linux-kernel - Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address

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Message-ID: <YvN9bKQ0XtUVJE7z@zn.tnic>
Date:   Wed, 10 Aug 2022 11:42:04 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
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Subject: Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address

On Wed, Aug 10, 2022 at 03:59:34AM +0000, Kalra, Ashish wrote:
> Actually SNP feature can be enabled globally, but SNP is activated on a per VM basis.
> 
> From the APM:
> The term SNP-enabled indicates that SEV-SNP is globally enabled in the SYSCFG 
> MSR. The term SNP-active indicates that SEV-SNP is enabled for a specific VM in the 
> SEV_FEATURES field of its VMSA

Aha, and I was wondering whether "globally" meant the RMP needs to cover
all physical memory but I guess that isn't the case:

"RMP-Covered: Checks that the target page is covered by the RMP. A page
is covered by the RMP if its corresponding RMP entry is below RMP_END.
Any page not covered by the RMP is considered a Hypervisor-Owned page."

> >You need to elaborate more here: a RMP fault can happen and then the
> >page can get unmapped? What is the exact scenario here?
>
> Yes, if the page gets unmapped while the RMP fault was being handled,
> will add more explanation here.

So what's the logic here to return 1, i.e., retry?

Why should a fault for a page that gets unmapped be retried? The fault
in that case should be ignored, IMO. It'll have the same effect to
return from do_user_addr_fault() there, without splitting but you need
to have a separate return value definition so that it is clear what
needs to happen. And that return value should be != 0 so that the
current check still works.

> Actually, the above computes an index into the RMP table.

What index in the RMP table?

> It is basically an index into the 4K page within the hugepage mapped
> in the RMP table or in other words an index into the RMP table entry
> for 4K page(s) corresponding to a hugepage.

So pte_index(address) and for 1G pages, pmd_index(address).

So no reinventing the wheel if we already have helpers for that.

> It is mainly a wrapper around__split_huge_pmd() for SNP use case where
> the host hugepage is split to be in sync with the RMP table.

I see what it is. And I'm saying this looks wrong. You're enforcing page
splitting to be a valid thing to do only for SEV machines. Why?

Why is

        if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT))
                return VM_FAULT_SIGBUS;

there at all?

This is generic code you're touching - not arch/x86/.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette