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 ].