Re: [PATCHv2 2/3] x86/tdx: Fix race between set_memory_encrypted() and load_unaligned_zeropad()

[Sathyanarayanan Kuppuswamy]

On 5/26/23 5:02 AM, Kirill A. Shutemov wrote:
> Touching privately mapped GPA that is not properly converted to private
> with MapGPA and accepted leads to unrecoverable exit to VMM.
> 
> load_unaligned_zeropad() can touch memory that is not owned by the
> caller, but just happened to next after the owned memory.

/s/to/to be ?

> This load_unaligned_zeropad() behaviour makes it important when kernel
> asks VMM to convert a GPA from shared to private or back. Kernel must
> never have a page mapped into direct mapping (and aliases) as private
> when the GPA is already converted to shared or when GPA is not yet
> converted to private.

I am wondering whether this issue exist in the AMD code? 

IMO, you can add some info on the window in set_memory_encrypted()
where this race exists.

> 
> guest.enc_status_change_prepare() called before adjusting direct mapping
> and therefore it is responsible for converting the memory to private.
> 
> guest.enc_status_change_finish() called after adjusting direct mapping
> and it converts the memory to shared.
> 
> It is okay to have a shared mapping of memory that is not converted
> properly. handle_mmio() knows how to deal with load_unaligned_zeropad()
> stepping on it.

> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Fixes: 7dbde7631629 ("x86/mm/cpa: Add support for TDX shared memory")
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  arch/x86/coco/tdx/tdx.c | 56 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 53 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index e146b599260f..59cc13e41aa6 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -840,6 +840,30 @@ static bool tdx_enc_status_changed(unsigned long vaddr, int numpages, bool enc)
>  	return true;
>  }
>  
> +static bool tdx_enc_status_change_prepare(unsigned long vaddr, int numpages,
> +					  bool enc)
> +{
> +	/*
> +	 * Only handle shared->private conversion here.
> +	 * See the comment in tdx_early_init().
> +	 */
> +	if (enc)
> +		return tdx_enc_status_changed(vaddr, numpages, enc);
> +	return true;
> +}
> +
> +static bool tdx_enc_status_change_finish(unsigned long vaddr, int numpages,
> +					 bool enc)
> +{
> +	/*
> +	 * Only handle private->shared conversion here.
> +	 * See the comment in tdx_early_init().
> +	 */
> +	if (!enc)
> +		return tdx_enc_status_changed(vaddr, numpages, enc);
> +	return true;
> +}
> +
>  void __init tdx_early_init(void)
>  {
>  	u64 cc_mask;
> @@ -867,9 +891,35 @@ void __init tdx_early_init(void)
>  	 */
>  	physical_mask &= cc_mask - 1;
>  
> -	x86_platform.guest.enc_cache_flush_required = tdx_cache_flush_required;
> -	x86_platform.guest.enc_tlb_flush_required   = tdx_tlb_flush_required;

I think you don't need to change the order here.

> -	x86_platform.guest.enc_status_change_finish = tdx_enc_status_changed;
> +	/*
> +	 * Touching privately mapped GPA that is not properly converted to
> +	 * private with MapGPA and accepted leads to unrecoverable exit
> +	 * to VMM.
> +	 *
> +	 * load_unaligned_zeropad() can touch memory that is not owned by
> +	 * the caller, but just happened to next after the owned memory.
> +	 * This load_unaligned_zeropad() behaviour makes it important when
> +	 * kernel asks VMM to convert a GPA from shared to private or back.
> +	 * Kernel must never have a page mapped into direct mapping (and
> +	 * aliases) as private when the GPA is already converted to shared or
> +	 * when GPA is not yet converted to private.
> +	 *
> +	 * guest.enc_status_change_prepare() called before adjusting direct
> +	 * mapping and therefore it is responsible for converting the memory
> +	 * to private.
> +	 *
> +	 * guest.enc_status_change_finish() called after adjusting direct
> +	 * mapping and it converts the memory to shared.
> +	 *
> +	 * It is okay to have a shared mapping of memory that is not converted
> +	 * properly. handle_mmio() knows how to deal with load_unaligned_zeropad()
> +	 * stepping on it.
> +	 */
> +	x86_platform.guest.enc_status_change_prepare = tdx_enc_status_change_prepare;
> +	x86_platform.guest.enc_status_change_finish  = tdx_enc_status_change_finish;
> +
> +	x86_platform.guest.enc_cache_flush_required  = tdx_cache_flush_required;
> +	x86_platform.guest.enc_tlb_flush_required    = tdx_tlb_flush_required;
>  
>  	pr_info("Guest detected\n");
>  }

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer