Can you explain this vulnerability to me?

This vulnerability is in the Linux kernel's Open vSwitch component related to the handling of the set(nsh(...)) action. The validation logic for this action is incorrect because it uses the same function meant for other NSH keys, but set(nsh(...)) has a different memory layout with nested attributes doubled in size when masked. This causes improper validation, confusion between flags, and leads to kernel crashes due to NULL pointer dereferences during validation. The functionality never worked correctly since its introduction, and attempts to use it result in validation failures or kernel crashes. The fix was to remove this unsupported functionality entirely.


How can this vulnerability impact me? :

This vulnerability can cause the Linux kernel to crash (kernel NULL pointer dereference) when processing certain Open vSwitch NSH actions, leading to denial of service. If an attacker or misconfigured system attempts to use the set(nsh(...)) action, it can cause system instability or crashes. Additionally, even if a flow with such an action is installed, it will not function correctly due to mixed-up keys and masks.


What immediate steps should I take to mitigate this vulnerability?

The vulnerability is caused by a never-working feature in the Linux kernel's openvswitch module related to setting NSH fields. The fix is to remove this unsupported functionality altogether. Immediate mitigation would be to update the Linux kernel to a version where this vulnerability is resolved by removing the set(nsh(...)) action support in openvswitch. Since the feature never worked properly, disabling or avoiding use of NSH set actions in openvswitch flows can also help mitigate the issue until an update is applied.


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