CVE-2024-26803

In the Linux kernel, the following vulnerability has been resolved: net:
veth: clear GRO when clearing XDP even when down veth sets NETIF_F_GRO
automatically when XDP is enabled, because both features use the same NAPI
machinery. The logic to clear NETIF_F_GRO sits in veth_disable_xdp() which
is called both on ndo_stop and when XDP is turned off. To avoid the flag
from being cleared when the device is brought down, the clearing is skipped
when IFF_UP is not set. Bringing the device down should indeed not modify
its features. Unfortunately, this means that clearing is also skipped when
XDP is disabled while the device is down. And there’s nothing on the open
path to bring the device features back into sync. IOW if user enables XDP,
disables it and then brings the device up we’ll end up with a stray GRO
flag set but no NAPI instances. We don’t depend on the GRO flag on the
datapath, so the datapath won’t crash. We will crash (or hang), however,
next time features are sync’ed (either by user via ethtool or peer changing
its config). The GRO flag will go away, and veth will try to disable the
NAPIs. But the open path never created them since XDP was off, the GRO flag
was a stray. If NAPI was initialized before we’ll hang in napi_disable().
If it never was we’ll crash trying to stop uninitialized hrtimer. Move the
GRO flag updates to the XDP enable / disable paths, instead of mixing them
with the ndo_open / ndo_close paths.