NULL Pointer Dereference Affecting kernel-rt-debug-kvm package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-rt-debug-kvm package and not the kernel-rt-debug-kvm package as distributed by Centos. See How to fix? for Centos:8 relevant fixed versions and status.

In the Linux kernel, the following vulnerability has been resolved:

ethtool: ioctl: fix potential NULL deref in ethtool_set_coalesce()

ethtool_set_coalesce() now uses both the .get_coalesce() and .set_coalesce() callbacks. But the check for their availability is buggy, so changing the coalesce settings on a device where the driver provides only one of the callbacks results in a NULL pointer dereference instead of an -EOPNOTSUPP.

Fix the condition so that the availability of both callbacks is ensured. This also matches the netlink code.

Note that reproducing this requires some effort - it only affects the legacy ioctl path, and needs a specific combination of driver options:

• have .get_coalesce() and .coalesce_supported but no .set_coalesce(), or
• have .set_coalesce() but no .get_coalesce(). Here eg. ethtool doesn't cause the crash as it first attempts to call ethtool_get_coalesce() and bails out on error.

• https://access.redhat.com/security/cve/CVE-2021-47556
• https://git.kernel.org/stable/c/0276af2176c78771da7f311621a25d7608045827
• https://git.kernel.org/stable/c/abfdd9e2f0f9699015d72317f74154d3e53664e6