CVE-2026-89968: nvmet-tcp: reject unsolicited H2CData PDUs

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

nvmet-tcp: reject unsolicited H2CData PDUs

nvmettcphandleh2cdatapdu() accepts an H2CData PDU after only checking that its TTAG is a valid in-range command index and that the command's data buffers are mapped. It never checks that the target has actually solicited that data by sending an R2T for the command.

A remote host can abuse this. It submits a write command that takes the R2T path and, before the target transmits the R2T, sends an H2CData PDU for that command's tag. The data completes the command early, and when the command then fails synchronously (e.g. a length mismatch caught by nvmetchecktransferlen()), it is completed a second time. Each completion calls nvmettcpqueueresponse(), so the same command is added to queue->resplist twice while it is still linked; the second llistadd() makes the node point to itself (lentry->next == lentry).

nvmettcpprocessresplist() then walks that self-referential node and adds the command to respsendlist twice. With CONFIGDEBUGLIST this trips the "listadd double add" check (kernel BUG); without it the loop never terminates and the nvmettcp workqueue wedges (soft-lockup). It is remotely triggerable and needs no authentication on an allowanyhost subsystem.

Track whether an R2T has been transmitted for a command and reject an H2CData PDU that arrives before it. The flag is cleared on command reuse (nvmettcpgetcmd() zeroes cmd->flags) and stays set across the multiple H2CData PDUs of a single solicited transfer.