On Wed, Oct 28, 2026 at 1:10 PM Benjamin Marzinski <[email protected]> wrote: > > On Mon, Oct 26, 2026 at 04:34:02PM +0530, Rahul Rao wrote: > > pool_ctr() takes separate specification strings for the metadata and data > > devices. However, different strings (or format representations such as > > /dev/ram0 and 1:0) can resolve to the same underlying block device. > > > > When the read_only feature is requested, the metadata device is opened > > read-only before the data device is opened read-write. If the devices > > alias, the second dm_get_device() call upgrades the existing device's > > mode. upgrade_mode() replaces the dm_dev and drops the old table device, > > leaving pool_ctr() with a stale metadata_dev pointer. The subsequent > > metadata_dev->bdev access triggers a KASAN use-after-free report. > > > > Acquire the read-write data device first so that acquiring the metadata > > device cannot trigger this mode upgrade. Then reject matching block > > devices, including those specified through different names, and release > > the devices in reverse acquisition order on error. > > What this patch fixes looks good to me, but there is still an issue > here. There is a similar problem in thin_ctr() if the pool and origin > device name strings are different but they alias. Unfortunately, this > style of fix doesn't work as well there. It is only a valid fix for a > thin-pool target because that is a DM_TARGET_SINGLETON. That means there > can't be another target that's part of the same table. This isn't the > case with the thin target. There can be other targets in the table, and > you can't force correct dm_get_device() ordering between them. Like > Mikulas said about a similar issue with dm-snapshot, this is probably > something that is better fixed in dm_get_device() than in its callers. > > -Ben
Agreed. Fixing in dm_get_device() looks like the right direction. A possible approach is to avoid upgrade_mode() and allow a dm_dev_internal to hold multiple references to table_devices of different modes. This way we can fix dm-thin by simply comparing the block_device pointers in pool_ctr() and thin_ctr() after dm_get_device() returns, and reject the table if they alias. dm-snapshot already has the block_device comparison, so once dm_get_device() is fixed it is covered too. I have a rough WIP that seems to confirm the idea, though the approach still needs refinement. > > > > > Reported-by: [email protected] > > Closes: https://syzkaller.appspot.com/bug?extid=83827f9a59abe148bff4 > > Fixes: 991d9fa02da0 ("dm: add thin provisioning target") > > Cc: [email protected] > > Signed-off-by: Rahul Rao <[email protected]> > > --- > > drivers/md/dm-thin.c | 22 ++++++++++++++-------- > > 1 file changed, 14 insertions(+), 8 deletions(-) > > > > diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c > > index 59392de7a477..52e47874048d 100644 > > --- a/drivers/md/dm-thin.c > > +++ b/drivers/md/dm-thin.c > > @@ -3326,18 +3326,24 @@ static int pool_ctr(struct dm_target *ti, unsigned > > int argc, char **argv) > > > > metadata_mode = BLK_OPEN_READ | > > ((pf.mode == PM_READ_ONLY) ? 0 : BLK_OPEN_WRITE); > > - r = dm_get_device(ti, argv[0], metadata_mode, &metadata_dev); > > + r = dm_get_device(ti, argv[1], BLK_OPEN_READ | BLK_OPEN_WRITE, > > &data_dev); > > if (r) { > > - ti->error = "Error opening metadata block device"; > > + ti->error = "Error getting data device"; > > goto out_unlock; > > } > > - warn_if_metadata_device_too_big(metadata_dev->bdev); > > > > - r = dm_get_device(ti, argv[1], BLK_OPEN_READ | BLK_OPEN_WRITE, > > &data_dev); > > + r = dm_get_device(ti, argv[0], metadata_mode, &metadata_dev); > > if (r) { > > - ti->error = "Error getting data device"; > > - goto out_metadata; > > + ti->error = "Error opening metadata block device"; > > + goto out_data; > > + } > > + > > + if (metadata_dev->bdev == data_dev->bdev) { > > + ti->error = "Metadata and data devices must be different"; > > + r = -EINVAL; > > + goto out; > > } > > + warn_if_metadata_device_too_big(metadata_dev->bdev); > > > > if (kstrtoul(argv[2], 10, &block_size) || !block_size || > > block_size < DATA_DEV_BLOCK_SIZE_MIN_SECTORS || > > @@ -3426,9 +3432,9 @@ static int pool_ctr(struct dm_target *ti, unsigned > > int argc, char **argv) > > out_free_pt: > > kfree(pt); > > out: > > - dm_put_device(ti, data_dev); > > -out_metadata: > > dm_put_device(ti, metadata_dev); > > +out_data: > > + dm_put_device(ti, data_dev); > > out_unlock: > > mutex_unlock(&dm_thin_pool_table.mutex); > > > > -- > > 2.43.0 > >
