CVE-2026-74577

Summary

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

net: mpls: initialize rtm_tos in mpls_getroute()

mpls_getroute() builds the RTM_NEWROUTE reply to an RTM_GETROUTE request by filling a struct rtmsg allocated from an skb whose data area is not zeroed (alloc_skb(NLMSG_GOODSIZE, …)). It sets every field of the header except rtm_tos:

r = nlmsg_data(nlh);
r->rtm_family	 = AF_MPLS;
r->rtm_dst_len	= 20;
r->rtm_src_len	= 0;
r->rtm_table	= RT_TABLE_MAIN;
r->rtm_type	= RTN_UNICAST;
r->rtm_scope	= RT_SCOPE_UNIVERSE;
r->rtm_protocol = rt->rt_protocol;
r->rtm_flags	= 0;

struct rtmsg has no padding, so the one uninitialised byte rtm_tos (offset 3) is copied straight to user space on recvmsg(), leaking a byte of uninitialised heap memory. This is in contrast to mpls_dump_route(), which fills the very same header and does set rtm_tos = 0.

Initialize rtm_tos to 0, matching mpls_dump_route().

Reproduced with KMSAN by adding an MPLS route and issuing a non-RTM_F_FIB_MATCH RTM_GETROUTE for its label:

BUG: KMSAN: kernel-infoleak in _copy_to_iter+0x36c/0x33f0 _copy_to_iter+0x36c/0x33f0 __skb_datagram_iter+0x196/0x12c0 skb_copy_datagram_iter+0x5b/0x210 netlink_recvmsg+0x37b/0xef0 … Uninit was created at: __alloc_skb+0x8ca/0x10e0 mpls_getroute+0x1280/0x3a40 rtnetlink_rcv_msg+0x1138/0x15a0 … Byte 19 of 64 is uninitialized

(byte 19 = nlmsghdr(16) + rtmsg offset 3 = rtm_tos)

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/95651461cf77cc6590fa08c87667717e5dcfa55d
• https://git.kernel.org/stable/c/1fea5ff0eb4aa7e951bb3d380248566c473aa377
• https://git.kernel.org/stable/c/a5cdd2407dd890f741f59b8367e4c6c101cce154
• https://git.kernel.org/stable/c/2dc2fffc704a4365cae1aae078ba62223aaeff93
• https://git.kernel.org/stable/c/295dd295e2137e10e9a5b1891d97e0f08de76f03

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