On Mon, Jun 01, 2020 at 04:13:45PM -0400, Jes Sorensen wrote: > Hi, > > I am working on adding fsverity support to RPM and I am hitting a tricky > problem. I am see this with RPM, but it really isn't specific to RPM, > and will apply to any method for distribution signatures. > > fsverity is currently hard-wiring the Merkle tree block size to > PAGE_SIZE. This is problematic for a number of reasons, in particular on > architectures that can be configured with different page sizes, such as > ARM, as well as the case where someone generates a shared 'common' > package to be used cross architectures (noarch package in RPM terms). > > For a package manager to be able to create a generic package with > signatures, it basically has to build a signature for every supported > page size of the target architecture. > > Chris Mason is working on adding fsverity support to btrfs, and I > understand he is supporting 4K as the default Merkle tree block size, > independent of the PAGE_SIZE. > > Would it be feasible to make ext4 and other file systems support 4K for > non 4K page sized systems and make that a general recommendation going > forward? > It's possible, but it will be a little difficult. We made a similar simplification for ext4 encryption initially, and it took a long time to remove it. (ext4 encryption was first supported in Linux v4.1. ext4 encryption didn't support block_size != PAGE_SIZE until Linux v5.5, following work by Chandan Rajendra and myself.) Fixing this would require a number of different changes: - Updating fscrypt_verify_bio() and fsverity_verity_page() to iterate through all data blocks in each page, and to handle sub-page Merkle tree blocks - Defining a mechanism other than PageChecked to mark Merkle tree blocks as verified. E.g., allocating an in-memory bitmap as part of the struct fsverity_info. This should also be optional, in the sense that we shouldn't use the memory for it when it's not needed. - Supporting fs-verity in block_read_full_page() in fs/buffer.c. There may be other changes needed too; those are just the obvious ones. Is this something that you or Chris is interested in working on? - Eric
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