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

From: Venu Busireddy <venu.busireddy@oracle.com>
To: Brijesh Singh <brijesh.singh@amd.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, linux-efi@vger.kernel.org,
	platform-driver-x86@vger.kernel.org, linux-coco@lists.linux.dev,
	linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Jim Mattson <jmattson@google.com>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Sergio Lopez <slp@redhat.com>, Peter Gonda <pgonda@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	David Rientjes <rientjes@google.com>,
	Dov Murik <dovmurik@linux.ibm.com>,
	Tobin Feldman-Fitzthum <tobin@ibm.com>,
	Borislav Petkov <bp@alien8.de>,
	Michael Roth <michael.roth@amd.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Andi Kleen <ak@linux.intel.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	tony.luck@intel.com, marcorr@google.com,
	sathyanarayanan.kuppuswamy@linux.intel.com
Subject: Re: [PATCH v8 15/40] x86/mm: Add support to validate memory when changing C-bit
Date: Tue, 4 Jan 2022 16:31:46 -0600	[thread overview]
Message-ID: <YdTK0h2ZKu4UxrhQ@dt> (raw)
In-Reply-To: <20211210154332.11526-16-brijesh.singh@amd.com>

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@amd.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