Re: [PATCH 00/45] arm Spectre fix backport review for LTS 4.9

[Will Deacon]

On Fri, Mar 02, 2018 at 05:02:32PM +0800, Alex Shi wrote:
> As testing the spectre bug fix, that's a good question. I also asked
> this question to original patch authors, like Marc. They said they just
> figure out these patches could block spectre or meltdown issue. From my
> side, I just reproduced the process internal spectre. But all fix on arm
> can not resolve the user space internal spectre. It can block from user
> to kernel or kernel to user spectre according the code purose. So I
> believe these patch could do their job. And arm cpu would drop the
> spectre branches if it has 20+ 'nop' instructions...

Since this is archived on a public list and I don't want people to rely on
this, no, you cannot rely on "20+ 'nop' instructions" to work around
spectre on arm/arm64. It might prevent a particular PoC working on a
particular SoC, but it's fragile at best.

Will