CVE-2026-72466: xprtrdma: Fix bcall rep leak and unbounded peek

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

xprtrdma: Fix bcall rep leak and unbounded peek

rpcrdmaisbcall() decodes a reply's first words to decide whether the frame is a backchannel call. Two issues in that decode path let a short or malformed reply leak the receive buffer and drain the Receive queue.

First, the speculative peek

p = xdrinlinedecode(xdr, 0); / five p++ reads follow /

asks xdrinlinedecode() for zero bytes, which returns xdr->p without consulting xdr->end. The five subsequent be32 reads can then walk up to 20 bytes past the wire payload into stale regbuf contents and misclassify the reply as a backchannel call.

Second, after the post-peek

p = xdrinlinedecode(xdr, 3 sizeof(p)); if (unlikely(!p)) return true;

the short-header arm returns true without calling rpcrdmabcreceivecall(). The contract with the caller is that a true return transfers ownership of rep to the backchannel path:

rpcrdmareplyhandler() if (rpcrdmaisbcall(rxprt, rep)) return; / bare return, skips outpost / ... outpost: rpcrdmapostrecvs(rxprt, credits + ...);

Because rpcrdmabcreceivecall() never ran, no one took rep, but rpcrdmareplyhandler still bare-returns past rpcrdmarepput() and rpcrdmapostrecvs(). The rep, with its persistently DMA-mapped receive buffer, is orphaned on rballreps and freed only at transport teardown. This completion reposts nothing, so its slot is reclaimed only when a later forward-channel reply reaches outpost and rpcrdmapostrecvs() allocates a fresh rep to backfill; absent that traffic the Receive queue drains and the peer's Sends draw RNR NAKs.

Fix by consulting xdr->end after the zero-length peek so the five be32 reads cannot run unless 20 bytes of wire payload remain. A byte-precise comparison against xdr->end is required because a non-4-aligned receive rounds the stream's word count up past the true payload. Also return false from the short-header arm so the reply falls through the normal outnorqst cleanup chain (rpcrdmarepput() plus rpcrdmapostrecvs()).