linux-kernel - Re: [PATCH 1/3] kexec: Do not map the kexec area as decrypted when SEV is active

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Message-ID: <20190325173239.GO12016@zn.tnic>
Date:   Mon, 25 Mar 2019 18:32:39 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Singh, Brijesh" <brijesh.singh@....com>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>
Cc:     lijiang <lijiang@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "dyoung@...hat.com" <dyoung@...hat.com>,
        "bhe@...hat.com" <bhe@...hat.com>
Subject: Re: [PATCH 1/3] kexec: Do not map the kexec area as decrypted when
 SEV is active

On Mon, Mar 25, 2019 at 05:17:55PM +0000, Singh, Brijesh wrote:
> By default all the memory regions are mapped encrypted. The
> set_memory_{encrypt,decrypt}() is a generic function which can be
> called explicitly to clear/set the encryption mask from the existing
> memory mapping. The mem_encrypt_active() returns true if either SEV or 
> SME is active. So the __set_memory_enc_dec() uses the
> memory_encrypt_active() check to ensure that the function is no-op when
> SME/SEV are not active.
> 
> Currently, the arch_kexec_post_alloc_pages() unconditionally clear the
> encryption mask from the kexec area. In case of SEV, we should not clear
> the encryption mask.

Brijesh, I know all that.

Please read what I said here at the end:

https://lkml.kernel.org/r/20190324150034.GH23289@zn.tnic

With this change, the code looks like this:

+       if (sme_active())
+               return set_memory_decrypted((unsigned long)vaddr, pages);

now in __set_memory_enc_dec via set_memory_decrypted():

        /* Nothing to do if memory encryption is not active */
        if (!mem_encrypt_active())
                return 0;


so you have:

	if (sme_active())

		...

		if (!mem_encrypt_active())


now maybe this is all clear to you and Tom but I betcha others will get
confused. Probably something like "well, what should be active now, SME,
SEV or memory encryption in general"?

I hope you're catching my drift.

So if you want to *not* decrypt memory in the SEV case, then doing something
like this should make it a bit more clear:


	if (sev_active())
		return;

	return set_memory_decrypted((unsigned long)vaddr, pages);

along with a comment *why* we're checking here.

But actually, I'd prefer if you had separate wrappers which are called
for SME and for SEV.

I'll let Tom chime in too.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.