The researchers from ETH Zurich COMSEC published a new speculative execution attack dubbed Retbleed (CVE-2022-29900 for AMD, CVE-2022-29901 for Intel).
It is a variant of the Spectre vulnerability and exploits return instructions, which were so far considered impractical to attack.
An unprivileged user can inject branch targets into kernel address space, and their demo leaks kernel memory on both vendors.
Intel Gen 6 - 8, from Skylake through Coffee Lake, are affected.
Also, AMD Zen 1, Zen 1+, and Zen 2 processors are affected.
Microsoft Windows is not affected, as they are already using Indirect Branch Restricted Speculation (IBRS).
For Linux a patch was pulled into the mainline kernel.
The measured performance loss was in the range of 14 to 39%.
Phoronix benchmarked the patched Linux 5.19 kernel on Core i7 8700K, Xeon E3-1260L v5, Threadripper 3960X and EPYC 7742 systems and called it one of the most performance-wrenching mitigations since the early Spectre/Meltdown days.
If you would rather keep the speed and the risk, retbleed=off on the kernel command line turns it off again.
Update 2022-09-14: In Linux 5.19 guests on VMware ESXi, the Intel mitigation costs up to 70% of compute performance.