Re: [PATCH 0/4] dm verity: add support for error correction

[Sami Tolvanen]

On Thu, Nov 05, 2015 at 08:34:04AM +0100, Milan Broz wrote:
> could you please elaborate why is all this needed? To extend support
> of some faulty flash chips?

This makes dm-verity more robust against corruption caused by either
hardware or software bugs, both of which we have seen in the past on
actual devices.

Note that unlike the error correction sometimes included in flash
storage devices, this doesn't merely protect against random bit flips,
it makes it possible to recover from several megabytes of corrupted
or lost data.

> Do you have some statistics that there are really such correctable errors
> in real devices?

Sorry, I don't have statistics to share at the moment.

> Anyway, I really do not understand layer separation here.

I should have elaborated more on this. Implementing this without
integrity checking would not be feasible for a few reasons:

  1. Being able to detect which blocks are corrupted allows us to
     avoid correcting valid blocks. Correcting errors is slow and
     this is the only way to keep performance acceptable.

  2. Due to a property of erasure codes, we can correct twice as
     many errors if we know where the errors are. Using the hash
     tree to detect corrupted blocks lets us locate erasures.

  3. Error correction algorithms may not produce valid output and
     without integrity checking, there's no reliable way to detect
     when we actually succeeded in correcting a block.

> Are we sure this combination does not create some unintended
> gap in integrity checking?

Yes, I'm sure. Corrupted blocks are integrity checked again after they
are corrected to make sure only valid data is allowed to pass.

> Why the integrity check should even try to do some
> error correction if there is an intentional integrity attack?

Most corruption is not malicious and being able to recover from it
makes the system more reliable. This doesn't make it any easier for an
attacker to break dm-verity. That would still require finding a hash
collision (or being able to modify the hash tree).

> The second question - why are you writing another separate tool
> for maintenance for dm-verity when there is veritysetup?

Our tool for generating error correction metadata is independent from
dm-verity. This data is also used by other software to correct errors
during software updates, for example. If there's interest, I can help
in adding this functionality to veritysetup.

Sami
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