linux-kernel - Re: [PATCH 3/3] virt: Add sev_secret module to expose confidential computing secrets

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Message-ID: <CADcWuH05vbFtJ1WYSs3d+_=TGzh-MitvAXp1__d1kGJJkvkWpQ@mail.gmail.com>
Date:   Mon, 23 Aug 2021 20:21:59 +0100
From:   Andrew Scull <ascull@...gle.com>
To:     Dov Murik <dovmurik@...ux.ibm.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Borislav Petkov <bp@...e.de>,
        Ashish Kalra <ashish.kalra@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@...ux.ibm.com>,
        Jim Cadden <jcadden@....com>, linux-coco@...ts.linux.dev,
        linux-security-module@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] virt: Add sev_secret module to expose confidential
 computing secrets

On Fri, 20 Aug 2021 at 19:36, Dov Murik <dovmurik@...ux.ibm.com> wrote:
>
>
>
> On 19/08/2021 16:02, Andrew Scull wrote:
> > On Mon, 16 Aug 2021 at 10:57, Ard Biesheuvel <ardb@...nel.org> wrote:
> >>
> >> On Fri, 13 Aug 2021 at 15:05, Andrew Scull <ascull@...gle.com> wrote:
> >>>
> >>> On Mon, Aug 09, 2021 at 07:01:57PM +0000, Dov Murik wrote:
>
> [...]
>
> >>>
> >>>> +static int sev_secret_unlink(struct inode *dir, struct dentry *dentry)
> >>>> +{
> >>>> +     struct sev_secret *s = sev_secret_get();
> >>>> +     struct inode *inode = d_inode(dentry);
> >>>> +     struct secret_entry *e = (struct secret_entry *)inode->i_private;
> >>>> +     int i;
> >>>> +
> >>>> +     if (e) {
> >>>> +             /* Zero out the secret data */
> >>>> +             memzero_explicit(e->data, secret_entry_data_len(e));
> >>>
> >>> Would there be a benefit in flushing these zeros?
> >>>
> >>
> >> Do you mean cache clean+invalidate? Better to be precise here.
> >
> > At least a clean, to have the zeros written back to memory from the
> > cache, in order to overwrite the secret.
> >
>
> I agree, but not sure how to implement this:
>
> I see there's an arch_wb_cache_pmem exported function which internally
> (in arch/x86/lib/usercopy_64.c) calls clean_cache_range which seems to
> do what we want (assume the secret can be longer than the cache line).
>
> But arch_wb_cache_pmem is declared in include/linux/libnvdimm.h and
> guarded with #ifdef CONFIG_ARCH_HAS_PMEM_API -- both seem not related to
> what I'm trying to do.
>
> I see there's an exported clflush_cache_range for x86 -- but that's a
> clean+flush if I understand correctly.

This would be perfectly correct, the invalidation is just unnecessary.

> Suggestions on how to approach? I can copy the clean_cache_range
> implementation into the sev_secret module but hopefully there's a better
> way to reuse.  Maybe export clean_cache_range in x86?

Exporting sounds much better than duplicating.

It looks like the clean-only instruction was added to x86 more
recently and with persistent memory as the intended application.

d9dc64f30 "x86/asm: Add support for the CLWB instruction" says:

"This should be used in favor of clflushopt or clflush in cases where
you require the cache line to be written to memory but plan to access
the data cache line to be written to memory but plan to access the
data"

I don't expect the secret table would be accessed with such frequency
that it would actually make a difference, but if it's just a quirk of
history that the clean-only version isn't exported, now seems as good
a time as any to change that!

> Since this is for SEV the solution can be x86-specific, but if there's a
> generic way I guess it's better (I think all of sev_secret module
> doesn't have x86-specific stuff).

arch_wb_cache_pmem is the closest to arch agnostic I've seen, but that
has it own problems :/