On Fri, Oct 21 2016 at 2:33pm -0400, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > Hi > > I found a bug in dm regarding the BLKFLSBUF ioctl. > > The BLKFLSBUF ioctl can be called on a block device and it flushes the > buffer cache. There is one exception - when it is called on ramdisk, it > actually destroys all ramdisk data (it works like a discard on the full > device). > > The device mapper passes this ioctl down to the underlying device, so when > the ioctl is called on a logical volume, it can be used to destroy the > underlying volume group if the volume group is on ramdisk. > > For example: > # modprobe brd rd_size=1048576 > # pvcreate /dev/ram0 > # vgcreate ram_vg /dev/ram0 > # lvcreate -L 16M -n ram_lv ram_vg > # blockdev --flushbufs /dev/ram_vg/ram_lv > --- and now the whole volume group is gone, all data on the > ramdisk were replaced with zeroes > > The BLKFLSBUF ioctl is only allowed with CAP_SYS_ADMIN, so there shouldn't > be security implications with this. > > Whan to do with it? The best thing would be to drop this special ramdisk > behavior and make the BLKFLSBUF ioctl flush the buffer cache on ramdisk > like on all other block devices. But there may be many users having > scripts that depend on this special behavior. > > Another possibility is to stop the device mapper from passing the > BLKFLSBUF ioctl down. If anything DM is being consistent with what the underlying device is meant to do. brd_ioctl() destroys the data in response to BLKFLSBUF.. I'm missing why this is a DM-specific problem. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
