OpenBSD Journal

More mitigations against speculative execution vulnerabilities

Contributed by rueda on from the gift-that-keeps-on-giving dept.

Philip Guenther (guenther@) and Bryan Steele (brynet@) have added more mitigations against speculative execution CPU vulnerabilities on the amd64 platform.

For "SpectreRSB" and earlier Spectre variants:

Module name:	src
Changes by:	2018/07/23 11:54:04

Modified files:
	sys/arch/amd64/amd64: locore.S 
	sys/arch/amd64/include: asm.h cpufunc.h frameasm.h 

Log message:
Do "Return stack refilling", based on the "Return stack underflow" discussion
and its associated appendix at
This should address at least some cases of "SpectreRSB" and earlier
Spectre variants; more commits to follow.

The refilling is done in the enter-kernel-from-userspace and
return-to-userspace-from-kernel paths, making sure to do it before
unblocking interrupts so that a successive interrupt can't get the
CPU to C code without doing this refill.  Per the link above, it
also does it immediately after mwait, apparently in case the low-power
CPU states of idle-via-mwait flush the RSB.

ok mlarkin@ deraadt@


Module name:	src
Changes by:	2018/07/23 20:42:25

Modified files:
	sys/arch/amd64/amd64: locore.S vector.S vmm_support.S 
	sys/arch/amd64/include: asm.h cpufunc.h 

Log message:
Also do RSB refilling when context switching, after vmexits, and
when vmlaunch or vmresume fails.

Follow the lead of clang and the intel recommendation and do an lfence
after the pause in the speculation-stop path for retpoline, RSB refill,
and meltover ASM bits.

ok kettenis@ deraadt@

"Mitigation G-2" for AMD processors:

Module name:	src
Changes by:	2018/07/23 17:25:03

Modified files:
	sys/arch/amd64/amd64: identcpu.c 
	sys/arch/amd64/include: specialreg.h 

Log message:
Add "Mitigation G-2" per AMD's Whitepaper "Software Techniques for
Managing Speculation on AMD Processors"

By setting MSR C001_1029[1]=1, LFENCE becomes a dispatch serializing

Tested on AMD FX-4100 "Bulldozer", and Linux guest in SVM vmd(8)

ok deraadt@ mlarkin@

(Comments are closed)

  1. By Amit Kulkarni (amitkulz) on

    As noted by brynet@ "Mitigation G-2" for AMD processors diff was also ok'd by guenther@



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