linux-kernel - Re: [Patch v3 05/14] x86/mm: Handle decryption/re-encryption of bss

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Message-ID: <4d27540e-691e-bd86-0f70-1faff39f7187@amd.com>
Date:   Wed, 16 Nov 2022 14:35:35 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Michael Kelley <mikelley@...rosoft.com>, hpa@...or.com,
        kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, luto@...nel.org, peterz@...radead.org,
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        hch@...radead.org, m.szyprowski@...sung.com, robin.murphy@....com,
        brijesh.singh@....com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com,
        Tianyu.Lan@...rosoft.com, kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, dan.j.williams@...el.com,
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        iommu@...ts.linux.dev
Subject: Re: [Patch v3 05/14] x86/mm: Handle decryption/re-encryption of
 bss_decrypted consistently

On 11/16/22 12:41, Michael Kelley wrote:
> Current code in sme_postprocess_startup() decrypts the bss_decrypted
> section when sme_me_mask is non-zero.  But code in
> mem_encrypt_free_decrytped_mem() re-encrypts the unused portion based
> on CC_ATTR_MEM_ENCRYPT.  In a Hyper-V guest VM using vTOM, these
> conditions are not equivalent as sme_me_mask is always zero when
> using vTOM.  Consequently, mem_encrypt_free_decrypted_mem() attempts
> to re-encrypt memory that was never decrypted.
> 
> Fix this in mem_encrypt_free_decrypted_mem() by conditioning the
> re-encryption on the same test for non-zero sme_me_mask.  Hyper-V
> guests using vTOM don't need the bss_decrypted section to be
> decrypted, so skipping the decryption/re-encryption doesn't cause
> a problem.
> 
> Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
> ---
>   arch/x86/mm/mem_encrypt_amd.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
> index 9c4d8db..5a51343 100644
> --- a/arch/x86/mm/mem_encrypt_amd.c
> +++ b/arch/x86/mm/mem_encrypt_amd.c
> @@ -513,10 +513,14 @@ void __init mem_encrypt_free_decrypted_mem(void)
>   	npages = (vaddr_end - vaddr) >> PAGE_SHIFT;
>   
>   	/*
> -	 * The unused memory range was mapped decrypted, change the encryption
> -	 * attribute from decrypted to encrypted before freeing it.
> +	 * If the unused memory range was mapped decrypted, change the encryption
> +	 * attribute from decrypted to encrypted before freeing it. Base the
> +	 * re-encryption on the same condition used for the decryption in
> +	 * sme_postprocess_startup(). Higher level abstractions, such as
> +	 * CC_ATTR_MEM_ENCRYPT, aren't necessarily equivalent in a Hyper-V VM
> +	 * using vTOM, where sme_me_mask is always zero.
>   	 */
> -	if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) {
> +	if (sme_get_me_mask()) {

To be consistent within this file, you should use sme_me_mask directly.

Thanks,
Tom

>   		r = set_memory_encrypted(vaddr, npages);
>   		if (r) {
>   			pr_warn("failed to free unused decrypted pages\n");