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

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Date:   Mon, 28 Nov 2022 02:52:28 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>, KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
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        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
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        "ak@...ux.intel.com" <ak@...ux.intel.com>,
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        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Subject: RE: [Patch v3 05/14] x86/mm: Handle decryption/re-encryption of
 bss_decrypted consistently

> 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")

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);
}