On Tue, Nov 20, 2012 at 1:13 PM, Alan Cox <a...@lxorguk.ukuu.org.uk> wrote: > On Tue, 20 Nov 2012 12:42:38 -0800 > Kees Cook <keesc...@chromium.org> wrote: > >> Since devtmpfs is writable, make the default noexec,nosuid as well. This >> protects from the case of a privileged process having an arbitrary file >> write flaw and an argumentless arbitrary execution (i.e. it would lack >> the ability to run "mount -o remount,exec,suid /dev"). > > Ok this looks crap on two levels. > > 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. > 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. The noexec and nosuid mount options were designed for this sort of thing, so I think that's where it should be handled. -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/
