Overview
About vulnerability
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 ].
Details
- Affected product:
- Debian 10 ELS
- Affected packages:
- linux @ 4.19.0