[PATCH] RDMA/rxe: validate num_sge/cur_sge before indexing wqe->dma.sge[]

[Ibrahim Hashimov <security@auditcode.ai>]

A user QP's send queue (qp->sq.queue) is a shared ring the userspace
application writes to directly via mmap (vmalloc_user(), see
rxe_queue.c). For such a QP, rxe_post_send() takes the qp->is_user
branch and only schedules the requester task -- it never validates or
copies the posted WQE:

	drivers/infiniband/sw/rxe/rxe_verbs.c:
		if (qp->is_user) {
			rxe_sched_task(&qp->send_task);
			...
		}

The requester then consumes the WQE in place straight out of that
mmap'd ring:

	rxe_req.c:	wqe = req_next_wqe(qp);
	rxe_req.c:	err = copy_data(qp->pd, 0, &wqe->dma,
					payload_addr(pkt), payload,
					RXE_FROM_MR_OBJ);

copy_data() immediately indexes the per-WQE sge array with the
attacker-controlled cur_sge field, before any bound is checked:

	rxe_mr.c:	struct rxe_sge *sge = &dma->sge[dma->cur_sge];
	rxe_mr.c:	...
	rxe_mr.c:	if (sge->length && (offset < sge->length)) {

dma->sge[] is a flex array whose real backing storage is exactly
qp->sq.max_sge entries per WQE slot (see rxe_qp.c, wqe_size computed
from max_sge at QP create time). Since a user QP's WQE bytes are
entirely attacker-supplied, both wqe->dma.num_sge and wqe->dma.cur_sge
can be set to arbitrary values independent of each other and of
max_sge. Only the *kernel*-QP post path bounds num_sge:

	rxe_verbs.c: validate_send_wr()
		if (num_sge > sq->max_sge) {
			rxe_err_qp(qp, "num_sge > max_sge\n");

but that function is only reachable from rxe_post_one_send() for
kernel-owned QPs; it is never consulted for a user QP's raw WQE.

The sibling receive path already has the equivalent guard, with the
literal comment documenting exactly why it is required:

	rxe_resp.c: get_srq_wqe()
		/* don't trust user space data */
		if (unlikely(wqe->dma.num_sge > srq->rq.max_sge)) {
			...
			rxe_dbg_qp(qp, "invalid num_sge in SRQ entry\n");
			return RESPST_ERR_MALFORMED_WQE;
		}

The send/requester path has no analogous check, so a local,
unprivileged user who can open /dev/infiniband/uverbs* and create a
user QP on a soft-RoCE (rxe) link can hand-craft a WQE in the shared
send queue with an out-of-range wqe->dma.cur_sge (or an oversized
wqe->dma.num_sge) and ring the send doorbell. rxe_requester() then
calls copy_data(), which dereferences &dma->sge[cur_sge] out of the
bounds of the per-WQE sge array -- a vmalloc out-of-bounds *read*
(confirmed via KASAN: "KASAN: vmalloc-out-of-bounds in copy_data"),
reliably panicking the kernel (local DoS). sge->addr itself is still
bounds-checked later by lookup_mr()/rxe_mr_copy(), so the primitive is
an OOB read of sge metadata, not an arbitrary read/write primitive.

Fix this the same way get_srq_wqe() already does for SRQ entries:
bound both fields pulled from the (possibly user-mapped) send queue
entry before they are ever used to index wqe->dma.sge[], right where
the requester fetches the next WQE off the ring in rxe_requester().
num_sge is capped at qp->sq.max_sge (matching the sibling SRQ check
and the kernel-QP validate_send_wr() check), and cur_sge is capped at
qp->sq.max_sge directly, since that is the true per-WQE array capacity
that copy_data() indexes into -- this also covers num_sge == 0 /
cur_sge == 0 local-op and zero-payload WQEs, which remain valid.

This is a long-standing bug in the rxe (soft-RoCE) driver: the
qp->is_user bypass in rxe_post_send() and the unbounded
&dma->sge[dma->cur_sge] indexing in copy_data() have been present
since the driver was introduced.

Runtime-verified on a v6.19 KASAN (CONFIG_KASAN_VMALLOC=y) stand: a
reproducer that posts a user QP send WQE with an out-of-range
cur_sge reliably tripped "KASAN: vmalloc-out-of-bounds in
copy_data" (an out-of-bounds read) before this patch, and no longer
triggers that report with the patch applied.

Fixes: 8700e3e7c485 ("Soft RoCE driver")
Cc: stable@vger.kernel.org
Signed-off-by: Ibrahim Hashimov <security@auditcode.ai>
Assisted-by: AuditCode-AI:2026.07
---
 drivers/infiniband/sw/rxe/rxe_req.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/infiniband/sw/rxe/rxe_req.c b/drivers/infiniband/sw/rxe/rxe_req.c
index 12d03f390b09..9fb2c49fb503 100644
--- a/drivers/infiniband/sw/rxe/rxe_req.c
+++ b/drivers/infiniband/sw/rxe/rxe_req.c
@@ -701,6 +701,22 @@ int rxe_requester(struct rxe_qp *qp)
 	if (unlikely(!wqe))
 		goto exit;
 
+	/*
+	 * Don't trust user space data: qp->sq.queue is a raw ring the
+	 * application writes directly for a user QP, so wqe->dma.num_sge
+	 * and wqe->dma.cur_sge must be bounds-checked the same way
+	 * get_srq_wqe() checks an SRQ entry's num_sge before it is used.
+	 * Otherwise copy_data() indexes wqe->dma.sge[wqe->dma.cur_sge]
+	 * with an unvalidated, attacker-controlled index/count and reads
+	 * out of bounds of the per-wqe sge array.
+	 */
+	if (unlikely(wqe->dma.num_sge > qp->sq.max_sge ||
+		     wqe->dma.cur_sge >= qp->sq.max_sge)) {
+		rxe_dbg_qp(qp, "invalid num_sge/cur_sge in send wqe\n");
+		wqe->status = IB_WC_LOC_QP_OP_ERR;
+		goto err;
+	}
+
 	if (rxe_wqe_is_fenced(qp, wqe)) {
 		qp->req.wait_fence = 1;
 		goto exit;
-- 
2.50.1 (Apple Git-155)