linux-kernel - Re: [PATCH v8 15/40] x86/mm: Add support to validate memory when changing C-bit

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Date:   Tue, 4 Jan 2022 16:31:46 -0600
From:   Venu Busireddy <venu.busireddy@...cle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.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>,
        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>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v8 15/40] x86/mm: Add support to validate memory when
 changing C-bit

On 2021-12-10 09:43:07 -0600, Brijesh Singh wrote:
> The set_memory_{encrypt,decrypt}() are used for changing the pages

s/set_memory_{encrypt,decrypt}/snp_set_memory_{shared,private}/

> from decrypted (shared) to encrypted (private) and vice versa.
> When SEV-SNP is active, the page state transition needs to go through
> additional steps.
> 
> If the page is transitioned from shared to private, then perform the
> following after the encryption attribute is set in the page table:
> 
> 1. Issue the page state change VMGEXIT to add the memory region in
>    the RMP table.
> 2. Validate the memory region after the RMP entry is added.
> 
> To maintain the security guarantees, if the page is transitioned from
> private to shared, then perform the following before encryption attribute
> is removed from the page table:
> 
> 1. Invalidate the page.
> 2. Issue the page state change VMGEXIT to remove the page from RMP table.
> 
> To change the page state in the RMP table, use the Page State Change
> VMGEXIT defined in the GHCB specification.
> 
> The GHCB specification provides the flexibility to use either 4K or 2MB
> page size in during the page state change (PSC) request. For now use the
> 4K page size for all the PSC until page size tracking is supported in the
> kernel.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>

[snip]

> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 2971aa280ce6..35c772bf9f6c 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -574,7 +574,7 @@ static void pvalidate_pages(unsigned long vaddr, unsigned int npages, bool valid
>  	}
>  }
>  
> -static void __init early_set_page_state(unsigned long paddr, unsigned int npages, enum psc_op op)
> +static void __init early_set_pages_state(unsigned long paddr, unsigned int npages, enum psc_op op)

Is there a need to change the name? "npages" can take a value of 1 too.
Hence, early_set_page_state() appears to be a better name!

> +		/*
> +		 * Page State Change VMGEXIT can pass error code through
> +		 * exit_info_2.
> +		 */

Collapse into one line?

> +void snp_set_memory_shared(unsigned long vaddr, unsigned int npages)
> +{
> +	if (!cc_platform_has(CC_ATTR_SEV_SNP))
> +		return;
> +
> +	pvalidate_pages(vaddr, npages, 0);

Replace '0' with "false"?

> +
> +	set_pages_state(vaddr, npages, SNP_PAGE_STATE_SHARED);
> +}
> +
> +void snp_set_memory_private(unsigned long vaddr, unsigned int npages)
> +{
> +	if (!cc_platform_has(CC_ATTR_SEV_SNP))
> +		return;
> +
> +	set_pages_state(vaddr, npages, SNP_PAGE_STATE_PRIVATE);
> +
> +	pvalidate_pages(vaddr, npages, 1);

Replace '1' with "true"?

Venu