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From: Olof Johansson <olof@lixom.net>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Gao Xiang <gaoxiang25@huawei.com>,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
	Michael Halcrow <mhalcrow@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fscrypt@vger.kernel.org, linux-integrity@vger.kernel.org,
	Mimi Zohar <zohar@linux.vnet.ibm.com>,
	Victor Hsieh <victorhsieh@google.com>
Subject: Re: [RFC PATCH 02/10] fs-verity: add data verification hooks for ->readpages()
Date: Sat, 1 Sep 2018 19:35:53 -0700	[thread overview]
Message-ID: <CAOesGMhGfesVQHHMpqW5r7gMOZccX4RsZZex6y-kfkmuA6K0ow@mail.gmail.com> (raw)
In-Reply-To: <20180825041647.GA726@sol.localdomain>

Hi,

On Fri, Aug 24, 2018 at 9:16 PM, Eric Biggers <ebiggers@kernel.org> wrote:
> Hi Gao,
>
> On Sat, Aug 25, 2018 at 10:29:26AM +0800, Gao Xiang wrote:
>> Hi,
>>
>> At last, I hope filesystems could select the on-disk position of hash tree and 'struct fsverity_descriptor'
>> rather than fixed in the end of verity files...I think if fs-verity preparing such support and interfaces could be better.....hmmm... :(
>
> In theory it would be a much cleaner design to store verity metadata separately
> from the data.  But the Merkle tree can be very large.  For example, a 1 GB file
> using SHA-512 would have a 16.6 MB Merkle tree.  So the Merkle tree can't be an
> extended attribute, since the xattrs API requires xattrs to be small (<= 64 KB),
> and most filesystems further limit xattr sizes in their on-disk format to as
> little as 4 KB.  Furthermore, even if both of these limits were to be increased,
> the xattrs functions (both the syscalls, and the internal functions that
> filesystems have) are all based around getting/setting the entire xattr value.
>
> Also when used with fscrypt, we want the Merkle tree and fsverity_descriptor to
> be encrypted, so they doesn't leak plaintext hashes.  And we want the Merkle
> tree to be paged into memory, just like the file contents, to take advantage of
> the usual Linux memory management.
>
> What we really need is *streams*, like NTFS has.  But the filesystems we're
> targetting don't support streams, nor does the Linux syscall interface have any
> API for accessing streams, nor does the VFS support them.
>
> Adding streams support to all those things would be a huge multi-year effort,
> controversial, and almost certainly not worth it just for fs-verity.
>
> So simply storing the verity metadata past i_size seems like the best solution
> for now.
>
> That being said, in the future we could pretty easily swap out the calls to
> read_mapping_page() with something else if a particular filesystem wanted to
> store the metadata somewhere else.  We actually even originally had a function
> ->read_metadata_page() in the filesystem's fsverity_operations, but it turned
> out to be unnecessary and I replaced it with directly calling
> read_mapping_page(), but it could be changed back at any time.

What about an xattr not to hold the Merkle tree, but to contain a
suitable reference to a file/inode+offset that contains it (+ toplevel
hash for said tree/file or the descriptor/struct)?

If you also expose said file in the directory structure, things such
as backups might be easier to handle. For where the tree is appended
to the file, you could self-reference.


-Olof

  parent reply	other threads:[~2018-09-02  6:49 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-24 16:16 [RFC PATCH 00/10] fs-verity: filesystem-level integrity protection Eric Biggers
2018-08-24 16:16 ` [RFC PATCH 01/10] fs-verity: add setup code, UAPI, and Kconfig Eric Biggers
2018-08-24 17:28   ` Randy Dunlap
2018-08-24 17:42   ` Colin Walters
2018-08-24 22:45     ` Theodore Y. Ts'o
2018-08-25  4:48     ` Eric Biggers
2018-09-14 13:15       ` Colin Walters
2018-09-14 16:21         ` Eric Biggers
2018-09-15 15:27           ` Theodore Y. Ts'o
2018-08-26 16:22   ` Chuck Lever
2018-08-26 17:17     ` Eric Biggers
2018-08-24 16:16 ` [RFC PATCH 02/10] fs-verity: add data verification hooks for ->readpages() Eric Biggers
2018-08-25  2:29   ` [f2fs-dev] " Gao Xiang
2018-08-25  3:45     ` Theodore Y. Ts'o
2018-08-25  4:00       ` Gao Xiang
2018-08-25  5:06         ` Theodore Y. Ts'o
2018-08-25  7:33           ` Gao Xiang
2018-08-25  7:55             ` Gao Xiang
2018-08-25  4:16     ` Eric Biggers
2018-08-25  6:31       ` Gao Xiang
2018-08-25  7:18         ` Eric Biggers
2018-08-25  7:43           ` Gao Xiang
2018-08-25 17:06             ` Theodore Y. Ts'o
2018-08-26 13:44               ` Gao Xiang
2018-09-02  2:35       ` Olof Johansson [this message]
2018-08-26 15:55   ` Chuck Lever
2018-08-26 17:04     ` Eric Biggers
2018-08-26 17:44       ` Gao Xiang
2018-08-24 16:16 ` [RFC PATCH 03/10] fs-verity: implement FS_IOC_ENABLE_VERITY ioctl Eric Biggers
2018-08-24 16:16 ` [RFC PATCH 04/10] fs-verity: implement FS_IOC_MEASURE_VERITY ioctl Eric Biggers
2018-08-24 16:16 ` [RFC PATCH 05/10] fs-verity: add SHA-512 support Eric Biggers
2018-08-24 16:16 ` [RFC PATCH 06/10] fs-verity: add CRC-32C support Eric Biggers
2018-08-24 16:16 ` [RFC PATCH 07/10] fs-verity: support builtin file signatures Eric Biggers
2018-08-24 16:16 ` [RFC PATCH 08/10] ext4: add basic fs-verity support Eric Biggers
2018-08-24 16:16 ` [RFC PATCH 09/10] ext4: add fs-verity read support Eric Biggers
2018-08-24 16:16 ` [RFC PATCH 10/10] f2fs: fs-verity support Eric Biggers
2018-08-25  5:54   ` [f2fs-dev] " Chao Yu
2018-08-26 17:35     ` Eric Biggers
2018-08-27 15:54       ` Chao Yu
2018-08-28  7:27         ` Jaegeuk Kim
2018-08-28  9:20           ` Chao Yu
2018-08-28 17:01             ` Jaegeuk Kim
2018-08-29  1:22               ` Chao Yu
2018-08-29  1:43                 ` Jaegeuk Kim
2018-08-31 20:05 ` [RFC PATCH 00/10] fs-verity: filesystem-level integrity protection Jan Lübbe
2018-08-31 21:39   ` Eric Biggers

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