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

[Sathyanarayanan Kuppuswamy]

Hi,

On 5/30/23 5:57 AM, Tom Lendacky wrote:
> On 5/29/23 19:57, Kirill A. Shutemov wrote:
>> On Fri, May 26, 2023 at 03:10:56PM -0700, Sathyanarayanan Kuppuswamy wrote:
>>>
>>>
>>> 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 ?
>>
>> Yep, my bad.
>>
>>>> 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.
>>
>> I don't think AMD affected by load_unaligned_zeropad() the same way as
>> Intel does. But I'm not sure.
>>
>> Tom, do you have any comments?
> 
> Right, shouldn't be an issue for SNP.

Thanks for confirming.

> 
> Thanks,
> Tom
> 
>>
>>>
>>>>
>>>> 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.
>>>
>>>>

Rest looks good to me.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>

>>>> 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.
>>
>> I wanted to emphasise that the comment is for _prepare/_finish callbacks
>> and I hoped re-order would help with this.
>>
>>>> -    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
>>

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer