linux-kernel - Re: Should Linux set the new constant-time mode CPU flags?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Yw0O6pQ7oKHGmQ0O@zx2c4.com>
Date:   Mon, 29 Aug 2022 15:09:30 -0400
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        Adam Langley <agl@...gle.com>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: Should Linux set the new constant-time mode CPU flags?

On Mon, Aug 29, 2022 at 06:08:07PM +0000, Eric Biggers wrote:
> I'd much prefer it being set unconditionally by default as well, as making
> everyone (both kernel and userspace) turn it on and off constantly would be a
> nightmare.
> 
> Note that Intel's documentation says that CPUs before Ice Lake behave as if
> DOITM is always set:
> 
>     "For Intel® Core™ family processors based on microarchitectures before Ice
>     Lake and Intel Atom® family processors based on microarchitectures before
>     Gracemont that do not enumerate IA32_UARCH_MISC_CTL, developers may assume
>     that the instructions listed here operate as if DOITM is enabled."
> 
> (It's a bit ambiguous, as it leaves the door open to IA32_UARCH_MISC_CTL being
> retroactively added to old CPUs.  But I assume that hasn't actually happened.)
> 
> So I think the logical approach is to unconditionally set DOITM by default, to
> fix this CPU bug in Ice Lake and later and just bring things back to the way
> they were in CPUs before Ice Lake.  With that as a baseline, we can then discuss
> whether it's useful to provide ways to re-enable this CPU bug / "feature", for
> people who want to get the performance boost (if one actually exists) of data
> dependent timing after carefully assessing the risks.
> 
> The other way around, of making everything insecure by default, seems like a
> really bad idea.

Right. It's actually kind of surprising that Intel didn't already do
this by default. Sure, maybe the Intel manual never explicitly
guaranteed constant time, but a heck of a lot of code relies on that
being the case.

Jason