Hi, On Wed, Jan 23, 2013 at 3:29 AM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > On Fri, 18 Jan 2013, Kasatkin, Dmitry wrote: > >> Hi Mikulas, >> >> Thanks for looking into it. >> >> On Thu, Jan 17, 2013 at 6:54 AM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: >> > Hi Dmitry >> > >> > I looked at dm-integrity. The major problem is that if crash happens when >> > data were written and checksum wasn't, the block has incorrect checksum >> > and can't be read again. >> > >> >> This is how it works. >> This is a purpose of integrity protection - do not allow "bad" content >> to load and use. >> >> But even with encryption it might happen that some blocks have been >> updated and some not. >> Even if reading the blocks succeeds, the content can be a mess from >> old and new blocks. > > dm-crypt encrypts each 512-byte sector individually, so (assuming that > there is no disk with sector size <512 bytes), it can't result in random > data. You read either new data or old data. I have not expressed correctly what I wanted to say. Basically a file might consists of several sectors where part of them will have an old content and part of them will have a new content. That "combination" content is a garbage... > >> This patch I sent out has one missing feature what I have not pushed yet. >> In the case of none-matching blocks, it just zeros blocks and returns >> no error (zero-on-mismatch). >> Writing to the block replaces the hmac. >> It works quite nicely. mkfs and fsck is able to read and write/fix the >> filesystem. > > But it causes silent data corruption for the user. So it's worse than > returning an error. Agree. Error is good as it was. > >> > How is this integrity target going to be used? Will you use it in an >> > environment where destroying data on crash doesn't matter? (can you >> > describe such environment?) >> > >> >> We are looking for possibility to use it in LSM based environment, >> where we do not want >> attacker could make offline modification of the filesystem and modify >> the TCB related stuff. > > What are the exact attach attack possibilities you are protecting against? > That is to protect against offline attacks only - when the system is down. LSM supposed to protect when system is running. > Can the attacker observe or modify the data while system is running? (for > example the data is accessed remotely over an unsecured network > connection?) Or is it only protecting against modifications when the > system is down? > Right. > Can the attacker modify the partition with hashes? - or do you store it in > another place that is supposed to be secure? > As also Will said in the next email.. That is not hashes, but HMACs. They are protected by the key and do not require secure storage. > What are you going to do if you get failed checksum because of a crash? > Integrity verification failed - return an error. No reason to run modified /sbin/init. I understand about unusable system, but this is to prevent running compromised system. >> > It could possibly be used with ext3 or ext4 with data=journal mode - in >> > this mode, the filesystem writes everything to journal and overwrites data >> > and metadata with copy from journal on reboot, so it wouldn't matter if a >> > block that was being written is unreadable after the reboot. But even with >> > data=journal there are still some corner cases where metadata are >> > overwritten without journaling (for example fsck or tune2fs utilities) - >> > and if a crash happens, it could make metadata unreadable. >> > >> >> In normal environment, if fsck crashes, it might corrupt file system >> in the same way. >> zero-on-mismatch makes block device still accessible/fixable for fsck. > > The problem is that it apmplifies filesystem damage. For example, suppose > that fsck is modifying an inode. You get a crash and on next reboot not > just one inode, but the whole block of inodes is unreadable (or replaced > with zeros). Fsck "fixes" it, but the user loses more files. > >From security perspective it is "unsafe" to run fsck. System has been compromised and fixing it by fsck might result in unexpected behavior. > > I am thinking about possibly rewriting it so that it has two hashes per > sector so that if either old or new data is read, at least one hash > matches and it won't result in data corruption. > This is an improving step. There are different performance issues about such approach. I was working on such solution before dm_bufio appeared. In own integrity data management it was so that I could know what block of primary integrity data is written and could issue write for secondary integrity data. Integrity data blocks writing/updating were handled independently of each other. It was easily possible to get "mirror" block and issue a write request. With bufio API I do not really see how target could be notified that buffer has been written. May be complete callback on like bio_end_io would be needed. You might advise on how to achieve it. But there are performance issues with that and it might be the next step. Thanks, Dmitry > Mikulas -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
