The msgs displayed by the kernel tells me that the fs is first mounted in RO mode and then in RW Mode. Why is it so and why is this necessary .

The way you worded the question sounds as if it's right out of a text book

My guess is that it mounts it first in RO mode because all it has to do is read the kernel parameters, and boot the necessary items, then remounts in RW mode to allow for changes to directories, and to files (such as mtab) and those also needing an IP and such.

That is just my guess though...

And why is it necessary? Well if I am even remotely close, then that would seem very secure to me. Your kernel, and only your kernel is read and acted upon during the RO process, thereby if a virus somehow infected your system, and attacked and replaced the kernel (hid itself as the kernel) then nothing horrible could actually be done, and the system would not get past the RO point (because the virus is not actually the kernel which requires certain obvious parameters to be executed properly). Again, just a theory, based off my theory.

Well the virus thing is a nice theory but it's the kernel that first mounts the root filesystem in RO mode. So if the virus replaced the kernel then the virus wouldn't even get access to the filesystem in readonly mode. Unless the virus would be a little smarter but in that case they could just as well access it in read write mode.

The kernel needs the root filesystem to continue it's boot process so that's why it gets mounted. The root filesystem can also be on readonly media like a cd, so sometimes it won't even be able to mount it in read/write mode. The remount in read/write mode is usually done by the boot scripts if it is necessary to have a root filesystem with read/write access.