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

Affected: Linux

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

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

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

if (header_check < 0) { dev_kfree_skb_irq(skb); vp->estats.rx_encaps_errors++; 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:

(*skbuff_vector) = NULL; mmsg_vector++; skbuff_vector++;

The verify_header() < 0 path skips that via continue, so the freed skb is left in skbuff_vector[] 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 verify_header() 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.

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