From nobody Sun Jun 14 18:59:20 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3CF3C30C35C; Sun, 5 Apr 2026 15:45:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775403904; cv=none; b=uJiPqhCo4HumPoqlvky21gcEOtRyJvq7GpmFjNBtD0Uk4q2iyJ26a/lA8GZXUd+6PGTFWxFXs/jc6xE2DMEPRAlBKWmwlKgjhLUj2tL2Si1Yy7iMz0Gw0zCIj1sidM/5AfXbDNJKFKnGKfM5g9TmbfHO/rALeLLjqPuRKfr+wsU= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775403904; c=relaxed/simple; bh=VEc5kchUdrjTQupcPPjRdL0NSXEa1++/a4O9isGQMPo=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type; b=mm3KxgL31xRboBvY6LSe6a8XOIgfXq4sa301jO1OhhzHEBFg8OzNyhz8oI9MpZYq6lES2VTEMVP21dCYsROkndeimkhwS1qBaNLEs7XjZ5JwJUhg7Eb+2QZFsaVcN9pc6PohwKwuglqxDtfZ6QWrmVrnbUzZbPpIc+OHR0Nh4VU= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JdFDT8zw; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JdFDT8zw" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 17EB1C116C6; Sun, 5 Apr 2026 15:45:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775403903; bh=VEc5kchUdrjTQupcPPjRdL0NSXEa1++/a4O9isGQMPo=; h=From:To:Cc:Subject:Date:From; b=JdFDT8zw42NtibtZGOLln0+GoCTPGRkW+l73EVX8ezjW2g5jYwArKUuUtYTB/oGP0 e3wWddNO9vAe5ai5AOiO05WK6vff06/glGFM4ABjGruWIDHvLeqQuL/rj8SqjlpDUK KJ9oJzWXhodaGwAIYOWNJIzwTUkH2dK+yGQuRR+ZYl+Waw808MknVdHBfTW0gP0OjC /AeS0v4Dxbv8ISTkKAcbk5o4vLHTi1GCP6LST7ygMm7k2mW2rOwIaZs6jI3taDTd2h vKYZbal1Jc8MWsJZgBuQ3vJ0kHZDKBxuyUJk0yz096PSyUdPsUoNTpu34rTa1IdF5x TL3Fv+5PObYCQ== From: Leon Romanovsky To: Jason Gunthorpe , Leon Romanovsky Cc: linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org, Chen Zhao , Parav Pandit Subject: [PATCH rdma-next] IB/core: Fix zero dmac race in neighbor resolution Date: Sun, 5 Apr 2026 18:44:55 +0300 Message-ID: <20260405-fix-dmac-race-v1-1-cfa1ec2ce54a@nvidia.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" X-Change-ID: 20260405-fix-dmac-race-1c57aa966d4d X-Mailer: b4 0.15-dev-18f8f Content-Transfer-Encoding: quoted-printable From: Chen Zhao dst_fetch_ha() checks nud_state without holding the neighbor lock, then copies ha under the seqlock. A race in __neigh_update() where nud_state is set to NUD_REACHABLE before ha is written allows dst_fetch_ha() to read a zero MAC address while the seqlock reports no concurrent writer. netevent_callback amplifies this by waking ALL pending addr_req workers when ANY neighbor becomes NUD_VALID. At scale (N peers resolving ARP concurrently), the hit probability scales as N^2, making it near-certain for large RDMA workloads. N(A): neigh_update(A) W(A): addr_resolve(A) | [sleep] | write_lock_bh(&A->lock) | | A->nud_state =3D NUD_REACHABLE | | // A->ha is still 0 | | [woken by netevent_cb() of | another neighbour] | | dst_fetch_ha(A) | | A->nud_state & NUD_VALID | | read_seqbegin(&A->ha_lock) | | snapshot =3D A->ha /* 0 */ | | read_seqretry(&A->ha_lock) | | return snapshot | seqlock(&A->ha_lock) | A->ha =3D mac_A /* too late */ | sequnlock(&A->ha_lock) | write_unlock_bh(&A->lock) The incorrect/zero mac is read and programmed in the device QP while it was not yet updated. This causes silent packet loss and eventual RETRY_EXC_ERR. Fix by holding the neighbor read lock across the nud_state check and ha copy in dst_fetch_ha(), ensuring it synchronizes with __neigh_update() which is updating while holding the write lock. Fixes: 92ebb6a0a13a ("IB/cm: Remove now useless rcu_lock in dst_fetch_ha") Signed-off-by: Chen Zhao Reviewed-by: Parav Pandit Signed-off-by: Leon Romanovsky --- Strictly speaking the commit in Fixes doesn't look like the one which caused the race, but it is most relevant one to put. --- drivers/infiniband/core/addr.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/drivers/infiniband/core/addr.c b/drivers/infiniband/core/addr.c index 866746695712a..6526fda8f9c0b 100644 --- a/drivers/infiniband/core/addr.c +++ b/drivers/infiniband/core/addr.c @@ -321,11 +321,14 @@ static int dst_fetch_ha(const struct dst_entry *dst, if (!n) return -ENODATA; =20 + read_lock_bh(&n->lock); if (!(n->nud_state & NUD_VALID)) { + read_unlock_bh(&n->lock); neigh_event_send(n, NULL); ret =3D -ENODATA; } else { neigh_ha_snapshot(dev_addr->dst_dev_addr, n, dst->dev); + read_unlock_bh(&n->lock); } =20 neigh_release(n); --- base-commit: 69db255d5fafb5651013c79e54c1d535fc5015fb change-id: 20260405-fix-dmac-race-1c57aa966d4d Best regards, -- =20 Leon Romanovsky