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

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Date:   Mon, 25 Mar 2019 18:17:26 +0000
From:   "Singh, Brijesh" <brijesh.singh@....com>
To:     Borislav Petkov <bp@...en8.de>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>
CC:     "Singh, Brijesh" <brijesh.singh@....com>,
        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 3/25/19 12:32 PM, Borislav Petkov wrote:
> 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);
> 


I see your point. I agree it can get confusing.


> 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.


Just a thought, maybe we can move the above if(sev_active()) check up in
kernel/kexec_core.c because we don't need to set/clear the encryption
masks when SEV is active so there is no need to call the wrapper.


> 
> I'll let Tom chime in too.
>