Linux Will End Up Disabling x86 PTI For AMD Processors - Update: Now Disabled

Written by Michael Larabel in Linux Kernel on 3 January 2018 at 12:45 PM EST. 45 Comments
While at the moment with the mainline Linux kernel Git tree AMD CPUs enable x86 PTI and are treated as "insecure" CPUs, the AMD patch for not setting X86_BUG_CPU_INSECURE will end up being honored.

The patch covered in the aforelinked article has not been merged through to Linus Torvalds' Git tree. Instead, as of a short time ago, is now living within the tip/tip.git tree. In there is also defaulting PAGE_TABLE_ISOLATION to on and other recent fixes around x86 Page Table Isolation (PTI) support.

But what remains to be seen is if this work will be pulled into Linux 4.15 Git or not. We're within three weeks of the executed debut of Linux 4.15.0 stable and it isn't clear if these tip changes will be requested to be pulled into Linux 4.15 or be postponed until the start of the Linux 4.16 kernel merge window, since the safe bulk of the x86 PTI work is already in Git master. Right now the branch name doesn't indicate it's in any fixes/urgent queue nor has there been any pull request yet asking Torvalds to take it into his repository: normally tip.git master is with material for linux-next.

So we'll have to see what ends up happening in the days ahead, but regardless, at least the "AMD patch" is now sitting within a known tree that will eventually flow into the mainline Linux tree whether it be 4.15 or 4.16.

Update: Linus Torvalds has now ended up pulling the latest PTI fixes that also include the change to disable page table isolation for now on all AMD CPUs. The commit is in mainline for Linux 4.15 along with a few basic fixes and ensuring PAGE_TABLE_ISOLATION is enabled by default.

Kernel developer Thomas Gleixner wrote in the pull request of disabling KPTI on AMD hardware, "Not necessarily a fix, but if AMD is so confident that they are not affected, then we should not burden users with the overhead."
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