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

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

[Thomas Hellström (VMware)]

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.

Changes since:
RFC:
- Make sme_me_mask port of _PAGE_CHG_MASK rather than using it by its own in
  pgprot_modify().

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>

[PATCH 1/2] x86: Don't let pgprot_modify() change the page encryption bit

[Thomas Hellström (VMware)]

From: Thomas Hellstrom <thellstrom@vmware.com>

When SEV or SME is enabled and active, vm_get_page_prot() typically
returns with the encryption bit set. This means that users of
pgprot_modify(, vm_get_page_prot()) (mprotect_fixup, do_mmap) typically
unintentionally sets encrypted page protection even on mmap'd coherent
memory where the mmap callback has cleared the bit. Fix this by not
allowing pgprot_modify() to change the encryption bit, similar to
how it's done for PAT bits.

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
---
 arch/x86/include/asm/pgtable.h       | 7 +++++--
 arch/x86/include/asm/pgtable_types.h | 2 +-
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 0bc530c4eb13..1e6bb4c25334 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -624,12 +624,15 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
 	return __pmd(val);
 }
 
-/* mprotect needs to preserve PAT bits when updating vm_page_prot */
+/*
+ * mprotect needs to preserve PAT and encryption bits when updating
+ * vm_page_prot
+ */
 #define pgprot_modify pgprot_modify
 static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
 {
 	pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK;
-	pgprotval_t addbits = pgprot_val(newprot);
+	pgprotval_t addbits = pgprot_val(newprot) & ~_PAGE_CHG_MASK;
 	return __pgprot(preservebits | addbits);
 }
 
diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index b5e49e6bac63..e13084b3d6cb 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -123,7 +123,7 @@
  */
 #define _PAGE_CHG_MASK	(PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |		\
 			 _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY |	\
-			 _PAGE_SOFT_DIRTY | _PAGE_DEVMAP)
+			 _PAGE_SOFT_DIRTY | _PAGE_DEVMAP | sme_me_mask)
 #define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)
 
 /*
-- 
2.20.1

[PATCH 2/2] dma-mapping: Fix dma_pgprot() for unencrypted coherent pages

[Thomas Hellström (VMware)]

From: Thomas Hellstrom <thellstrom@vmware.com>

When force_dma_unencrypted() returns true, the linear kernel map of the
coherent pages have had the encryption bit explicitly cleared and the
page content is unencrypted. Make sure that any additional PTEs we set
up to these pages also have the encryption bit cleared by having
dma_pgprot() return a protection with the encryption bit cleared in this
case.

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
---
 kernel/dma/mapping.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index b0038ca3aa92..2b499dcae74f 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -157,6 +157,8 @@ EXPORT_SYMBOL(dma_get_sgtable_attrs);
  */
 pgprot_t dma_pgprot(struct device *dev, pgprot_t prot, unsigned long attrs)
 {
+	if (force_dma_unencrypted(dev))
+		prot = pgprot_decrypted(prot);
 	if (dev_is_dma_coherent(dev) ||
 	    (IS_ENABLED(CONFIG_DMA_NONCOHERENT_CACHE_SYNC) &&
              (attrs & DMA_ATTR_NON_CONSISTENT)))
-- 
2.20.1

Re: [PATCH 0/2] Fix SEV user-space mapping of unencrypted coherent memory

[Ingo Molnar]

* Thomas Hellström (VMware) <thomas_os@shipmail.org> wrote:

> 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.
> 
> Changes since:
> RFC:
> - Make sme_me_mask port of _PAGE_CHG_MASK rather than using it by its own in
>   pgprot_modify().

Could you please add a "why is this patch-set needed", not just describe 
the "what does this patch set do"? I've seen zero discussion in the three 
changelogs of exactly why we'd want this, which drivers and features are 
affected and in what way, etc.

It's called a "fix" but doesn't explain what bad behavior it fixes.

Thanks,

	Ingo

Re: [PATCH 0/2] Fix SEV user-space mapping of unencrypted coherent memory

[Thomas Hellström (VMware)]

On 9/11/19 7:59 AM, Ingo Molnar wrote:
> * Thomas Hellström (VMware) <thomas_os@shipmail.org> wrote:
>
>> 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.
>>
>> Changes since:
>> RFC:
>> - Make sme_me_mask port of _PAGE_CHG_MASK rather than using it by its own in
>>    pgprot_modify().
> Could you please add a "why is this patch-set needed", not just describe
> the "what does this patch set do"? I've seen zero discussion in the three
> changelogs of exactly why we'd want this, which drivers and features are
> affected and in what way, etc.
>
> It's called a "fix" but doesn't explain what bad behavior it fixes.
>
> Thanks,
>
> 	Ingo

I'll update the changelog to be more clear about that.

Thanks,

Thomas