[openssl-users] request for TLBleed information / non-constant-time vulnerabilities

Michael Wojcik Michael.Wojcik at microfocus.com
Fri Jul 27 13:35:55 UTC 2018

> From: openssl-users [mailto:openssl-users-bounces at openssl.org] On Behalf
> Of Michael R. Hines via openssl-users
> Sent: Thursday, July 26, 2018 14:49
> Our team is trying to get an accurate understanding of whether or not
> cryptographic libraries are vulnerable to the kind of non-constant-time
> attack used by exploits such as the one recently documented here:
> https://www.vusec.net/wp-content/uploads/2018/07/tlbleed-author-
> preprint.pdf

That's easy: Yes. The attack in the published paper is against a cryptographic library (libgcrypt), so at least one cryptographic library is vulnerable.

More generally, TLBleed is not a software vulnerability, and as far as I'm aware no practical software mitigations have been shown for it. Therefore cryptographic libraries, like all other software, are vulnerable.

The TLBleed authors note that their specific attack can be prevented by disabling hyperthreading (a system configuration mitigation), or by aggressively partitioning the target process address space (which would require massive code changes and would probably not be feasible for libraries, or for most applications). Beyond that we have only the usual mitigating factors: the attacker must be local and the attack requires substantial effort.

(I'm only commenting on TLBleed here because I'm not sure what you mean by "non-constant-time attack". TLBleed isn't a timing side channel, so what does constant time have to do with the question?)

> Unfortunately, Intel has not provided much guidance in this area but has
> indicated that software mitigation can and should be implemented by
> libraries like OpenSSL.

Intel is spreading FUD, because they know perfectly well that microarchitecture side channel vulnerabilities are a big PR problem. So they're doing whatever they can to minimize the issue.

AMD similarly are pretending that just because no one's demonstrated a TLB side channel on their processors, that they don't have to worry about the possibilities.

> We're also not currently aware of any open CVEs
> or embargos active for this particular side-channel attack.

Well, no, because the manufacturers are claiming there is no problem, or if there is that it's someone else's.

More importantly, as the TLBleed authors, and the authors of the original Spectre paper, and many other researchers have pointed out, microarchitecture side channels are a large class of vulnerabilities. Spot defenses against particular variants rarely help protect against other variants. Microarchitecture side channel attacks will be with us for a long time.

Michael Wojcik
Distinguished Engineer, Micro Focus

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