Holy crap, tried this on my S 3 too (custom rom Cyanogenmod derived MIUI) with the same result. I guess chmoding it to the proper permissions (0600) will cause all kinds of different issues, won't it?
Just tested this. Details first - Custom ROM (based on ICS). Stock google camera app (based on ICS).
"chmod 0600 /dev/exynos-mem" works well. The file permissions then shows crw------- as expected. The camera app still works fine. I tried recording videos and capturing pictures and was successful (1080p & 8MPx settings). Looking at the file permission after launching the camera app still shows no change (i.e. if it was 0600, it remains 0600). However, when I reboot the phone, the file's permission returns to 666. I have done this entire process three times just to be sure, and the file's permissions always returns to the default values.
Might need to look into it further, but frankly I don't have the time nowadays.
This file is located on the initrd on the boot image, however, so you have to pull the boot image, split it, extract the initrd, change the uevent file, repack it, and flash it.
I've done that, and it sticks on boot now.
The proper solution is to obviously limit the DMA to regions of memory only needed by the camera. From my albeit cursory glance of the issue, it seems like you can pretty much write to any region of memory.
I have to imagine an exploit of that gaping a security hole is not very far away given the huge install base of samsung exynos devices.
EDIT - I still don't understand. The camera seems to be completely controlled by black magic. I have not recorded, but opening/closing the camera, then changing the permissions seems to allow you to continue using the camera app even with (0600).