On Wed, Jan 23, 2013 at 8:09 AM, Will Drewry <redpig@xxxxxxxxxxxxx> wrote: > On Tue, Jan 22, 2013 at 5:29 PM, 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. > > With respect to the use of "integrity", you may want to consider > something like dm-integrity-hmac to disambiguate from the BIO > integrity naming. It's why I proposed the somewhat obtuse "verity" > name for the other data-integrity target. > >>> 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. >> >>> 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. >> >>> > 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? >> >> 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? >> >> Can the attacker modify the partition with hashes? - or do you store it in >> another place that is supposed to be secure? > > Given that HMACs are being used to authenticate blocks, I'd assume, > until corrected, that the HMACs aren't required to be on secure > storage. To that end, it seems like there is a distinct risk that an > attacker could use old data blocks and old HMACs to construct an > "authentic" dm-integrity target that doesn't match anything the > user/TPM ever saw in aggregate before. Perhaps I missed something > when I skimmed the code, but it doesn't seem trivial to version the > data or bind them to a large enough group of adjacent blocks without > paying more computational costs (like using a Merkle tree with an > HMAC'd root node). Technically, all the blocks would still be > authentic, but the ordering in time and space wouldn't be. I'd love to > know what ideas you have for that, or if that sort of attack is out of > scope? For ordering in space, inclusion of the sector index in the > HMAC might help. > Hello, Yes. You are right. All is about computational and IO costs. "In time" is really hard to manage. The key is the same and there is a possibility to replace the blocks with older block. But this is a case with encryption as well. right? "in space" - it is easier. As you said sector index might be used like with encryption. Please have a look to dm_int_calc_hmac(). It uses already offset in calculations.. err = crypto_shash_init(&desc.shash); if (!err) err = crypto_shash_update(&desc.shash, digest, size); if (!err) err = crypto_shash_finup(&desc.shash, (u8 *)&offset, sizeof(offset), hmac); Thanks, Dmitry > thanks! > will > >> What are you going to do if you get failed checksum because of a crash? >> >>> > 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. >> >> >> 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. >> >> 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
