Re: [PATCH] nbd: provide a way for userspace processes to identify device backends

[Ming Lei]

On Tue, May 18, 2021 at 01:22:19PM +0530, Prasanna Kalever wrote:
> On Tue, May 18, 2021 at 6:00 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> >
> > Hello Prasanna,
> >
> > On Thu, Apr 29, 2021 at 03:58:28PM +0530, Prasanna Kumar Kalever wrote:
> > > Problem:
> > > On reconfigure of device, there is no way to defend if the backend
> > > storage is matching with the initial backend storage.
> > >
> > > Say, if an initial connect request for backend "pool1/image1" got
> > > mapped to /dev/nbd0 and the userspace process is terminated. A next
> > > reconfigure request within NBD_ATTR_DEAD_CONN_TIMEOUT is allowed to
> > > use /dev/nbd0 for a different backend "pool1/image2"
> > >
> > > For example, an operation like below could be dangerous:
> >
> 
> Hello Ming,
> 
> > Can you explain a bit why it is dangerous?
> 
> Yes, sure. Please check the below comments inline,
> 
> >
> > >
> > > $ sudo rbd-nbd map --try-netlink rbd-pool/ext4-image
> > > /dev/nbd0
> > > $ sudo blkid /dev/nbd0
> > > /dev/nbd0: UUID="bfc444b4-64b1-418f-8b36-6e0d170cfc04" TYPE="ext4"
> 
> On Map the rbd-nbd attempting to send NBD_CMD_CONNECT, for backend
> 'rbd-pool/ext4-image'. Post which kernel will allocate a new device
> say /dev/nbd0 for backend file rbd-pool/ext4-image (format:
> <pool>/<backendfile>)
> 
> > > $ sudo pkill -9 rbd-nbd
> 
> Assume normally or abnormally the userspace process (rbd-nbd here) is
> terminated, but then as per the settings the device /dev/nbd0 is not
> returned immediately, the kernel will wait for the
> NBD_ATTR_DEAD_CONN_TIMEOUT to expire.
> 
> At this point two things could be possible:
> 1. if there is a reconfigure request from userspace within the timeout
> then the kernel might reassign the same device /dev/nbd0.
> 2. if the timeout has expired, then the device will be relieved.
> 
> > > $ sudo rbd-nbd attach --try-netlink --device /dev/nbd0 rbd-pool/xfs-image
> > > /dev/nbd0
> > > $ sudo blkid /dev/nbd0
> > > /dev/nbd0: UUID="d29bf343-6570-4069-a9ea-2fa156ced908" TYPE="xfs"
> 
> On attach the rbd-nbd attempt to send NBD_CMD_RECONFIGURE, after which
> the kernel will assign '--device /dev/nbd0' to specified backend.
> 
> But there is a chance that userspace processes might accidentally send
> NBD_CMD_RECONFIGURE claiming for /dev/nbd0 for a different backend
> (rbd-pool/xfs-image instead of original rbd-pool/ext4-image).
> Currently, there is no mechanism to verify if the backend provided
> later with attach(NBD_CMD_RECONFIGURE) is matching with the one
> provided originally with map(NBD_CMD_CONNECT) before granting for a
> attach or reconfigure.
> 
> For example in the above-explained scenario:
> Assume EXT4 on rbd-pool/ext4-image was mounted, after attach (Note:
> device /dev/nbd0 is reconfigured to a different backend file) XFS on
> rbd-pool/xfs-image would get corrupted. If there was an application
> using /dev/nbd0 directly (as a raw block volume), it wouldn't be happy
> either.

OK, got it. If I understand correctly, what you need is to not allow
reconfigure if the nbd disk is mounted, right?

> 
> > >
> > > Solution:
> > > Provide a way for userspace processes to keep some metadata to identify
> > > between the device and the backend, so that when a reconfigure request is
> > > made, we can compare and avoid such dangerous operations.
> > >
> > > With this solution, as part of the initial connect request, backend
> > > path can be stored in the sysfs per device config, so that on a reconfigure
> > > request it's easy to check if the backend path matches with the initial
> > > connect backend path.
> > >
> > > Please note, ioctl interface to nbd will not have these changes, as there
> > > won't be any reconfigure.
> >
> > BTW, loop has similar issue, and patch of 'block: add a sequence number to disks'
> > is added for addressing this issue, what do you think of that generic
> > approach wrt. this nbd's issue? such as used the exposed sysfs sequence number
> > for addressing this issue?
> >
> > https://lore.kernel.org/linux-block/YH81n34d2G3C4Re+@gardel-login/#r
> 
> If I understand the changes and the background of the fix correctly, I
> think with that fix author is trying to monotonically increase the seq
> number and add it to the disk on every single device map/attach and
> expose it through the sysfs, which will help the userspace processes
> further to correlate events for particular and specific devices that
> reuse the same loop device.
> 
> Coming back to my changes:
> I think here with this fix, we are trying to solve a different
> problem. The fix with this patch accepts a cookie or a backend string
> (could be file-path or whatever id userspace choose to provide) from
> userspace at the time of map and stores it in the sysfs
> /sys/block/nbdX/backend path and persists it until unmap is issued on
> the device (meaning that identity stays throughout the life cycle of
> that device, no matter how many detach and attaches happen).

Your solution needs change from userspace side, so it isn't flexible.

> If there
> is a detach request in between (not unmap) then on the next attach
> (reconfigure request to reuse the same device) the stored
> cookie/UUID/backend-string will stand as a reference to verify if the
> newly passed backend is matching with the actual backend passed at map
> time to avoid any misconfigurations by accident and to safely proceed
> with attach.

We can avoid reconfigure if the nbd disk is opened exclusively, such as
mount, please see if the following patch can solve your probelm:

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 4ff71b579cfc..045532a68d1e 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2063,6 +2063,11 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
 		return -EINVAL;
 	}
 
+	/* avoid changing device under exclusive owner */
+	ret = bd_prepare_to_claim(nbd->disk->part0, nbd_genl_reconfigure);
+	if (ret)
+		goto out_no_abort_claim;
+
 	mutex_lock(&nbd->config_lock);
 	config = nbd->config;
 	if (!test_bit(NBD_RT_BOUND, &config->runtime_flags) ||
@@ -2141,6 +2146,8 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
 		}
 	}
 out:
+	bd_abort_claiming(nbd->disk->part0, nbd_genl_reconfigure);
+out_no_abort_claim:
 	mutex_unlock(&nbd->config_lock);
 	nbd_config_put(nbd);
 	nbd_put(nbd);


Thanks,
Ming