Linux Preps For New AMD Zen 6 BTB CTX Isolation Security Feature

A recently posted Linux kernel patch has revealed a new security improvement with Zen 6 processors. There is a new security defense following the Speculative Return Stack Overflow (SRSO) vulnerability that affected Zen 1 to Zen 4 generations of AMD processors. With Zen 6 is the introduction of Branch Target Buffer (BTB) protection to isolate different contexts from one another. AMD Linux engineer Borislav Petkovv explained on a kernel patch that Zen 6 brings BTB protection to isolate different user/kernel and guest/host contexts from one another: "Zen6 has BTB protection which isolates the different contexts (user/kernel, guest/host) from one another. This makes the SafeRET mitigation there unnecessary leaving the user/user and guest/guest attack vectors open, whose protection is handled by the Spectre v2 mitigation setting to do IBPB on a context switch. Detect that setting and report it with a new mitigation string." The patch is queued up into tip/tip.git's x86/bugs Git branch. Presumably it will be submitted for the Linux v7.4 kernel cycle unless it ends up getting picked up as part of "fixes" for the current Linux v7.3 cycle. Beyond this kernel patch I haven't seen any more information about the branch target buffer context "BTB CTX" isolation or other security changes with AMD Zen 6 CPUs.