linux-kernel - Re: [Patch v3 05/14] x86/mm: Handle decryption/re-encryption of bss

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Message-ID: <fbf2cdcc-4ff7-b466-a6af-7a147f3bc94d@amd.com>
Date:   Mon, 28 Nov 2022 08:15:58 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dexuan Cui <decui@...rosoft.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Sathyanarayanan Kuppuswamy 
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Subject: Re: [Patch v3 05/14] x86/mm: Handle decryption/re-encryption of
 bss_decrypted consistently

On 11/27/22 20:52, Dexuan Cui wrote:
>> From: Michael Kelley (LINUX) <mikelley@...rosoft.com>
>> Sent: Thursday, November 17, 2022 6:56 PM
>> [...]
>>
>> But I had not thought about TDX.  In the TDX case, it appears that
>> sme_postprocess_startup() will not decrypt the bss_decrypted section.
>> The corresponding mem_encrypt_free_decrypted_mem() is a no-op unless
>> CONFIG_AMD_MEM_ENCRYPT is set.  But maybe if someone builds a
>> kernel image that supports both TDX and AMD encryption, it could break
>> at runtime on a TDX system.  I would also note that on a TDX system
>> without CONFIG_AMD_MEM_ENCRYPT, the unused memory in the
>> bss_decrypted section never gets freed.
> 
> On a TDX system *with* CONFIG_AMD_MEM_ENCRYPT, the unused
> memory in the bss_decrypted section also never gets freed due to the
> below "return;"
> 
> I'd suggest a Fixes tag should be added to make sure the distro vendors
> notice the patch and backport it :-)
> 
> BTW, I just posted a similar patch as I didn't notice this patch. I have
> replied to my patch email, asking people to ignore my patch.
> 
> Fixes: b3f0907c71e0 ("x86/mm: Add .bss..decrypted section to hold shared variables")

I think the Fixes: tag should really be:

e9d1d2bb75b2 ("treewide: Replace the use of mem_encrypt_active() with cc_platform_has()")

since mem_encrypt_active() used to return sme_me_mask, so the checks were
balanced at that point.

Thanks,
Tom

> 
> void __init mem_encrypt_free_decrypted_mem(void)
> {
>          unsigned long vaddr, vaddr_end, npages;
>          int r;
> 
>          vaddr = (unsigned long)__start_bss_decrypted_unused;
>          vaddr_end = (unsigned long)__end_bss_decrypted;
>          npages = (vaddr_end - vaddr) >> PAGE_SHIFT;
> 
>          /*
>           * The unused memory range was mapped decrypted, change the encryption
>           * attribute from decrypted to encrypted before freeing it.
>           */
>          if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) {
>                  r = set_memory_encrypted(vaddr, npages);
>                  if (r) {
>                          pr_warn("failed to free unused decrypted pages\n");
>                          return;
>                  }
>          }
> 
>          free_init_pages("unused decrypted", vaddr, vaddr_end);
> }
>