[PATCH v2 13/16] xen-blkback: Implement diskseq checks

Demi Marie Obenour
2023-05-30T20:33:35Z

This allows specifying a disk sequence number in XenStore. If it does
not match the disk sequence number of the underlying device, the device
will not be exported and a warning will be logged. Userspace can use
this to eliminate race conditions due to major/minor number reuse.
Old kernels do not support the new syntax, but a later patch will allow
userspace to discover that the new syntax is supported.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
drivers/block/xen-blkback/xenbus.c | 112 +++++++++++++++++++++++------
1 file changed, 89 insertions(+), 23 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 4807af1d58059394d7a992335dabaf2bc3901721..9c3eb148fbd802c74e626c3d7bcd69dcb09bd921 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -24,6 +24,7 @@ struct backend_info {
struct xenbus_watch backend_watch;
unsigned major;
unsigned minor;
+ unsigned long long diskseq;
char *mode;
};

@@ -479,7 +480,7 @@ static void xen_vbd_free(struct xen_vbd *vbd)

static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
unsigned major, unsigned minor, int readonly,
- int cdrom)
+ bool cdrom, u64 diskseq)
{
struct xen_vbd *vbd;
struct block_device *bdev;
@@ -507,6 +508,26 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
xen_vbd_free(vbd);
return -ENOENT;
}
+
+ if (diskseq) {
+ struct gendisk *disk = bdev->bd_disk;
+
+ if (unlikely(disk == NULL)) {
+ pr_err("%s: device %08x has no gendisk\n",
+ __func__, vbd->pdevice);
+ xen_vbd_free(vbd);
+ return -EFAULT;
+ }
+
+ if (unlikely(disk->diskseq != diskseq)) {
+ pr_warn("%s: device %08x has incorrect sequence "
+ "number 0x%llx (expected 0x%llx)\n",
+ __func__, vbd->pdevice, disk->diskseq, diskseq);
+ xen_vbd_free(vbd);
+ return -ENODEV;
+ }
+ }
+
vbd->size = vbd_sz(vbd);

if (cdrom || disk_to_cdi(vbd->bdev->bd_disk))
@@ -707,6 +728,9 @@ static void backend_changed(struct xenbus_watch *watch,
int cdrom = 0;
unsigned long handle;
char *device_type;
+ char *diskseq_str = NULL;
+ int diskseq_len;
+ unsigned long long diskseq;

pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);

@@ -725,10 +749,46 @@ static void backend_changed(struct xenbus_watch *watch,
return;
}

- if (be->major | be->minor) {
- if (be->major != major || be->minor != minor)
- pr_warn("changing physical device (from %x:%x to %x:%x) not supported.\n",
- be->major, be->minor, major, minor);
+ diskseq_str = xenbus_read(XBT_NIL, dev->nodename, "diskseq", &diskseq_len);
+ if (IS_ERR(diskseq_str)) {
+ int err = PTR_ERR(diskseq_str);
+ diskseq_str = NULL;
+
+ /*
+ * If this does not exist, it means legacy userspace that does not
+ * support diskseq.
+ */
+ if (unlikely(!XENBUS_EXIST_ERR(err))) {
+ xenbus_dev_fatal(dev, err, "reading diskseq");
+ return;
+ }
+ diskseq = 0;
+ } else if (diskseq_len <= 0) {
+ xenbus_dev_fatal(dev, -EFAULT, "diskseq must not be empty");
+ goto fail;
+ } else if (diskseq_len > 16) {
+ xenbus_dev_fatal(dev, -ERANGE, "diskseq too long: got %d but limit is 16",
+ diskseq_len);
+ goto fail;
+ } else if (diskseq_str[0] == '0') {
+ xenbus_dev_fatal(dev, -ERANGE, "diskseq must not start with '0'");
+ goto fail;
+ } else {
+ char *diskseq_end;
+ diskseq = simple_strtoull(diskseq_str, &diskseq_end, 16);
+ if (diskseq_end != diskseq_str + diskseq_len) {
+ xenbus_dev_fatal(dev, -EINVAL, "invalid diskseq");
+ goto fail;
+ }
+ kfree(diskseq_str);
+ diskseq_str = NULL;
+ }
+
+ if (be->major | be->minor | be->diskseq) {
+ if (be->major != major || be->minor != minor || be->diskseq != diskseq)
+ pr_warn("changing physical device (from %x:%x:%llx to %x:%x:%llx)"
+ " not supported.\n",
+ be->major, be->minor, be->diskseq, major, minor, diskseq);
return;
}

@@ -756,29 +816,35 @@ static void backend_changed(struct xenbus_watch *watch,

be->major = major;
be->minor = minor;
+ be->diskseq = diskseq;

err = xen_vbd_create(be->blkif, handle, major, minor,
- !strchr(be->mode, 'w'), cdrom);
-
- if (err)
- xenbus_dev_fatal(dev, err, "creating vbd structure");
- else {
- err = xenvbd_sysfs_addif(dev);
- if (err) {
- xen_vbd_free(&be->blkif->vbd);
- xenbus_dev_fatal(dev, err, "creating sysfs entries");
- }
- }
+ !strchr(be->mode, 'w'), cdrom, diskseq);

if (err) {
- kfree(be->mode);
- be->mode = NULL;
- be->major = 0;
- be->minor = 0;
- } else {
- /* We're potentially connected now */
- xen_update_blkif_status(be->blkif);
+ xenbus_dev_fatal(dev, err, "creating vbd structure");
+ goto fail;
}
+
+ err = xenvbd_sysfs_addif(dev);
+ if (err) {
+ xenbus_dev_fatal(dev, err, "creating sysfs entries");
+ goto free_vbd;
+ }
+
+ /* We're potentially connected now */
+ xen_update_blkif_status(be->blkif);
+ return;
+
+free_vbd:
+ xen_vbd_free(&be->blkif->vbd);
+fail:
+ kfree(diskseq_str);
+ kfree(be->mode);
+ be->mode = NULL;
+ be->major = 0;
+ be->minor = 0;
+ be->diskseq = 0;
}

/*