Re: [PATCH Part1 v5 30/38] x86/compressed/64: store Confidential Computing blob address in bootparams

[Michael Roth <michael.roth@amd.com>]

On Fri, Aug 27, 2021 at 04:15:14PM +0200, Borislav Petkov wrote:
> On Fri, Aug 20, 2021 at 10:19:25AM -0500, Brijesh Singh wrote:
> > From: Michael Roth <michael.roth@amd.com>
> > 
> > When the Confidential Computing blob is located by the boot/compressed
> > kernel, store a pointer to it in bootparams->cc_blob_address to avoid
> > the need for the run-time kernel to rescan the EFI config table to find
> > it again.
> > 
> > Since this function is also shared by the run-time kernel, this patch
> 
> Here's "this patch" again... but you know what to do.
> 
> > also adds the logic to make use of bootparams->cc_blob_address when it
> > has been initialized.
> > 
> > Signed-off-by: Michael Roth <michael.roth@amd.com>
> > Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> > ---
> >  arch/x86/kernel/sev-shared.c | 40 ++++++++++++++++++++++++++----------
> >  1 file changed, 29 insertions(+), 11 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> > index 651980ddbd65..6f70ba293c5e 100644
> > --- a/arch/x86/kernel/sev-shared.c
> > +++ b/arch/x86/kernel/sev-shared.c
> > @@ -868,7 +868,6 @@ static enum es_result vc_handle_rdtsc(struct ghcb *ghcb,
> >  	return ES_OK;
> >  }
> >  
> > -#ifdef BOOT_COMPRESSED
> >  static struct setup_data *get_cc_setup_data(struct boot_params *bp)
> >  {
> >  	struct setup_data *hdr = (struct setup_data *)bp->hdr.setup_data;
> > @@ -888,6 +887,16 @@ static struct setup_data *get_cc_setup_data(struct boot_params *bp)
> >   *   1) Search for CC blob in the following order/precedence:
> >   *      - via linux boot protocol / setup_data entry
> >   *      - via EFI configuration table
> > + *   2) If found, initialize boot_params->cc_blob_address to point to the
> > + *      blob so that uncompressed kernel can easily access it during very
> > + *      early boot without the need to re-parse EFI config table
> > + *   3) Return a pointer to the CC blob, NULL otherwise.
> > + *
> > + * For run-time/uncompressed kernel:
> > + *
> > + *   1) Search for CC blob in the following order/precedence:
> > + *      - via linux boot protocol / setup_data entry
> 
> Why would you do this again if the boot/compressed kernel has already
> searched for it?

In some cases it's possible to boot directly to kernel proper without
going through decompression kernel (e.g. CONFIG_PVH), so this is to allow
a way for boot loaders of this sort to provide a CC blob without relying
on EFI. It could be relevant for things like fast/virtualized containers.

> 
> > + *      - via boot_params->cc_blob_address
> 
> Yes, that is the only thing you need to do in the runtime kernel - see
> if cc_blob_address is not 0. And all the work has been done by the
> decompressor kernel already.
> 
> >   *   2) Return a pointer to the CC blob, NULL otherwise.
> >   */
> >  static struct cc_blob_sev_info *sev_snp_probe_cc_blob(struct boot_params *bp)
> > @@ -897,9 +906,11 @@ static struct cc_blob_sev_info *sev_snp_probe_cc_blob(struct boot_params *bp)
> >  		struct setup_data header;
> >  		u32 cc_blob_address;
> >  	} *sd;
> > +#ifdef __BOOT_COMPRESSED
> >  	unsigned long conf_table_pa;
> >  	unsigned int conf_table_len;
> >  	bool efi_64;
> > +#endif
> 
> That function turns into an unreadable mess with that #ifdef
> __BOOT_COMPRESSED slapped everywhere.
> 
> It seems the cleanest thing to do is to do what we do with
> acpi_rsdp_addr: do all the parsing in boot/compressed/ and pass it on
> through boot_params. Kernel proper simply reads the pointer.
> 
> Which means, you can stick all that cc_blob figuring out functionality
> in arch/x86/boot/compressed/sev.c instead.

Most of the #ifdef'ery is due to the EFI scan, so I moved that part out
to a separate helper, snp_probe_cc_blob_efi(), that lives in
boot/compressed.sev.c. Still not pretty, but would this be acceptable?

/*
 * For boot/compressed kernel:
 *
 *   1) Search for CC blob in the following order/precedence:
 *      - via linux boot protocol / setup_data entry
 *      - via EFI configuration table
 *   2) If found, initialize boot_params->cc_blob_address to point to the
 *      blob so that uncompressed kernel can easily access it during very
 *      early boot without the need to re-parse EFI config table
 *   3) Return a pointer to the CC blob, NULL otherwise.
 *
 * For run-time/uncompressed kernel:
 *
 *   1) Search for CC blob in the following order/precedence:
 *      - via boot_params->cc_blob_address
 *      - via linux boot protocol / setup_data entry
 *   2) Return a pointer to the CC blob, NULL otherwise.
 */
static struct cc_blob_sev_info *sev_snp_probe_cc_blob(struct boot_params *bp)
{
        struct cc_blob_sev_info *cc_info = NULL;
        struct cc_setup_data *sd;

#ifndef __BOOT_COMPRESSED
        /*
         * CC blob isn't in setup_data, see if boot kernel passed it via
         * boot_params.
         */
        if (bp->cc_blob_address) {
                cc_info = (struct cc_blob_sev_info *)(unsigned long)bp->cc_blob_address;
                goto out_verify;
        }
#endif

        /* Try to get CC blob via setup_data */
        sd = get_cc_setup_data(bp);
        if (sd) {
                cc_info = (struct cc_blob_sev_info *)(unsigned long)sd->cc_blob_address;
                goto out_verify;
        }

#ifdef __BOOT_COMPRESSED
        cc_info = snp_probe_cc_blob_efi(bp);
#endif

out_verify:
        /* CC blob should be either valid or not present. Fail otherwise. */
        if (cc_info && cc_info->magic != CC_BLOB_SEV_HDR_MAGIC)
                sev_es_terminate(1, GHCB_SNP_UNSUPPORTED);

#ifdef __BOOT_COMPRESSED
        /*
         * Pass run-time kernel a pointer to CC info via boot_params for easier
         * access during early boot.
         */
        bp->cc_blob_address = (u32)(unsigned long)cc_info;
#endif

        return cc_info;
}

> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
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