Users of 32-bit Linux distributions have another reason to consider an upgrade following the news that their kernels will not be patched against the latest speculative execution attack, known as Retbleed . The attack, classified as CVE-2022-29900 and CVE-2022-29901 , has led to a number of Linux patches, according to reporting from Phoronix .
A variant of Spectre , Retbleed exploits one of the mitigations against such attacks, and affects particular x86-64 (AMD Zen 1/1+/2 and Intel Core 6th through 8th Gen). It has been addressed in a 64-bit Linux kernel update (and Windows users should already have received mitigations). But the amount of work involved means that the 32-bit Linux kernel won’t be getting the same treatment.
The attack exploits retpolines, a mitigation against Spectre introduced in 2018 that’s a portmanteau word of ‘return’ and ‘trampoline’. The original attack used indirect branches in a processor’s speculative execution system to steal information from the system memory through branch mispredictions leaking data. But retpolines replaced these branches with a return that uses a never-executed infinite loop to prevent a processor from speculating on the target of an indirect jump.
These retpolines can now be bypassed by the new exploit, leading to arbitrary information leaking from a computer’s memory. While at least one proof-of-concept exploit using Retbleed has been coded, it’s unknown if the exploit is at large on the internet.
Responding to queries on the lore.kernel.org mailing list, Intel’s Pawan Gupta wrote: "Intel is not aware of production environments that use 32-bit mode on Skylake-gen CPUs. So this should not be a concern.”
His colleague Peter Zijlstra added: "Yeah, so far nobody cared to fix 32-bit. If someone *realllllly* cares and wants to put the effort in I suppose I'll review the patches, but seriously, you shouldn't be running 32-bit kernels on Skylake / Zen based systems, that's just silly."
So the message is clear: If you’re running a 32-bit Linux distro on a Skylake-vintage CPU, stop it immediately and upgrade to 64bit.
