CVE-2026-74478: um: vector: fix use-after-free in vector_mmsg_rx()

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

um: vector: fix use-after-free in vectormmsgrx()

When vectormmsgrx() discards a packet whose overlay header fails verifyheader(), it frees the skb and continues the loop:

if (headercheck < 0) { devkfreeskbirq(skb); vp->estats.rxencapserrors++; continue; }

The normal and short-packet paths fall through to the bottom of the loop body, which clears the consumed slot and advances the cursors:

(skbuffvector) = NULL; mmsgvector++; skbuffvector++;

The verifyheader() < 0 path skips that via continue, so the freed skb is left in skbuffvector[] and the cursors do not advance. The next iteration reads the same slot, gets the freed skb, and frees it again, producing a refcount underflow / use-after-free in the RX path.

Discard the slot the same way the other paths do before continuing.

Only transports whose verifyheader() can return negative are affected: GRE and L2TPv3 do so on a cookie/session-id mismatch (raw/tap do not), so any peer on such a transport can trigger it without authentication.