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Re: [dm-devel] dm-integrity: integrity protection device-mapper target


On Wed, Jan 23, 2013 at 3:29 AM, Mikulas Patocka <mpatocka redhat com> 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 redhat com> 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?


> 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.



> Mikulas

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