On 26/11/12 22:18, Andrei POPESCU wrote:
On Lu, 26 nov 12, 21:47:36, Amit wrote:Yes the above would work in most cases but in the case I am dealing with, the filesystem is not mounted yet. For example, I plug in a USB drive. Before it is mounted, there is a /dev/sd[x] node. I can open this node and write anything I want, thereby corrupting the filesystem on that device.Not unless you are 'root' or member of group 'floppy': $ ls -l /dev/sdb1 brw-rw---T 1 root floppy 8, 17 nov 27 00:14 /dev/sdb1 You could just tweak the relevant udev rule to create the device nodes as root:root or root:disk (like hard drives), since root would be able to circumvent any protection and 'disk' is almost the same as 'root'.
I just tested a basic udev rule which sets read-only permissions on any usb disk when inserted. Obviously root would be able to change those permissions, but I don't believe any command (other than chmod) would override read-only for the device file.
dom@oz:~$ cat /etc/udev/rules.d/99-usb-ro.rules:
#Make USB storage devices read only
KERNEL=="sd*",ACTION=="add",ENV{ID_BUS}=="usb",MODE="0444"
dom@oz:~$#(plugs in usb storage device)
dom@oz:~$ ls -l /dev/sd*
brw-rw---T 1 root disk 8, 0 Nov 27 03:16 /dev/sda
brw-rw---T 1 root disk 8, 1 Nov 27 03:16 /dev/sda1
brw-rw---T 1 root disk 8, 2 Nov 27 03:16 /dev/sda2
br--r--r-T 1 root floppy 8, 16 Nov 27 07:12 /dev/sdb
br--r--r-T 1 root floppy 8, 17 Nov 27 07:12 /dev/sdb1
dom@oz:~$
The rule may need a bit of tweaking perhaps. I'm not a udev expert by a
long way.
-- Dom