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

Michael Roth
2021-08-30T15:56:23Z

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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