On Tue, Nov 20, 2012 at 3:53 PM, Alan Cox <a...@lxorguk.ukuu.org.uk> wrote: >> > 1. Why not just have your userspace mount -o remount the file system this >> > way already in early boot. (and if you trojanned boot that early then any >> > supposed security gain is already lost) >> >> That's certainly possible, but I am hoping to avoid adding any extra >> boot time. The kernel is responsible for this mount, so its flags >> should be configurable, resulting in no time penalty anywhere. > > You just broke my bullshitometer > > It's a single syscall from your init binary, its microseconds.
Whatever, I still see it as a needless inefficiency. >> > 2. If you want to do this right then you need to work out what you are >> > trying to prevent. Your devtmpfs can force file permissions on the >> > underlying device nodes by having its own operation handling for chmod. >> > >> > At that point you can force permissions on anything that you want to >> > avoid floating around that filesystem with other rights, while not >> > touching it on device or directory nodes where the meaning is different. >> > >> > In its current form however it appears to be a kernel implementation of >> > "mount is too hard". >> >> This change also stops mmap() with PROT_EXEC which a chmod handler >> wouldn't be able to do. > > You don't want to stop mmap with PROT_EXEC on /dev/mem as that breaks a > load of stuff, you want to stop people adding stuff to that file system > and executing it. Well, initially the latter, yes. But as it turns out, setting noexec also stops PROT_EXEC on /dev/mem. Since the systems I'm building for all use KMS, there's no need to execute regions of /dev/mem (e.g. VESA BIOS init, etc). -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
