Bug 2492121 (CVE-2026-52940)

Summary: CVE-2026-52940 kernel: tun: zero the whole vnet header in tun_put_user()
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's `tun` driver. An unprivileged user can exploit this vulnerability by setting the virtual network (vnet) header size to 24 bytes. This action causes the kernel to copy partially initialized stack memory to userspace when reading non-tunnel packets, leading to the disclosure of 14 bytes of kernel stack memory. This information disclosure could potentially expose sensitive kernel data.
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Description OSIDB Bzimport 2026-06-24 08:03:01 UTC 
In the Linux kernel, the following vulnerability has been resolved:

tun: zero the whole vnet header in tun_put_user()

tun_put_user() declares an on-stack struct virtio_net_hdr_v1_hash_tunnel
without zeroing it. For a non-tunnel skb, virtio_net_hdr_tnl_from_skb()
only initializes the first 10 bytes (sizeof(struct virtio_net_hdr)),
leaving bytes 10..23 (num_buffers and the hash/tunnel fields) as stack
garbage.

An unprivileged user can set the vnet header size to 24 with
TUNSETVNETHDRSZ, so __tun_vnet_hdr_put() copies all 24 bytes of the
partially-initialized struct to userspace, leaking 14 bytes of kernel
stack on every read of a non-tunnel packet.

Fix it the same way tun_get_user() already does by zeroing the whole
header right after declaration.
Comment 1 Mauro Matteo Cascella 2026-06-24 09:32:31 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026062434-CVE-2026-52940-734c@gregkh/T