linux-ext4 - Re: Proposal: A new fs-verity interface

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Message-ID: <20190110181836.GA20467@magnolia>
Date:   Thu, 10 Jan 2019 10:18:36 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        Eric Biggers <ebiggers@...nel.org>,
        linux-fscrypt@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: Proposal: A new fs-verity interface

On Thu, Jan 10, 2019 at 12:15:00AM -0500, Theodore Y. Ts'o wrote:
> The following approach is based in Darrick's suggestion:
> 
> int ioctl(fd, FS_IOC_ENABLE_VERITY, struct fsverity_arg *arg);
> 
> struct fsverity_arg {
>        int fsv_donor_fd;

Explicitly sized fields and padding here, please.  ISTR there are a few
arches that don't have alignment requirements which will make this
messy.

>        u64 fsv_offset;
>        u64 fsv_size;

You might want to allocate some reserved space for flags in case you
ever decide you need it, but otherwise it seems fine to me...

--D

> };
> 
> fsv_offset and fsz_size must be a multiple of the file system block
> size.  If the ioctl comples successfully, as a side effect the
> donor_fd will have a hole punch operation on the specified range.  In
> other words, the equivalent of operation of fallocate(fsv_donor_fd,
> FALLOC_FL_PUNCH_HOLE, fsv_offset, fsv_size), and the file specified by
> fd will be protected using fsverity.
> 
> It will be legal for fsv_donor_fd == fd, so this interface is a
> superset of the original FS_IOC_ENABLE_VERITY ioctl.
> 
> This will hopefully make Christoph and Dave happy because the
> interface does not presuppose how ext4 and f2fs will implement
> fsverity behind the scenes.  However, it does not forbid it, and the
> net cost is that ext4 and f2fs will have to implement code which
> transplants the blocks from the donor_fd to fd in the case where
> donor_fd != fd --- and in the case where blocks are encrypted using
> fscrypt, we will have to decrypt the blocks from donor_fd and possibly
> re-encrypt then in fd's per-file key, which means we'll have to add
> extra complexity to implement the decrypt and re-encrypt passing
> through the page cache.
> 
> But if this helps resolve Christoph and Dave's objections, it
> shouldn't be _too_ much extra complexity.  Before we go ahead an
> implement it, though, I'd appreciate a confirmation that this will
> indeed actually resolve their complaints.
> 
> Thanks,
> 
> 					- Ted