linux-kernel - Re: [REGRESSION] Unable to unlock encrypted disk starting with kernel 5.19-rc1+

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Date:   Tue, 28 Jun 2022 09:52:28 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>,
        Alexandre Messier <alex@...ssier.org>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Andrew.Cooper3@...rix.com, mingo@...hat.com,
        dave.hansen@...ux.intel.com, x86@...nel.org,
        regressions@...ts.linux.dev
Subject: Re: [REGRESSION] Unable to unlock encrypted disk starting with kernel
 5.19-rc1+

First of all, thank you for bisecting this!  I know those are a lot of work.

That XSAVEC patch modifies the AVX register save/restore code.  There is
a set of x86 AES acceleration instructions called AES-NI.  Those
instructions use the AVX registers.  So, it's at least a plausible
connection between that patch and your symptoms.  But, I don't think
anyone's been able to reproduce what you're seeing yet.

The kernel XSAVE buffer formats also differ slightly between AMD and
Intel.  That *should* be OK, but it might explain why I can't reproduce
this.

If you get a chance, could you apply this (ugly hackish) patch to the
userspace 'cryptsetup' utility and run it?

	https://sr71.net/~dave/intel/cryptsetup-memcmp.patch

On Ubuntu at least, it was as simple as:

	apt-get source cryptsetup
	apt-get build-dep cryptsetup
	cd cryptsetup-1.6.6
	./configure
	make

Then I could run:

	./src/cryptsetup benchmark --cipher=aes-xts --key-size=512
and
	./src/cryptsetup benchmark --cipher=aes-xts --key-size=256

With that patch applied, you should see some output like:

# ./src/cryptsetup benchmark --cipher=aes-xts --key-size=512
# Tests are approximate using memory only (no storage IO).
memcmp12: 0
memcmp23: 0
memcmp13: 0
memcmp12: -173
memcmp23: 173
memcmp13: 0
#  Algorithm | Key |  Encryption |  Decryption
     aes-xts   512b  4592.2 MiB/s  4192.0 MiB/s

The "memcmp13:" lines should both be 0.  That means that an encryption
and decryption cycle didn't change the data.  You *might* have to run
this in a loop if there's some kind of bad timing involved in triggering
the bug.

If you see a "memcmp13:" with something other than 0, that will narrow
things down and means we'll have a pretty quick reproducer that doesn't
involve luks which should speed things along.