Re: [PATCH v7 2/8] efi: Add embedded peripheral firmware support

[Hans de Goede]

Hi Luis,

Thank you for the reviews and sorry for being a bit slow to respind.

On 11-10-2019 16:48, Luis Chamberlain wrote:
> On Fri, Oct 04, 2019 at 04:50:50PM +0200, Hans de Goede wrote:
> > +static int __init efi_check_md_for_embedded_firmware(
> > +	efi_memory_desc_t *md, const struct efi_embedded_fw_desc *desc)
> > +{
> > +	const u64 prefix = *((u64 *)desc->prefix);
> > +	struct sha256_state sctx;
> > +	struct embedded_fw *fw;
> > +	u8 sha256[32];
> > +	u64 i, size;
> > +	void *map;
> > +
> > +	size = md->num_pages << EFI_PAGE_SHIFT;
> > +	map = memremap(md->phys_addr, size, MEMREMAP_WB);
>
> Since our limitaiton is the init process must have mostly finished,
> it implies early x86 boot code cannot use this, what measures can we
> take to prevent / check for such conditions to be detected and
> gracefully errored out?

As with all (EFI) early boot code, there simply is a certain order
in which things need to be done. This needs to happen after the basic
mm is setup, but before efi_free_boot_services() gets called, there
isn't really a way to check for all these conditions. As with all
early boot code, people making changes need to be careful to not
break stuff.

> > +	if (!map) {
> > +		pr_err("Error mapping EFI mem at %#llx\n", md->phys_addr);
> > +		return -ENOMEM;
> > +	}
> > +
> > +	size -= desc->length;
>
> Remind me again, why we decrement the size here?

Basically this is another way of writing:

	for (i = 0; (i + desc->length) < size; i += 8) {

> I was going to ask if we didn't need a:
>
> if (desc->length > size) {
> 	memunmap(map);
> 	return -EINVAL;
> }

That is a good point, unlikely but still a good point,
so I guess that writing:

	for (i = 0; (i + desc->length) < size; i += 8) {

Instead would better as that avoids the need for that check.
I will fix this for the next version.

Regards,

Hans

> > +	for (i = 0; i < size; i += 8) {
> > +		u64 *mem = map + i;
> > +
> > +		if (*mem != prefix)
> > +			continue;
> > +
> > +		sha256_init(&sctx);
> > +		sha256_update(&sctx, map + i, desc->length);
> > +		sha256_final(&sctx, sha256);
> > +		if (memcmp(sha256, desc->sha256, 32) == 0)
> > +			break;
> > +	}
> > +	if (i >= size) {
> > +		memunmap(map);
> > +		return -ENOENT;
> > +	}
> > +
> > +	pr_info("Found EFI embedded fw '%s'\n", desc->name);
>
> Otherwise looks good.
>
>    Luis