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Fri, 10 Oct 2025 01:09:02 -0700 (PDT) X-Google-Smtp-Source: AGHT+IF6MmakUZRFgwAiuScJI9gru6tPPAJEKkB0mpdEKWKLWgqJ4dbClzuAVkwqsASebSsNiHDHtA== X-Received: by 2002:a17:906:ee89:b0:b3c:896:abf5 with SMTP id a640c23a62f3a-b50aa393bc7mr1142557466b.25.1760083741964; Fri, 10 Oct 2025 01:09:01 -0700 (PDT) Received: from fedora ([2a02:8308:b104:2c00:7718:da55:8b6:8dcc]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b55d95257c8sm171260166b.77.2025.10.10.01.09.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 10 Oct 2025 01:09:01 -0700 (PDT) From: Ondrej Mosnacek To: Josef Bacik , Jens Axboe Cc: linux-block@vger.kernel.org, nbd@other.debian.org, linux-security-module@vger.kernel.org, selinux@vger.kernel.org, Ming Lei Subject: [PATCH v2] nbd: override creds to kernel when calling sock_{send,recv}msg() Date: Fri, 10 Oct 2025 10:09:00 +0200 Message-ID: <20251010080900.1680512-1-omosnace@redhat.com> X-Mailer: git-send-email 2.51.0 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit sock_{send,recv}msg() internally calls security_socket_{send,recv}msg(), which does security checks (e.g. SELinux) for socket access against the current task. However, _sock_xmit() in drivers/block/nbd.c may be called indirectly from a userspace syscall, where the NBD socket access would be incorrectly checked against the calling userspace task (which simply tries to read/write a file that happens to reside on an NBD device). To fix this, temporarily override creds to kernel ones before calling the sock_*() functions. This allows the security modules to recognize this as internal access by the kernel, which will normally be allowed. A way to trigger the issue is to do the following (on a system with SELinux set to enforcing): ### Create nbd device: truncate -s 256M /tmp/testfile nbd-server localhost:10809 /tmp/testfile ### Connect to the nbd server: nbd-client localhost ### Create mdraid array mdadm --create -l 1 -n 2 /dev/md/testarray /dev/nbd0 missing After these steps, assuming the SELinux policy doesn't allow the unexpected access pattern, errors will be visible on the kernel console: [ 142.204243] nbd0: detected capacity change from 0 to 524288 [ 165.189967] md: async del_gendisk mode will be removed in future, please upgrade to mdadm-4.5+ [ 165.252299] md/raid1:md127: active with 1 out of 2 mirrors [ 165.252725] md127: detected capacity change from 0 to 522240 [ 165.255434] block nbd0: Send control failed (result -13) [ 165.255718] block nbd0: Request send failed, requeueing [ 165.256006] block nbd0: Dead connection, failed to find a fallback [ 165.256041] block nbd0: Receive control failed (result -32) [ 165.256423] block nbd0: shutting down sockets [ 165.257196] I/O error, dev nbd0, sector 2048 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.257736] Buffer I/O error on dev md127, logical block 0, async page read [ 165.258263] I/O error, dev nbd0, sector 2048 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.259376] Buffer I/O error on dev md127, logical block 0, async page read [ 165.259920] I/O error, dev nbd0, sector 2048 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.260628] Buffer I/O error on dev md127, logical block 0, async page read [ 165.261661] ldm_validate_partition_table(): Disk read failed. [ 165.262108] I/O error, dev nbd0, sector 2048 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.262769] Buffer I/O error on dev md127, logical block 0, async page read [ 165.263697] I/O error, dev nbd0, sector 2048 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.264412] Buffer I/O error on dev md127, logical block 0, async page read [ 165.265412] I/O error, dev nbd0, sector 2048 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.265872] Buffer I/O error on dev md127, logical block 0, async page read [ 165.266378] I/O error, dev nbd0, sector 2048 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.267168] Buffer I/O error on dev md127, logical block 0, async page read [ 165.267564] md127: unable to read partition table [ 165.269581] I/O error, dev nbd0, sector 0 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.269960] Buffer I/O error on dev nbd0, logical block 0, async page read [ 165.270316] I/O error, dev nbd0, sector 0 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.270913] Buffer I/O error on dev nbd0, logical block 0, async page read [ 165.271253] I/O error, dev nbd0, sector 0 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 2 [ 165.271809] Buffer I/O error on dev nbd0, logical block 0, async page read [ 165.272074] ldm_validate_partition_table(): Disk read failed. [ 165.272360] nbd0: unable to read partition table [ 165.289004] ldm_validate_partition_table(): Disk read failed. [ 165.289614] nbd0: unable to read partition table The corresponding SELinux denial on Fedora/RHEL will look like this (assuming it's not silenced): type=AVC msg=audit(1758104872.510:116): avc: denied { write } for pid=1908 comm="mdadm" laddr=::1 lport=32772 faddr=::1 fport=10809 scontext=system_u:system_r:mdadm_t:s0-s0:c0.c1023 tcontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 tclass=tcp_socket permissive=0 The respective backtrace looks like this: @security[mdadm, -13, handshake_exit+221615650 handshake_exit+221615650 handshake_exit+221616465 security_socket_sendmsg+5 sock_sendmsg+106 handshake_exit+221616150 sock_sendmsg+5 __sock_xmit+162 nbd_send_cmd+597 nbd_handle_cmd+377 nbd_queue_rq+63 blk_mq_dispatch_rq_list+653 __blk_mq_do_dispatch_sched+184 __blk_mq_sched_dispatch_requests+333 blk_mq_sched_dispatch_requests+38 blk_mq_run_hw_queue+239 blk_mq_dispatch_plug_list+382 blk_mq_flush_plug_list.part.0+55 __blk_flush_plug+241 __submit_bio+353 submit_bio_noacct_nocheck+364 submit_bio_wait+84 __blkdev_direct_IO_simple+232 blkdev_read_iter+162 vfs_read+591 ksys_read+95 do_syscall_64+92 entry_SYSCALL_64_after_hwframe+120 ]: 1 The issue has started to appear since commit 060406c61c7c ("block: add plug while submitting IO"). Cc: Ming Lei Link: https://bugzilla.redhat.com/show_bug.cgi?id=2348878 Fixes: 060406c61c7c ("block: add plug while submitting IO") Signed-off-by: Ondrej Mosnacek --- Changes in v2: * Move put_cred() after destroy_workqueue() in nbd_cleanup() to avoid a UAF * Add some more details into the commit message * Add a Fixes: tag v1: https://lore.kernel.org/linux-block/20251009134542.1529148-1-omosnace@redhat.com/ drivers/block/nbd.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c index 6463d0e8d0cef..3903186e8a4e4 100644 --- a/drivers/block/nbd.c +++ b/drivers/block/nbd.c @@ -52,6 +52,7 @@ static DEFINE_IDR(nbd_index_idr); static DEFINE_MUTEX(nbd_index_mutex); static struct workqueue_struct *nbd_del_wq; +static struct cred *nbd_cred; static int nbd_total_devices = 0; struct nbd_sock { @@ -554,6 +555,7 @@ static int __sock_xmit(struct nbd_device *nbd, struct socket *sock, int send, int result; struct msghdr msg = {} ; unsigned int noreclaim_flag; + const struct cred *old_cred; if (unlikely(!sock)) { dev_err_ratelimited(disk_to_dev(nbd->disk), @@ -562,6 +564,8 @@ static int __sock_xmit(struct nbd_device *nbd, struct socket *sock, int send, return -EINVAL; } + old_cred = override_creds(nbd_cred); + msg.msg_iter = *iter; noreclaim_flag = memalloc_noreclaim_save(); @@ -586,6 +590,8 @@ static int __sock_xmit(struct nbd_device *nbd, struct socket *sock, int send, memalloc_noreclaim_restore(noreclaim_flag); + revert_creds(old_cred); + return result; } @@ -2669,7 +2675,15 @@ static int __init nbd_init(void) return -ENOMEM; } + nbd_cred = prepare_kernel_cred(&init_task); + if (!nbd_cred) { + destroy_workqueue(nbd_del_wq); + unregister_blkdev(NBD_MAJOR, "nbd"); + return -ENOMEM; + } + if (genl_register_family(&nbd_genl_family)) { + put_cred(nbd_cred); destroy_workqueue(nbd_del_wq); unregister_blkdev(NBD_MAJOR, "nbd"); return -EINVAL; @@ -2724,6 +2738,7 @@ static void __exit nbd_cleanup(void) /* Also wait for nbd_dev_remove_work() completes */ destroy_workqueue(nbd_del_wq); + put_cred(nbd_cred); idr_destroy(&nbd_index_idr); unregister_blkdev(NBD_MAJOR, "nbd"); } -- 2.51.0