[PATCH v2] nbd: override creds to kernel when calling sock_{send,recv}msg()

Fri Oct 10 08:42:41 UTC 2025

On Fri, Oct 10, 2025 at 10:09:00AM +0200, Ondrej Mosnacek 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>

Looks fine:

Reviewed-by: Ming Lei <ming.lei at redhat.com>
Tested-by: Ming Lei <ming.lei at redhat.com>

Thanks,
Ming