Re: [Patch v4 04/13] x86/mm: Handle decryption/re-encryption of bss_decrypted consistently

From: Borislav Petkov <bp@alien8.de>
To: Michael Kelley <mikelley@microsoft.com>
Cc: hpa@zytor.com, kys@microsoft.com, haiyangz@microsoft.com,
	wei.liu@kernel.org, decui@microsoft.com, luto@kernel.org,
	peterz@infradead.org, davem@davemloft.net, edumazet@google.com,
	kuba@kernel.org, pabeni@redhat.com, lpieralisi@kernel.org,
	robh@kernel.org, kw@linux.com, bhelgaas@google.com,
	arnd@arndb.de, hch@infradead.org, m.szyprowski@samsung.com,
	robin.murphy@arm.com, thomas.lendacky@amd.com,
	brijesh.singh@amd.com, tglx@linutronix.de, mingo@redhat.com,
	dave.hansen@linux.intel.com, Tianyu.Lan@microsoft.com,
	kirill.shutemov@linux.intel.com,
	sathyanarayanan.kuppuswamy@linux.intel.com, ak@linux.intel.com,
	isaku.yamahata@intel.com, dan.j.williams@intel.com,
	jane.chu@oracle.com, seanjc@google.com, tony.luck@intel.com,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	linux-hyperv@vger.kernel.org, netdev@vger.kernel.org,
	linux-pci@vger.kernel.org, linux-arch@vger.kernel.org,
	iommu@lists.linux.dev
Subject: Re: [Patch v4 04/13] x86/mm: Handle decryption/re-encryption of bss_decrypted consistently
Date: Thu, 29 Dec 2022 13:17:48 +0100	[thread overview]
Message-ID: <Y62FbJ1rZ6TVUgml@zn.tnic> (raw)
In-Reply-To: <1669951831-4180-5-git-send-email-mikelley@microsoft.com>

On Thu, Dec 01, 2022 at 07:30:22PM -0800, 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_decrypted_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.

Lemme simplify the formulations a bit:

"sme_postprocess_startup() decrypts the bss_decrypted ection when me_mask
sme_is non-zero.

mem_encrypt_free_decrypted_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.

So check sme_me_mask in mem_encrypt_free_decrypted_mem() too.

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."

> Fixes: e9d1d2bb75b2 ("treewide: Replace the use of mem_encrypt_active() with cc_platform_has()")

So when you say Fixes - this is an issue only for vTOM-using VMs and
before yours, there were none. And yours needs more enablement than just
this patch.

So does this one really need to be backported to stable@?

I'm asking because there's AI which will pick it up based on this Fixes
tag up but that AI is still not that smart to replace us all. :-)

-- 
Regards/Gruss,
    Boris.

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