When a filesystem is mounted read-only, make the underlying
block device read-only too. This helps prevent an attacker
who is able to change permissions on the files in /dev
(for example, symlink attack) from modifying the block device.
In particular, this change would have stopped the LG Thrill / Optimus
3D rooting exploit
(http://vulnfactory.org/blog/2012/02/26/rooting-the-lg-thrill-optimus-3d/)
as that exploit modified the raw block device corresponding to /system.
This change also makes UID=0 less powerful. Block devices cannot
be made writable again without CAP_SYS_ADMIN, so an escalation
to UID=0 by itself doesn't give full root access.
adb/mount: Prior to mounting something read-write, remove the
read-only restrictions on the underlying block device. This avoids
messing up developer workflows.
Change-Id: I135098a8fe06f327336f045aab0d48ed9de33807
