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Miller" Cc: Hidayath Khan , Simon Horman , linux-kernel@vger.kernel.org, netdev@vger.kernel.org, linux-s390@vger.kernel.org References: <20260821-b4-disp-3a6e8695-v2-0-37597ff723a8@proton.me> <20260821-b4-disp-3a6e8695-v2-1-37597ff723a8@proton.me> Content-Language: en-US From: Alexandra Winter In-Reply-To: <20260821-b4-disp-3a6e8695-v2-1-37597ff723a8@proton.me> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-GUID: BF6zlPgnwNa7mrkLfaLOMU-iGAUMta5h X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODI4MDA4NiBTYWx0ZWRfX5S0+zjSuXrt2 4VisFqFbWkVR+dNJ/eeXwJnSo+mAqHo1YFGLOa/DLR/Kpkm+cKrSAD1HkzwYUXxacN/K5qb7HHF J3IT1cuQUqvrh6noGDK2YRuaRrnOnFWglmyhIcm2VX4AeVxaE+HnjT6JgHZU49tJ8ekNxFB7mmP J+d0uqy3fgLbgDQ0NhnAAk7ZwM26ncvYMxDhbrLTV/Ob12/FO/EYXHTZyt9oH3QIw5hKSYgJM/q DZGHTWeoWiip+ID4nHPPxqwMw3//+B7/yrn+vDIo7j1+jPVJ9FT+it2bA1Ym9L2pzjdC63u51TZ YJmIMi8kGEoeyNBHtZYBpDufZfV9aeeK7hOPx9EQ8qc3EpD3CPA/ovijkCMaet99dKB5dZWZj0i 3bgY9QNdrE52R+WfWZKT2U1PtESMMYB7Qoil+Ysqz4MC+a81DPmbvgUaHv0ixKTz214gdtBPNUZ HxP1c8lcrX1zDqJuKdQ== X-Authority-Analysis: v=2.4 cv=Y/nIdBeN c=1 sm=1 tr=0 ts=6a915d84 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=iQ6ETzBq9ecOQQE5vZCe:22 a=zuM-BvYbtpJmjVirTZoA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Info: AW1haW4tMjYwODI4MDA4NiBTYWx0ZWRfX7BBrUGiGuUNG I6LgNYsMfmLd6EP+625cUpj2z30WTnQ0bGyijCYnhuenvYJZ6vkwNXTn9vAq0327xHXT5d8Q5Oy RB1YYdZE4ZeyP2wFqOpETRj4ipEswFo= X-Proofpoint-ORIG-GUID: 3p_voacDNfRY4kBA6Thl4rRyZRQISzq6 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-28_02,2026-08-27_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 impostorscore=0 priorityscore=1501 adultscore=0 bulkscore=0 suspectscore=0 malwarescore=0 clxscore=1015 lowpriorityscore=0 phishscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608280086 On 21.08.26 13:17, Bryam Vargas via B4 Relay wrote: > From: Bryam Vargas > > iucv_sock_recvmsg() sends AF_IUCV_FLAG_WIN without testing the transport, > but that flag exists only on HiperSockets. On a classic z/VM socket > iucv->hs_dev is NULL and iucv_send_ctrl() sizes the skb from > LL_RESERVED_SPACE(iucv->hs_dev), so the read goes through NULL. It is one > recvmsg() away for an unprivileged process on a socket of its own: > SO_MSGLIMIT accepts 1, so msglimit / 2 is 0 and msg_recv never leaves 0 on > a classic socket. The read lands in mapped lowcore on a default kernel and > the socket takes a spurious disconnect; with relocate_lowcore it faults. Hi Bryam, FYI, I tried to reproduce the fault with boot parameter relocate_lowcore. It did not run into a page fault. My AI says: "relocate_lowcore moves the lowcore mapping from 0x0 to 0x70000. It does not necessarily unmap all of page 0." So a garbage read still happens. The bug is very valid and I can reproduce the spurious disconnect, and that it is fixed with the proposed fix. But you may want to adapt the commit message.