linux-kernel - Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map when adding them to the RMP table

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Message-ID: <20240112194849.GDZaGXoTeYkx3GYSsQ@fat_crate.local>
Date: Fri, 12 Jan 2024 20:48:49 +0100
From: Borislav Petkov <bp@...en8.de>
To: Michael Roth <michael.roth@....com>
Cc: x86@...nel.org, kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
	linux-mm@...ck.org, linux-crypto@...r.kernel.org,
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	pankaj.gupta@....com, liam.merwick@...cle.com, zhi.a.wang@...el.com,
	Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map
 when adding them to the RMP table

On Sat, Dec 30, 2023 at 10:19:39AM -0600, Michael Roth wrote:
>  static int rmpupdate(u64 pfn, struct rmp_state *state)
>  {
>  	unsigned long paddr = pfn << PAGE_SHIFT;
> -	int ret;
> +	int ret, level, npages;
>  
>  	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
>  		return -ENODEV;
>  
> +	level = RMP_TO_PG_LEVEL(state->pagesize);
> +	npages = page_level_size(level) / PAGE_SIZE;
> +
> +	/*
> +	 * If the kernel uses a 2MB directmap mapping to write to an address,
> +	 * and that 2MB range happens to contain a 4KB page that set to private
> +	 * in the RMP table, an RMP #PF will trigger and cause a host crash.
> +	 *
> +	 * Prevent this by removing pages from the directmap prior to setting
> +	 * them as private in the RMP table.
> +	 */
> +	if (state->assigned && invalidate_direct_map(pfn, npages))
> +		return -EFAULT;

Well, the comment says one thing but the code does not do quite what the
text says:

* where is the check that pfn is part of the kernel direct map?

* where is the check that this address is part of a 2M PMD in the direct
map so that the situation is even given that you can have a 4K private
PTE there?

What this does is simply invalidate @npages unconditionally.

Then, __set_pages_np() already takes a number of pages and a start
address. Why is this thing iterating instead of sending *all* npages in
one go?

Yes, you'd need two new helpers, something like:

set_pages_present(struct page *page, int numpages)
set_pages_non_present(struct page *page, int numpages)

which call the lowlevel counterparts.

For some reason I remember asking this a while ago...

-- 
Regards/Gruss,
    Boris.

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