CVE-2026-72029: net: wwan: iosm: bound device offsets in the MUX downlink decoder

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

net: wwan: iosm: bound device offsets in the MUX downlink decoder

muxdladbdecode() walks a chain of aggregated datagram tables using offsets and lengths taken from the modem. firsttableindex, nexttableindex, tablelength, datagramindex and datagramlength are all device supplied le values. Only firsttableindex was checked, and only for being non zero. The decoder then formed adth = block + adthindex and read the table header and the datagram entries with no bound against the received skb. A modem that reports an index or a length past the downlink buffer makes the decoder read out of bounds.

The buffer is IPCMEMMAXDLMUXLITEBUFSIZE and skb->len is at most that, so skb->len is the real limit, but none of these in band offsets were checked against it.

The table chain is also followed with no forward progress check. The loop takes the next table from adth->nexttableindex and stops only when that reaches zero. A modem can stage two tables that point at each other, so the loop never ends. It runs in softirq and clones the skb on every pass.

Validate every device offset and length against skb->len before use. The block header must fit. Each table header, on entry and after every nexttableindex, must lie inside the skb. The datagram table must fit. Each datagram index and length must stay inside the skb. The header padding must not exceed the datagram length so the receive length does not wrap. Require each nexttableindex to move forward so the chain cannot cycle.

This was reproduced under KASAN as a slab out of bounds read on a normal downlink receive once the iosm net device is up.