Re: [RFC PATCH] efi_loader: clean up uefi secure boot image verification logic

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

Hi Ilias,

On Mon, Jan 24, 2022 at 05:36:20PM +0200, Ilias Apalodimas wrote:
> From: Ilias Apalodimas <apalos@gmail.com>
> 
> We currently distinguish between signed and non signed PE/COFF
> executables while trying to authenticate signatures and/or sha256
> hashes in db and dbx.  That code duplication can be avoided.

Thank you for cleaning up the code.
The change you made here looks good.
I want you to revise some wordings for clarification.

> On sha256 hashes we don't really care if the image is signed or
> not.

The sentence above might sound a bit misleading?

> The logic can be implemented in
>  1. Is the sha256 of the image in dbx

Please explicitly mention that the image will be rejected
in this case unlike the other cases below.

>  2. Is the image signed with a certificate that's found in db and
>     not in dbx
>  3. The image carries a cert which is signed by a cert in db (and
>     not in dbx, and the image can be verified against the former)

(2) and (3) are the rules applied only if the image is signed. So,
   2. If the image is signed,
      2-1. ...
      2-2. ...

In addition, your (3) should be described in a similar way as (2).
For instance,
   3. Is the image signed with a certificate that is carried in
      the image and the cert is signed by another cert in the image
      and so on, or in db (... ).

(although it is difficult to describe the logic precisely.)

>  4. Is the sha256 of the image in db
> So weed out the 'special' case handling unsigned images.
> 
> While at it move the checking of a hash outside the certificate
> verification loop which isn't really needed and check for the hash
> only once.

It might be my preference, but I would like to add
  assert(ret == false);
before efi_signature_lookup_digest(regs, db) to indicate this is the last
resort to authenticate the image.

> Also allow a mix of signatures and hashes in db instead
> of explicitly breaking out the sha verification loop if a signature
> happens to be added first.

I don't think that your change gives us an extra case of "allowing".
Please elaborate if I misunderstand something.

> It's worth noting that (2) only works if the certificate is self
> signed,  but that canb be fixed in a following patch.

Yes, please.

-Takahiro Akashi

> Signed-off-by: Ilias Apalodimas <apalos@gmail.com>
> ---
>  lib/efi_loader/efi_image_loader.c | 84 ++++++-------------------------
>  lib/efi_loader/efi_signature.c    |  2 +-
>  2 files changed, 15 insertions(+), 71 deletions(-)
> 
> diff --git a/lib/efi_loader/efi_image_loader.c b/lib/efi_loader/efi_image_loader.c
> index 255613eb72ba..53d54ca42f5c 100644
> --- a/lib/efi_loader/efi_image_loader.c
> +++ b/lib/efi_loader/efi_image_loader.c
> @@ -516,63 +516,16 @@ err:
>  }
>  
>  #ifdef CONFIG_EFI_SECURE_BOOT
> -/**
> - * efi_image_unsigned_authenticate() - authenticate unsigned image with
> - * SHA256 hash
> - * @regs:	List of regions to be verified
> - *
> - * If an image is not signed, it doesn't have a signature. In this case,
> - * its message digest is calculated and it will be compared with one of
> - * hash values stored in signature databases.
> - *
> - * Return:	true if authenticated, false if not
> - */
> -static bool efi_image_unsigned_authenticate(struct efi_image_regions *regs)
> -{
> -	struct efi_signature_store *db = NULL, *dbx = NULL;
> -	bool ret = false;
> -
> -	dbx = efi_sigstore_parse_sigdb(L"dbx");
> -	if (!dbx) {
> -		EFI_PRINT("Getting signature database(dbx) failed\n");
> -		goto out;
> -	}
> -
> -	db = efi_sigstore_parse_sigdb(L"db");
> -	if (!db) {
> -		EFI_PRINT("Getting signature database(db) failed\n");
> -		goto out;
> -	}
> -
> -	/* try black-list first */
> -	if (efi_signature_lookup_digest(regs, dbx)) {
> -		EFI_PRINT("Image is not signed and its digest found in \"dbx\"\n");
> -		goto out;
> -	}
> -
> -	/* try white-list */
> -	if (efi_signature_lookup_digest(regs, db))
> -		ret = true;
> -	else
> -		EFI_PRINT("Image is not signed and its digest not found in \"db\" or \"dbx\"\n");
> -
> -out:
> -	efi_sigstore_free(db);
> -	efi_sigstore_free(dbx);
> -
> -	return ret;
> -}
> -
>  /**
>   * efi_image_authenticate() - verify a signature of signed image
>   * @efi:	Pointer to image
>   * @efi_size:	Size of @efi
>   *
> - * A signed image should have its signature stored in a table of its PE header.
> + * An image should have its signature stored in a table of its PE header.
>   * So if an image is signed and only if if its signature is verified using
> - * signature databases, an image is authenticated.
> - * If an image is not signed, its validity is checked by using
> - * efi_image_unsigned_authenticated().
> + * signature databases or if it's sha256 is found in db an image is
> + * authenticated.
> + *
>   * TODO:
>   * When AuditMode==0, if the image's signature is not found in
>   * the authorized database, or is found in the forbidden database,
> @@ -608,14 +561,7 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
>  	if (!efi_image_parse(new_efi, efi_size, &regs, &wincerts,
>  			     &wincerts_len)) {
>  		EFI_PRINT("Parsing PE executable image failed\n");
> -		goto err;
> -	}
> -
> -	if (!wincerts) {
> -		/* The image is not signed */
> -		ret = efi_image_unsigned_authenticate(regs);
> -
> -		goto err;
> +		goto out;
>  	}
>  
>  	/*
> @@ -624,18 +570,18 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
>  	db = efi_sigstore_parse_sigdb(L"db");
>  	if (!db) {
>  		EFI_PRINT("Getting signature database(db) failed\n");
> -		goto err;
> +		goto out;
>  	}
>  
>  	dbx = efi_sigstore_parse_sigdb(L"dbx");
>  	if (!dbx) {
>  		EFI_PRINT("Getting signature database(dbx) failed\n");
> -		goto err;
> +		goto out;
>  	}
>  
>  	if (efi_signature_lookup_digest(regs, dbx)) {
>  		EFI_PRINT("Image's digest was found in \"dbx\"\n");
> -		goto err;
> +		goto out;
>  	}
>  
>  	/*
> @@ -729,20 +675,18 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
>  		/* try white-list */
>  		if (efi_signature_verify(regs, msg, db, dbx)) {
>  			ret = true;
> -			break;
> +			goto out;
>  		}
>  
>  		EFI_PRINT("Signature was not verified by \"db\"\n");
>  
> -		if (efi_signature_lookup_digest(regs, db)) {
> -			ret = true;
> -			break;
> -		}
> -
> -		EFI_PRINT("Image's digest was not found in \"db\" or \"dbx\"\n");
>  	}
>  
> -err:
> +	if (efi_signature_lookup_digest(regs, db))
> +		ret = true;
> +	else
> +		EFI_PRINT("Image's digest was not found in \"db\" or \"dbx\"\n");
> +out:
>  	efi_sigstore_free(db);
>  	efi_sigstore_free(dbx);
>  	pkcs7_free_message(msg);
> diff --git a/lib/efi_loader/efi_signature.c b/lib/efi_loader/efi_signature.c
> index 3243e2c60de0..8fa82f8cea4c 100644
> --- a/lib/efi_loader/efi_signature.c
> +++ b/lib/efi_loader/efi_signature.c
> @@ -176,7 +176,7 @@ bool efi_signature_lookup_digest(struct efi_image_regions *regs,
>  		if (guidcmp(&siglist->sig_type, &efi_guid_sha256)) {
>  			EFI_PRINT("Digest algorithm is not supported: %pUs\n",
>  				  &siglist->sig_type);
> -			break;
> +			continue;
>  		}
>  
>  		if (!efi_hash_regions(regs->reg, regs->num, &hash, &size)) {
> -- 
> 2.30.2
>