CVE-2024-14040

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

net: nexthop: Increase weight to u16

In CLOS networks, as link failures occur at various points in the network, ECMP weights of the involved nodes are adjusted to compensate. With high fan-out of the involved nodes, and overall high number of nodes, a (non-)ECMP weight ratio that we would like to configure does not fit into 8 bits. Instead of, say, 255:254, we might like to configure something like 1000:999. For these deployments, the 8-bit weight may not be enough.

To that end, in this patch increase the next hop weight from u8 to u16.

Increasing the width of an integral type can be tricky, because while the code still compiles, the types may not check out anymore, and numerical errors come up. To prevent this, the conversion was done in two steps. First the type was changed from u8 to a single-member structure, which invalidated all uses of the field. This allowed going through them one by one and audit for type correctness. Then the structure was replaced with a vanilla u16 again. This should ensure that no place was missed.

The UAPI for configuring nexthop group members is that an attribute NHA_GROUP carries an array of struct nexthop_grp entries:

struct nexthop_grp {
	__u32	id;	  /* nexthop id - must exist */
	__u8	weight;   /* weight of this nexthop */
	__u8	resvd1;
	__u16	resvd2;
};

The field resvd1 is currently validated and required to be zero. We can lift this requirement and carry high-order bits of the weight in the reserved field:

struct nexthop_grp {
	__u32	id;	  /* nexthop id - must exist */
	__u8	weight;   /* weight of this nexthop */
	__u8	weight_high;
	__u16	resvd2;
};

Keeping the fields split this way was chosen in case an existing userspace makes assumptions about the width of the weight field, and to sidestep any endianness issues.

The weight field is currently encoded as the weight value minus one, because weight of 0 is invalid. This same trick is...