netdev - RE: [Patch v3 05/14] x86/mm: Handle decryption/re-encryption of bss

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Date:   Fri, 18 Nov 2022 02:55:32 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>, KY Srinivasan <kys@...rosoft.com>,
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Subject: RE: [Patch v3 05/14] x86/mm: Handle decryption/re-encryption of
 bss_decrypted consistently

From: Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@...ux.intel.com>
> 
> On 11/16/22 10:41 AM, 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.
> >
> 
> Do you think it needs Fixes tag?
> 

At least for my purposes, it doesn't.  The original assumption that non-zero
sme_me_mask and CC_ATTR_MEM_ENCRYPT are equivalent was valid until
this patch series where Hyper-V guests are reporting CC_ATTR_MEM_ENCRYPT
as "true" but sme_me_mask is zero.  This patch series won't be backported,
so the old assumption remains valid for older kernels.  There's no benefit in
backporting the change.

But I had not thought about TDX.  In the TDX case, it appears that
sme_postprocess_startup() will not decrypt the bss_decrypted section.
The corresponding mem_encrypt_free_decrypted_mem() is a no-op unless
CONFIG_AMD_MEM_ENCRYPT is set.  But maybe if someone builds a
kernel image that supports both TDX and AMD encryption, it could break
at runtime on a TDX system.  I would also note that on a TDX system
without CONFIG_AMD_MEM_ENCRYPT, the unused memory in the
bss_decrypted section never gets freed.

But check my logic. :-)   I'm not averse to adding the Fixes: tag if there's a
scenario for TDX where doing the backport will solve a real problem.

And thanks for reviewing the code!

Michael