linux-kernel - Re: [Patch v4 04/13] x86/mm: Handle decryption/re-encryption of bss

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Message-ID: <Y62FbJ1rZ6TVUgml@zn.tnic>
Date:   Thu, 29 Dec 2022 13:17:48 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Michael Kelley <mikelley@...rosoft.com>
Cc:     hpa@...or.com, kys@...rosoft.com, haiyangz@...rosoft.com,
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Subject: Re: [Patch v4 04/13] x86/mm: Handle decryption/re-encryption of
 bss_decrypted consistently

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