Re: Re: [RFC PATCH 02/10] fs-verity: add data verification hooks for ->readpages()

From: Gao Xiang <hsiangkao@aol.com>
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: Eric Biggers <ebiggers@kernel.org>,
	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@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>,
	Gao Xiang <gaoxiang25@huawei.com>
Subject: Re: Re: [RFC PATCH 02/10] fs-verity: add data verification hooks for ->readpages()
Date: Sun, 26 Aug 2018 21:44:04 +0800	[thread overview]
Message-ID: <a7ee7fb5-26a0-6737-9246-93d6d7432a42@aol.com> (raw)
In-Reply-To: <20180825170624.GB10619@thunk.org>

Hi Ted,

Sorry for the late reply...

On 2018/8/26 1:06, Theodore Y. Ts'o wrote:
> On Sat, Aug 25, 2018 at 03:43:43PM +0800, Gao Xiang wrote:
>>> I don't know of any plan to use fs-verity on Android's system partition or to
>>> replace dm-verity on the system partition.  The use cases so far have been
>>> verifying files on /data, like APK files.
>>>
>>> So I don't think you need to support fs-verity in EROFS.
>>
>> Thanks for your information about fs-verity, that is quite useful for us
>> Actually, I was worrying about that these months...  :)
> 
> I'll be even clearer --- I can't *imagine* any situation where it
> would make sense to use fs-verity on the Android system partition.
> Remember, for OTA to work the system image has to be bit-for-bit
> identical to the official golden image for that release.  So the
> system image has to be completely locked down from any modification
> (to data or metadata), and that means dm-verity and *NOT* fs-verity.

I think so mainly because of the security reason you said above.

In addition, I think it is mandatory that the Android system partition
should also _never_ suffer from filesystem corrupted by design (expect
for the storage device corrupt or malware), therefore I think the
bit-for-bit read-only, and identical-verity requirement is quite strong
for Android, which will make the Android system steady and as solid as
rocks.

But I need to make sure my personal thoughts through this topic. :)

> 
> The initial use of fs-verity (as you can see if you look at AOSP) will
> be to protect a small number of privileged APK's that are stored on
> the data partition.  Previously, they were verified when they were
> downloaded, and never again.
> 
> Part of the goal which we are trying to achieve here is that even if
> the kernel gets compromised by a 0-day, a successful reboot should
> restore the system to a known state.  That is, the secure bootloader
> checks the signature of the kernel, and then in turn, dm-verity will
> verify the root Merkle hash protecting the system partition, and
> fs-verity will protect the privileged APK's.  If malware modifies any
> these components in an attempt to be persistent, the modifications
> would be detected, and the worst it could do is to cause subsequent
> reboots to fail until the phone's software could be reflashed.
> 

Yeah, I have seen the the fs-verity presentation and materials from
Android bootcamp and other official channels before.

Thanks for your kindly detailed explanation. :)

Best regards,
Gao Xiang

> Cheers,
> 
> 					- Ted
> 