Re: WIP: verity support for overlayfs

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On Wed, 8 Mar 2023 at 16:29, Alexander Larsson <alexl@xxxxxxxxxx> wrote:
> As was recently discussed in the various threads about composefs we
> want the ability to specify a fs-verity digest for metacopy files,
> such that the lower file used for the data is guaranteed to have the
> specified digest.
> I wrote an initial version of this here:
> I would like some feedback on this approach. Does it make sense?
> For context, here is the main commit text:
> This adds support for a new overlay xattr "overlay.verity", which
> contains a fs-verity digest. This is used for metacopy files, and
> whenever the lowerdata file is accessed overlayfs can verify that
> the data file fs-verity digest matches the expected one.
> By default this is ignored, but if the mount option "verity_policy" is
> set to "validate" or "require", then all accesses validate any
> specified digest. If you use "require" it additionally fails to access
> metacopy file if the verity xattr is missing.
> The digest is validated during ovl_open() as well as when the lower file
> is copied up. Additionally the overlay.verity xattr is copied to the
> upper file during a metacopy operation, in order to later do the validation
> of the digest when the copy-up happens.

Hmm, so what exactly happens if the file is copied up and then
modified?  The verification will fail, no?


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