linux-kernel - [PATCH v4 0/2] Fix SEV user-space mapping of unencrypted coherent memory

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Date:   Wed,  4 Mar 2020 12:45:25 +0100
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     x86@...nel.org, Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Thomas Hellström <thomas_os@...pmail.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Christian König <christian.koenig@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: [PATCH v4 0/2] Fix SEV user-space mapping of unencrypted coherent memory

This patchset fixes dma_mmap_coherent() mapping of unencrypted memory in
otherwise encrypted environments, where it would incorrectly map that memory as
encrypted.

With SEV and sometimes with SME encryption, The dma api coherent memory is
typically unencrypted, meaning the linear kernel map has the encryption
bit cleared. However, default page protection returned from vm_get_page_prot()
has the encryption bit set. So to compute the correct page protection we need
to clear the encryption bit.

Also, in order for the encryption bit setting to survive across do_mmap() and
mprotect_fixup(), We need to make pgprot_modify() aware of it and not touch it.
Therefore make sme_me_mask part of _PAGE_CHG_MASK and make sure
pgprot_modify() preserves also cleared bits that are part of _PAGE_CHG_MASK,
not just set bits. The use of pgprot_modify() is currently quite limited and
easy to audit.

(Note that the encryption status is not logically encoded in the pfn but in
the page protection even if an address line in the physical address is used).

The patchset has seen some sanity testing by exporting dma_pgprot() and
using it in the vmwgfx mmap handler with SEV enabled.

As far as I can tell there are no current users of dma_mmap_coherent() with
SEV or SME encryption which means that there is no need to CC stable.

Changes since:
RFC:
- Make sme_me_mask port of _PAGE_CHG_MASK rather than using it by its own in
  pgprot_modify().
v1:
- Clarify which use-cases this patchset actually fixes.
v2:
- Use _PAGE_ENC instead of sme_me_mask in the definition of _PAGE_CHG_MASK
v3:
- Added RB from Dave Hansen.

Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Christoph Hellwig <hch@...radead.org>
Cc: Christian König <christian.koenig@....com>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Tom Lendacky <thomas.lendacky@....com>