CVE-2026-89477: sctp: fix NULL deref on untransmitted RECONF completion

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

sctp: fix NULL deref on untransmitted RECONF completion

sctpprocessstrresetoutreq(), sctpprocessstrresetaddstrmout() and sctpprocessstrresetresp() complete a pending stream reconfiguration request by stopping the reconf timer on the transport it was sent on:

t = asoc->strresetchunk->transport; if (timerdelete(&t->reconftimer)) sctptransportput(t);

chunk->transport is assigned by sctppacketappendchunk() when the chunk is appended to an outbound packet, and sctpoutqflushctrl() arms the reconf timer at that same point. A request already published in asoc->strresetchunk but not yet transmitted has neither, so completing it dereferences NULL.

Two ways to get there. sctpsendasconfdelip() sets asoc->srcoutofasocok without sending anything when the address being removed is the association's last one, and sctpoutqflushctrl() then leaves every non-ASCONF control chunk queued; as only sctpprocessasconfack() clears that flag, it persists. An unprivileged process that removes such an address and then asks for a stream reset panics the kernel from softirq. A peer needs neither ASCONF nor local help: sctpcmdinterpreter() uncorks the outqueue only once the whole packet has been processed, so a reply built while walking a RECONF chunk stays untransmitted for the rest of that walk, and one RECONF chunk carrying [Incoming SSN Reset Request, Outgoing SSN Reset Request, Response] -- or two RECONF chunks in one packet -- reaches the same dereference.

KASAN: null-ptr-deref in range [0x00000000000001e8-0x00000000000001ef] RIP: 0010:timerdelete+0x67/0x110 Call Trace: <IRQ> sctpprocessstrresetaddstrmout (net/sctp/stream.c:832) sctpsfdoreconf (net/sctp/smstatefuns.c:4212) sctpdosm (net/sctp/smsideeffect.c:1172) sctpassocbhrcv (net/sctp/associola.c:1044) sctprcv (net/sctp/input.c:243) iplocaldeliver (net/ipv4/ipinput.c:262) processbacklog (net/core/dev.c:6680) </IRQ>

A response can only acknowledge a request that was actually sent, so do not match asoc->strresetchunk while chunk->transport is NULL. Guarding the lookup covers all three completion sites.