linux-kernel - Re: [PATCH Part2 v5 06/45] x86/sev: Invalid pages from direct map when adding it to RMP table

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Date:   Wed, 29 Sep 2021 16:34:24 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 06/45] x86/sev: Invalid pages from direct map
 when adding it to RMP table

On Fri, Aug 20, 2021 at 10:58:39AM -0500, Brijesh Singh wrote:
> Subject: Re: [PATCH Part2 v5 06/45] x86/sev: Invalid pages from direct map when adding it to RMP table

That subject needs to have a verb. I think that verb should be
"Invalidate".

> The integrity guarantee of SEV-SNP is enforced through the RMP table.
> The RMP is used with standard x86 and IOMMU page tables to enforce memory
> restrictions and page access rights. The RMP check is enforced as soon as
> SEV-SNP is enabled globally in the system. When hardware encounters an
> RMP checks failure, it raises a page-fault exception.
> 
> The rmp_make_private() and rmp_make_shared() helpers are used to add
> or remove the pages from the RMP table.

> Improve the rmp_make_private() to
> invalid state so that pages cannot be used in the direct-map after its
> added in the RMP table, and restore to its default valid permission after
> the pages are removed from the RMP table.

That sentence needs rewriting into proper english.

The more important thing is, though, this doesn't talk about *why*
you're doing this: you want to remove pages from the direct map when
they're in the RMP table because something might modify the page and
then the RMP check will fail?

Also, set_direct_map_invalid_noflush() simply clears the Present and RW
bit of a pte.

So what's up?

> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
>  arch/x86/kernel/sev.c | 61 ++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 60 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 8627c49666c9..bad41deb8335 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -2441,10 +2441,42 @@ int psmash(u64 pfn)
>  }
>  EXPORT_SYMBOL_GPL(psmash);
>  
> +static int restore_direct_map(u64 pfn, int npages)

restore_pages_in_direct_map()

> +{
> +	int i, ret = 0;
> +
> +	for (i = 0; i < npages; i++) {
> +		ret = set_direct_map_default_noflush(pfn_to_page(pfn + i));
> +		if (ret)
> +			goto cleanup;
> +	}

So this is looping over a set of virtually contiguous pages, I presume,
and if so, you should add a function called

	set_memory_p_rw()

to arch/x86/mm/pat/set_memory.c which does

	return change_page_attr_set(&addr, numpages, __pgprot(_PAGE_PRESENT | _PAGE_RW), 0);

so that you can do all pages in one go.

> +
> +cleanup:
> +	WARN(ret > 0, "Failed to restore direct map for pfn 0x%llx\n", pfn + i);
> +	return ret;
> +}
> +
> +static int invalid_direct_map(unsigned long pfn, int npages)

invalidate_pages_in_direct_map()

or so.

> +{
> +	int i, ret = 0;
> +
> +	for (i = 0; i < npages; i++) {
> +		ret = set_direct_map_invalid_noflush(pfn_to_page(pfn + i));

Same as above but that helper should do the reverse:

set_memory_np_ro()
{
	return change_page_attr_clear(&addr, numpages, __pgprot(_PAGE_PRESENT | _PAGE_RW), 0);
}

Btw, please add those helpers in a separate patch.

Thx.

-- 
Regards/Gruss,
    Boris.

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