[PATCH v2] nbd: override creds to kernel when calling sock_{send, recv}msg()
Stephen Smalley
stephen.smalley.work at gmail.com
Fri Oct 10 12:08:14 UTC 2025
On Fri, Oct 10, 2025 at 4:09 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>
> 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 <ming.lei at redhat.com>
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2348878
> Fixes: 060406c61c7c ("block: add plug while submitting IO")
> Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
Acked-by: Stephen Smalley <stephen.smalley.work at gmail.com>
> ---
>
> 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
>
>
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