On 2015-11-17 14:16, Seth Forshee wrote:
From a completeness perspective, you'd also need to hook into DM, MD, and bcache to handle their backing devices. There's not much we could do about iSCSI/ATAoE/NBD devices, and I think being able to restrict stuff calling mount from a userns to only be able to use FUSE would still be useful (FWIW, GRUB2 has a tool to use FUSE for testing it's own filesystem drivers, which I use regularly when I just need a read-only mount).On Tue, Nov 17, 2015 at 02:02:09PM -0500, Austin S Hemmelgarn wrote:On 2015-11-17 12:55, Al Viro wrote:On Tue, Nov 17, 2015 at 11:25:51AM -0600, Seth Forshee wrote:Shortly after that I plan to follow with support for ext4. I've been fuzzing ext4 for a while now and it has held up well, and I'm currently working on hand-crafted attacks. Ted has commented privately (to others, not to me personally) that he will fix bugs for such attacks, though I haven't seen any public comments to that effect._Static_ attacks, or change-image-under-mounted-fs attacks?To properly protect against attacks on mounted filesystems, we'd need some new concept of a userspace immutable file (that is, one where nobody can write to it except the kernel, and only the kernel can change it between regular access and this new state), and then have the kernel set an image (or block device) to this state when a filesystem is mounted from it (this introduces all kinds of other issues too however, for example stuff that allows an online fsck on the device will stop working, as will many un-deletion tools).Yeah, Serge and I were just tossing that idea around on irc. If we can make that work then it's probably the best solution. From a naive perspective it seems like all we really have to do is make the block device inode immutable to userspace when it is mounted. And the parent block device if it's a partition, which might be a bit troublesome. We'd have to ensure that writes couldn't happen via any fds already open when the device was mounted too. We'd need some cooperation from the loop driver too I suppose, to make sure the file backing the loop device is also immutable.
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