Re: [PATCH v2 1/4] x86/bugs: Add SRSO_USER_KERNEL_NO support

Borislav Petkov
2024-12-10T15:37:30Z

On Mon, Dec 09, 2024 at 10:53:31PM -0800, Josh Poimboeuf wrote:
> The presence of SRSO_USER_KERNEL_NO should indeed change the default,
> but if the user requests "safe_ret" specifically, shouldn't we give it
> to them?

Hardly a valid use case except for debugging but if you're doing that, you can
change the kernel too.

Because we just fixed this and if some poor soul has left
spec_rstack_overflow=safe-ret in her /etc/default/grub (it has happened to me
a bunch on my test boxes), she'll never get her performance back and that
would be a yucky situation.

> That would be more consistent with how we handle requested
> mitigations.

Yeah, but if there were a point to this, I guess. We don't really need this
mitigation as there's nothing to mitigate on the user/kernel transition
anymore.

> Also it doesn't really make sense to add a printk here as the mitigation
> will be printed at the end of the function.

This is us letting the user know that we don't need Safe-RET anymore and we're
falling back. But I'm not that hung up on that printk...

Thx.

--
Regards/Gruss,
Boris.