CVE-2026-80634netfilter: flowtable: avoid num_encaps underflow on bridge VLAN untag

Official description

In the Linux kernel, the following vulnerability has been resolved: netfilter: flowtable: avoid num_encaps underflow on bridge VLAN untag The DEV_PATH_BR_VLAN_UNTAG case post-decrements info->num_encaps inside WARN_ON_ONCE(). num_encaps is u8, so if it's already 0 the decrement still happens and wraps it to 255. The break only leaves the inner switch -- a later path entry can set info->indev back to a real device, and we end up returning with num_encaps == 255. nft_dev_forward_path() then walks info.encap[] (size 2) up to num_encaps, which means an OOB stack read and a bogus count copied into the route descriptor. Should only happen on a malformed bridge path stack, hence the WARN, but worth handling sanely. Move the decrement out of the WARN. [ While at this, remove the WARN_ON_ONCE since this can only happen with a buggy bridge path stack --pablo ].

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/9a4e3a9017ba3da2de187b72ed6545641a0d0a84
• https://git.kernel.org/stable/c/1b52aeb40c9f04bf50eeb4ea09142139d163e239
• https://git.kernel.org/stable/c/ee8440bb000fff4ef8acbfc99652851e9ca0a35b
• https://git.kernel.org/stable/c/547ea9f567498e4accc89e7816264f15a1a0629f
• https://git.kernel.org/stable/c/2f55fa28011c97d6495d5787808db10a8c2d690d
• https://git.kernel.org/stable/c/e052f920773b73be49eb4d8702a9f85de7464363