Re: [PATCH v2 1/3] cryptodisk: make the password getter and additional argument to recover_key

[Glenn Washburn]

On Fri, 13 Nov 2020 14:25:08 -0800
James Bottomley <jejb@linux.ibm.com> wrote:

> For AMD SEV environments, the grub boot password has to be retrieved
> from a given memory location rather than prompted for.  This means
> that the standard password getter needs to be replaced with one that
> gets the passphrase from the SEV area and uses that instead.  Adding
> the password getter as a passed in argument to recover_key() makes
> this possible.
> 
> Signed-off-by: James Bottomley <jejb@linux.ibm.com>
> 
> ---
> 
> v2: add conditional prompting to geli.c
> ---
>  grub-core/disk/cryptodisk.c |  2 +-
>  grub-core/disk/geli.c       | 12 +++++++-----
>  grub-core/disk/luks.c       | 12 +++++++-----
>  grub-core/disk/luks2.c      | 12 +++++++-----
>  include/grub/cryptodisk.h   |  6 +++++-
>  5 files changed, 27 insertions(+), 17 deletions(-)
> 
> diff --git a/grub-core/disk/cryptodisk.c b/grub-core/disk/cryptodisk.c
> index a3d672f68..682f5a55d 100644
> --- a/grub-core/disk/cryptodisk.c
> +++ b/grub-core/disk/cryptodisk.c
> @@ -997,7 +997,7 @@ grub_cryptodisk_scan_device_real (const char
> *name, grub_disk_t source) if (!dev)
>        continue;
>      
> -    err = cr->recover_key (source, dev);
> +    err = cr->recover_key (source, dev, grub_password_get);
>      if (err)
>      {
>        cryptodisk_close (dev);
> diff --git a/grub-core/disk/geli.c b/grub-core/disk/geli.c
> index e9d23299a..3fece3f4a 100644
> --- a/grub-core/disk/geli.c
> +++ b/grub-core/disk/geli.c
> @@ -398,7 +398,8 @@ configure_ciphers (grub_disk_t disk, const char
> *check_uuid, }
>  
>  static grub_err_t
> -recover_key (grub_disk_t source, grub_cryptodisk_t dev)
> +recover_key (grub_disk_t source, grub_cryptodisk_t dev,
> +          grub_passwd_cb *password_get)
>  {
>    grub_size_t keysize;
>    grub_uint8_t digest[GRUB_CRYPTO_MAX_MDLEN];
> @@ -438,11 +439,12 @@ recover_key (grub_disk_t source,
> grub_cryptodisk_t dev) tmp = NULL;
>    if (source->partition)
>      tmp = grub_partition_get_name (source->partition);
> -  grub_printf_ (N_("Enter passphrase for %s%s%s (%s): "),
> source->name,
> -             source->partition ? "," : "", tmp ? : "",
> -             dev->uuid);
> +  if (password_get == grub_password_get)
> +    grub_printf_ (N_("Enter passphrase for %s%s%s (%s): "),
> source->name,
> +               source->partition ? "," : "", tmp ? : "",
> +               dev->uuid);
>    grub_free (tmp);
> -  if (!grub_password_get (passphrase, MAX_PASSPHRASE))
> +  if (!password_get (passphrase, MAX_PASSPHRASE))
>      return grub_error (GRUB_ERR_BAD_ARGUMENT, "Passphrase not
> supplied"); 
>    /* Calculate the PBKDF2 of the user supplied passphrase.  */
> diff --git a/grub-core/disk/luks.c b/grub-core/disk/luks.c
> index 59702067a..165f4a6bd 100644
> --- a/grub-core/disk/luks.c
> +++ b/grub-core/disk/luks.c
> @@ -152,7 +152,8 @@ configure_ciphers (grub_disk_t disk, const char
> *check_uuid, 
>  static grub_err_t
>  luks_recover_key (grub_disk_t source,
> -               grub_cryptodisk_t dev)
> +               grub_cryptodisk_t dev,
> +               grub_passwd_cb *password_get)
>  {
>    struct grub_luks_phdr header;
>    grub_size_t keysize;
> @@ -187,11 +188,12 @@ luks_recover_key (grub_disk_t source,
>    tmp = NULL;
>    if (source->partition)
>      tmp = grub_partition_get_name (source->partition);
> -  grub_printf_ (N_("Enter passphrase for %s%s%s (%s): "),
> source->name,
> -            source->partition ? "," : "", tmp ? : "",
> -            dev->uuid);
> +  if (password_get == grub_password_get)
> +       grub_printf_ (N_("Enter passphrase for %s%s%s (%s): "),
> source->name,
> +                     source->partition ? "," : "", tmp ? : "",
> +                     dev->uuid);

I don't really like this added if statement.  It feels hackish.  I also
don't have an unambiguously better alternative.

What feels like an imperfect alternative is to define a function
grub_password_prompt(char buf[], unsigned buf_size, const char
*prompt), which calls grub_password_get after displaying the string
prompt.  Typedef grub_passwd_cb to grub_password_prompt's signature.
Then some grub_passwd_cb implementations can decide when not to use the
given prompt.  Perhaps grub_password_prompt should have snprintf's
signature to be more complete and combine the grub_printf_ and
grub_password_get.

Any other ideas?

Glenn