[PATCH] x86/tdx: Preserve shared bit on mprotect()

[PATCH] x86/tdx: Preserve shared bit on mprotect()

[Kirill A. Shutemov]

The TDX guest platform takes one bit from the physical address to
indicate if the page is shared (accessible by VMM). This bit is not part
of the physical_mask and is not preserved during mprotect(). As a
result, the 'shared' bit is lost during mprotect() on shared mappings.

_COMMON_PAGE_CHG_MASK specifies which PTE bits need to be preserved
during modification. AMD includes 'sme_me_mask' in the define to
preserve the 'encrypt' bit.

To cover both Intel and AMD cases, include 'cc_mask' in
_COMMON_PAGE_CHG_MASK instead of 'sme_me_mask'.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Fixes: 41394e33f3a0 ("x86/tdx: Extend the confidential computing API to support TDX guests")
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Chris Oo <cho@microsoft.com>
Cc: Dexuan Cui <decui@microsoft.com>
---
 arch/x86/include/asm/pgtable_types.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index 3f648ffdfbe5..7dd2fdfacff3 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -148,7 +148,7 @@
 #define _COMMON_PAGE_CHG_MASK	(PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |	\
 				 _PAGE_SPECIAL | _PAGE_ACCESSED |	\
 				 _PAGE_DIRTY_BITS | _PAGE_SOFT_DIRTY |	\
-				 _PAGE_DEVMAP | _PAGE_ENC | _PAGE_UFFD_WP)
+				 _PAGE_DEVMAP | _PAGE_CC | _PAGE_UFFD_WP)
 #define _PAGE_CHG_MASK	(_COMMON_PAGE_CHG_MASK | _PAGE_PAT)
 #define _HPAGE_CHG_MASK (_COMMON_PAGE_CHG_MASK | _PAGE_PSE | _PAGE_PAT_LARGE)
 
@@ -173,6 +173,7 @@ enum page_cache_mode {
 };
 #endif
 
+#define _PAGE_CC		(_AT(pteval_t, cc_mask))
 #define _PAGE_ENC		(_AT(pteval_t, sme_me_mask))
 
 #define _PAGE_CACHE_MASK	(_PAGE_PWT | _PAGE_PCD | _PAGE_PAT)
-- 
2.43.0

Re: [PATCH] x86/tdx: Preserve shared bit on mprotect()

[Edgecombe, Rick P]

On Fri, 2024-04-12 at 22:12 +0300, Kirill A. Shutemov wrote:
> The TDX guest platform takes one bit from the physical address to
> indicate if the page is shared (accessible by VMM). This bit is not part
> of the physical_mask and is not preserved during mprotect(). As a
> result, the 'shared' bit is lost during mprotect() on shared mappings.
> 
> _COMMON_PAGE_CHG_MASK specifies which PTE bits need to be preserved
> during modification. AMD includes 'sme_me_mask' in the define to
> preserve the 'encrypt' bit.
> 
> To cover both Intel and AMD cases, include 'cc_mask' in
> _COMMON_PAGE_CHG_MASK instead of 'sme_me_mask'.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Fixes: 41394e33f3a0 ("x86/tdx: Extend the confidential computing API to
> support TDX guests")
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Chris Oo <cho@microsoft.com>
> Cc: Dexuan Cui <decui@microsoft.com>

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

So does this mean there is shared memory mapped to userspace? Or is this a
theoretical correctness thing?

Re: [PATCH] x86/tdx: Preserve shared bit on mprotect()

[kirill.shutemov]

On Fri, Apr 12, 2024 at 08:48:56PM +0000, Edgecombe, Rick P wrote:
> On Fri, 2024-04-12 at 22:12 +0300, Kirill A. Shutemov wrote:
> > The TDX guest platform takes one bit from the physical address to
> > indicate if the page is shared (accessible by VMM). This bit is not part
> > of the physical_mask and is not preserved during mprotect(). As a
> > result, the 'shared' bit is lost during mprotect() on shared mappings.
> > 
> > _COMMON_PAGE_CHG_MASK specifies which PTE bits need to be preserved
> > during modification. AMD includes 'sme_me_mask' in the define to
> > preserve the 'encrypt' bit.
> > 
> > To cover both Intel and AMD cases, include 'cc_mask' in
> > _COMMON_PAGE_CHG_MASK instead of 'sme_me_mask'.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Fixes: 41394e33f3a0 ("x86/tdx: Extend the confidential computing API to
> > support TDX guests")
> > Cc: Tom Lendacky <thomas.lendacky@amd.com>
> > Cc: Chris Oo <cho@microsoft.com>
> > Cc: Dexuan Cui <decui@microsoft.com>
> 
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> 
> So does this mean there is shared memory mapped to userspace? Or is this a
> theoretical correctness thing?

Drivers can do this. Things like VFIO, I guess.

I think I should have credited Chris for reporting and testing the problem:

Reported-and-tested-by: Chris Oo <cho@microsoft.com>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

Re: [PATCH] x86/tdx: Preserve shared bit on mprotect()

[Kuppuswamy Sathyanarayanan]

On 4/12/24 12:12 PM, Kirill A. Shutemov wrote:
> The TDX guest platform takes one bit from the physical address to
> indicate if the page is shared (accessible by VMM). This bit is not part
> of the physical_mask and is not preserved during mprotect(). As a
> result, the 'shared' bit is lost during mprotect() on shared mappings.
>
> _COMMON_PAGE_CHG_MASK specifies which PTE bits need to be preserved
> during modification. AMD includes 'sme_me_mask' in the define to
> preserve the 'encrypt' bit.
>
> To cover both Intel and AMD cases, include 'cc_mask' in
> _COMMON_PAGE_CHG_MASK instead of 'sme_me_mask'.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Fixes: 41394e33f3a0 ("x86/tdx: Extend the confidential computing API to support TDX guests")
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Chris Oo <cho@microsoft.com>
> Cc: Dexuan Cui <decui@microsoft.com>
> ---

Looks good to me.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

>  arch/x86/include/asm/pgtable_types.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 3f648ffdfbe5..7dd2fdfacff3 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -148,7 +148,7 @@
>  #define _COMMON_PAGE_CHG_MASK	(PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |	\
>  				 _PAGE_SPECIAL | _PAGE_ACCESSED |	\
>  				 _PAGE_DIRTY_BITS | _PAGE_SOFT_DIRTY |	\
> -				 _PAGE_DEVMAP | _PAGE_ENC | _PAGE_UFFD_WP)
> +				 _PAGE_DEVMAP | _PAGE_CC | _PAGE_UFFD_WP)
>  #define _PAGE_CHG_MASK	(_COMMON_PAGE_CHG_MASK | _PAGE_PAT)
>  #define _HPAGE_CHG_MASK (_COMMON_PAGE_CHG_MASK | _PAGE_PSE | _PAGE_PAT_LARGE)
>  
> @@ -173,6 +173,7 @@ enum page_cache_mode {
>  };
>  #endif
>  
> +#define _PAGE_CC		(_AT(pteval_t, cc_mask))
>  #define _PAGE_ENC		(_AT(pteval_t, sme_me_mask))
>  
>  #define _PAGE_CACHE_MASK	(_PAGE_PWT | _PAGE_PCD | _PAGE_PAT)

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer