From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Dave Chinner <david@fromorbit.com>,
Christoph Hellwig <hch@infradead.org>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>,
Eric Biggers <ebiggers@kernel.org>,
linux-fscrypt@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-ext4@vger.kernel.org,
linux-f2fs-devel@lists.sourceforge.net,
linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org,
Jaegeuk Kim <jaegeuk@kernel.org>,
Victor Hsieh <victorhsieh@google.com>,
Chandan Rajendra <chandan@linux.vnet.ibm.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2 01/12] fs-verity: add a documentation file
Date: Wed, 19 Dec 2018 14:30:05 -0500 [thread overview]
Message-ID: <20181219193005.GB6889@mit.edu> (raw)
In-Reply-To: <20181219071420.GC2628@infradead.org> <20181219021953.GD31274@dastard>
On Wed, Dec 19, 2018 at 01:19:53PM +1100, Dave Chinner wrote:
> Putting metadata in user files beyond EOF doesn't work with XFS's
> post-EOF speculative allocation algorithms.
>
> i.e. Filesystem design/algorithms often assume that the region
> beyond EOF in user files is a write-only region. e.g. We can allow
> extents beyond EOF to be uninitialised because they are in a write
> only region of the file and so there's no possibility of stale data
> exposure. Unfortunately, putting filesystem/security metadata beyond
> EOF breaks these assumptions - it's no longer a write-only region.
On Tue, Dec 18, 2018 at 11:14:20PM -0800, Christoph Hellwig wrote:
> Filesystems already use blocks beyond EOF for preallocation, either
> speculative by the file system itself, or explicitly by the user with
> fallocate. I bet you will run into bugs with your creative abuse
> sooner or later. Indepnd of that the interface simply is gross, which
> is enough of a reason not to merge it.
Both of these concerns aren't applicable for fs-verity because the
entire file will be read-only. So there will be no preallocation or
fallocation going on --- or allowed --- for a file which is protected
by fs-verity. Since no writes are allowed at all, it won't break any
file systems' assumptions about "write-only regions".
As far as whether it's "gross" --- that's a taste question, and I
happen to think it's more "clever" than "gross". It allows for a very
simple implementation, *leveraging* the fact that the file will never
change --- and especially, grow in length. So why not use the space
after EOF?
The alternative requires adding Solaris-style alternate data streams
support. Whether or not ADS is a good idea or just an invitation to
malware authors[1] is something which can be debated, but my position
is it's unnecessary given the requirements of fs-verity. And avoiding
such complexity is a *good* thing, not a bad thing.
[1] https://www.deepinstinct.com/2018/06/12/the-abuse-of-alternate-data-stream-hasnt-disappeared/
- Ted
next prev parent reply other threads:[~2018-12-19 19:30 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-01 22:52 [PATCH v2 00/12] fs-verity: read-only file-based authenticity protection Eric Biggers
2018-11-01 22:52 ` [PATCH v2 01/12] fs-verity: add a documentation file Eric Biggers
2018-12-12 9:14 ` Christoph Hellwig
2018-12-12 20:26 ` Eric Biggers
2018-12-13 20:22 ` Christoph Hellwig
2018-12-14 4:48 ` Eric Biggers
2018-12-17 16:49 ` Christoph Hellwig
2018-12-17 18:32 ` Eric Biggers
2018-12-19 7:09 ` Christoph Hellwig
2018-12-17 20:00 ` Darrick J. Wong
2018-12-19 0:16 ` Theodore Y. Ts'o
2018-12-19 2:19 ` Dave Chinner
2018-12-19 19:30 ` Theodore Y. Ts'o [this message]
2018-12-19 21:35 ` Dave Chinner
2018-12-20 22:01 ` Theodore Y. Ts'o
2018-12-21 7:04 ` Christoph Hellwig
2018-12-21 10:06 ` Richard Weinberger
2018-12-21 15:47 ` Theodore Y. Ts'o
2018-12-21 15:53 ` Matthew Wilcox
2018-12-21 16:28 ` Theodore Y. Ts'o
2018-12-21 16:34 ` Matthew Wilcox
2018-12-21 19:13 ` Linus Torvalds
2018-12-22 4:17 ` Theodore Y. Ts'o
2018-12-22 22:47 ` Linus Torvalds
2018-12-23 4:34 ` Theodore Y. Ts'o
2018-12-23 4:10 ` Matthew Wilcox
2018-12-23 4:45 ` Theodore Y. Ts'o
2019-01-04 20:41 ` Daniel Colascione
2018-12-19 7:14 ` Christoph Hellwig
2018-12-19 7:11 ` Christoph Hellwig
[not found] ` <CAHk-=wiB8vGbje+NgNkMZupHsZ_cqg6YEBV+ZXSF4wnywFLRHQ@mail.gmail.com>
2018-12-19 7:19 ` Christoph Hellwig
2018-12-14 5:17 ` Theodore Y. Ts'o
2018-12-14 5:39 ` Eric Biggers
2018-12-17 16:52 ` Christoph Hellwig
2018-12-17 19:15 ` Eric Biggers
2018-12-21 16:11 ` Matthew Wilcox
2018-11-01 22:52 ` [PATCH v2 02/12] fs-verity: add setup code, UAPI, and Kconfig Eric Biggers
2018-11-01 22:52 ` [PATCH v2 03/12] fs-verity: add MAINTAINERS file entry Eric Biggers
2018-11-01 22:52 ` [PATCH v2 04/12] fs-verity: add data verification hooks for ->readpages() Eric Biggers
2018-11-01 22:52 ` [PATCH v2 05/12] fs-verity: implement FS_IOC_ENABLE_VERITY ioctl Eric Biggers
2018-11-01 22:52 ` [PATCH v2 06/12] fs-verity: implement FS_IOC_MEASURE_VERITY ioctl Eric Biggers
2018-11-01 22:52 ` [PATCH v2 07/12] fs-verity: add SHA-512 support Eric Biggers
2018-11-01 22:52 ` [PATCH v2 08/12] fs-verity: add CRC-32C support Eric Biggers
2018-11-01 22:52 ` [PATCH v2 09/12] fs-verity: support builtin file signatures Eric Biggers
2018-11-01 22:52 ` [PATCH v2 10/12] ext4: add basic fs-verity support Eric Biggers
2018-11-02 9:43 ` Chandan Rajendra
2018-11-06 1:25 ` Eric Biggers
2018-11-06 6:52 ` Chandan Rajendra
2018-11-05 21:05 ` Andreas Dilger
2018-11-06 1:11 ` Eric Biggers
2018-11-01 22:52 ` [PATCH v2 11/12] ext4: add fs-verity read support Eric Biggers
2018-11-01 22:52 ` [PATCH v2 12/12] f2fs: fs-verity support Eric Biggers
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