From: Eric Biggers [mailto:ebiggers@kernel.org]
Sent: Monday, November 15, 2021 8:05 PM
On Mon, Nov 15, 2021 at 08:49:41AM +0000, Roberto Sassu wrote:

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From: Eric Biggers [mailto:ebiggers@kernel.org]
Sent: Friday, November 12, 2021 8:12 PM
On Fri, Nov 12, 2021 at 01:44:11PM +0100, Roberto Sassu wrote:

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Make the necessary modifications to support fsverity in tmpfs.

First, implement the fsverity operations (in a similar way of f2fs). These
operations make use of shmem_read_mapping_page() instead of
read_mapping_page() to handle the case where the page has been

swapped

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out.

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The fsverity descriptor is placed at the end of the file and its location
is stored in an xattr.

Second, implement the ioctl operations to enable, measure and read

fsverity

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metadata.

Lastly, add calls to fsverity functions, to ensure that fsverity-relevant
operations are checked and handled by fsverity (file open, attr set, inode
evict).

Fsverity support can be enabled through the kernel configuration and
remains enabled by default for every tmpfs filesystem instantiated (there
should be no overhead, unless fsverity is enabled for a file).

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>

I don't see how this makes sense at all.  The point of fs-verity is to avoid
having to hash the whole file when verifying it.  However, obviously the

whole

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file still has to be hashed to build the Merkle tree in the first place.  That
makes sense for a persistent filesystem where a file can be written once and
verified many times.  I don't see how it makes sense for tmpfs, where files

have

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to be re-created on every boot.  You might as well just hash the whole file.

The point of adding fsverity support for tmpfs was to being able to do
integrity enforcement with just one mechanism, given that I was
planning to do integrity verification with reference values loaded
to the kernel with DIGLIM [1].

With an LSM such as IPE [2], integrity verification would consist in
querying the fsverity digest with DIGLIM and allowing the operation
if the digest was found. With fsverity support in tmpfs, this can be
done from the very beginning of the boot process.

Using regular file digests would be also possible but this requires
loading with DIGLIM both fsverity and non-fsverity reference values.
It would also require two separate mechanisms for calculating
the file digest depending on the filesystem. It could be done, but
I thought it was easier to add support for fsverity in tmpfs.

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Also, you didn't implement actually verifying the data (by calling
fsverity_verify_page()), so this patch doesn't really do anything anyway.

Yes, at the end I didn't add it. Probably the only place where
calling fsverity_verify_page() would make sense is when a page
is swapped in (assuming that the swap device is untrusted).

I tried to add a call in shmem_swapin_page() but fsverity complained
due to the fact that the page was already up to date, and also
rejected the page. I will check it better.

It sounds like you really only care about calculating fs-verity file digests.
That's just an algorithm for hashing a file, so it could just be implemented in
generic code that operates on any file on any filesystem, like how IMA
implemennts full file hashing for any file.  There isn't a need for any special
filesystem support to do this.

Initially I thought the same. Then, I realized that fsverity is much more
than that. Fsverity could be seen as a sort of property enforcer, it provides
a property associated to the file (the fsverity digest) and ensures that
the property remains the same while the system is running. In addition,
it takes advantage of the page cache to avoid remeasuring an up to date
page.

This remove some burden from LSMs. IPE would have just to compare
the fsverity digest with that in the policy (or just query it with DIGLIM).
Not taking into consideration the specific filesystem, not having to
fall back to the new fsverity measurement function, and avoiding to
preserve the fsverity property by itself, would make the LSM
implementation very simple.

Roberto

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