[PATCH v2 3/6] efi_loader: don't load signature database from file
AKASHI Takahiro
takahiro.akashi at linaro.org
Fri Aug 27 06:51:39 CEST 2021
On Fri, Aug 27, 2021 at 01:49:41PM +0900, AKASHI Takahiro wrote:
> On Fri, Aug 27, 2021 at 06:42:39AM +0200, Heinrich Schuchardt wrote:
> > On 8/27/21 6:12 AM, AKASHI Takahiro wrote:
> > > On Thu, Aug 26, 2021 at 03:48:02PM +0200, Heinrich Schuchardt wrote:
> > > > The UEFI specification requires that the signature database may only be
> > > > stored in tamper-resistant storage. So these variable may not be read
> > > > from an unsigned file.
> > >
> > > I don't have a strong opinion here, but it seems to be too restrictive.
> > > Nobody expects that file-based variable implementation is *safe*.
> > > Leave it as it is so that people can easily experiment secure boot.
> >
> > If the prior boot stage checks the integrity of the U-Boot binary, the
> > file based implementation becomes 'safe' with this patch.
>
> How safe (or secure) is it? That is a question.
> What is your thread model?
Obviously, thread -> threat
>
> -Takahiro Akashi
>
>
> > Users can still experiment with secure boot by setting the secure boot
> > variables via the efidebug command.
> >
> > I cannot see a use case for having the secure boot data base on an
> > insecure medium.
> >
> > Best regards
> >
> > Heinrich
> >
> > >
> > > -Takahiro Akashi
> > >
> > >
> > > > Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
> > > > ---
> > > > v2:
> > > > no change
> > > > ---
> > > > include/efi_variable.h | 5 +++-
> > > > lib/efi_loader/efi_var_common.c | 2 --
> > > > lib/efi_loader/efi_var_file.c | 41 ++++++++++++++++++++-------------
> > > > lib/efi_loader/efi_variable.c | 2 +-
> > > > 4 files changed, 30 insertions(+), 20 deletions(-)
> > > >
> > > > diff --git a/include/efi_variable.h b/include/efi_variable.h
> > > > index 4623a64142..2d97655e1f 100644
> > > > --- a/include/efi_variable.h
> > > > +++ b/include/efi_variable.h
> > > > @@ -161,10 +161,13 @@ efi_status_t __maybe_unused efi_var_collect(struct efi_var_file **bufp, loff_t *
> > > > /**
> > > > * efi_var_restore() - restore EFI variables from buffer
> > > > *
> > > > + * Only if @safe is set secure boot related variables will be restored.
> > > > + *
> > > > * @buf: buffer
> > > > + * @safe: restoring from tamper-resistant storage
> > > > * Return: status code
> > > > */
> > > > -efi_status_t efi_var_restore(struct efi_var_file *buf);
> > > > +efi_status_t efi_var_restore(struct efi_var_file *buf, bool safe);
> > > >
> > > > /**
> > > > * efi_var_from_file() - read variables from file
> > > > diff --git a/lib/efi_loader/efi_var_common.c b/lib/efi_loader/efi_var_common.c
> > > > index cf7afecd60..b0c5b672c5 100644
> > > > --- a/lib/efi_loader/efi_var_common.c
> > > > +++ b/lib/efi_loader/efi_var_common.c
> > > > @@ -32,10 +32,8 @@ static const struct efi_auth_var_name_type name_type[] = {
> > > > {u"KEK", &efi_global_variable_guid, EFI_AUTH_VAR_KEK},
> > > > {u"db", &efi_guid_image_security_database, EFI_AUTH_VAR_DB},
> > > > {u"dbx", &efi_guid_image_security_database, EFI_AUTH_VAR_DBX},
> > > > - /* not used yet
> > > > {u"dbt", &efi_guid_image_security_database, EFI_AUTH_VAR_DBT},
> > > > {u"dbr", &efi_guid_image_security_database, EFI_AUTH_VAR_DBR},
> > > > - */
> > > > };
> > > >
> > > > static bool efi_secure_boot;
> > > > diff --git a/lib/efi_loader/efi_var_file.c b/lib/efi_loader/efi_var_file.c
> > > > index de076b8cbc..c7c6805ed0 100644
> > > > --- a/lib/efi_loader/efi_var_file.c
> > > > +++ b/lib/efi_loader/efi_var_file.c
> > > > @@ -148,9 +148,10 @@ error:
> > > > #endif
> > > > }
> > > >
> > > > -efi_status_t efi_var_restore(struct efi_var_file *buf)
> > > > +efi_status_t efi_var_restore(struct efi_var_file *buf, bool safe)
> > > > {
> > > > struct efi_var_entry *var, *last_var;
> > > > + u16 *data;
> > > > efi_status_t ret;
> > > >
> > > > if (buf->reserved || buf->magic != EFI_VAR_FILE_MAGIC ||
> > > > @@ -160,21 +161,29 @@ efi_status_t efi_var_restore(struct efi_var_file *buf)
> > > > return EFI_INVALID_PARAMETER;
> > > > }
> > > >
> > > > - var = buf->var;
> > > > last_var = (struct efi_var_entry *)((u8 *)buf + buf->length);
> > > > - while (var < last_var) {
> > > > - u16 *data = var->name + u16_strlen(var->name) + 1;
> > > > -
> > > > - if (var->attr & EFI_VARIABLE_NON_VOLATILE && var->length) {
> > > > - ret = efi_var_mem_ins(var->name, &var->guid, var->attr,
> > > > - var->length, data, 0, NULL,
> > > > - var->time);
> > > > - if (ret != EFI_SUCCESS)
> > > > - log_err("Failed to set EFI variable %ls\n",
> > > > - var->name);
> > > > - }
> > > > - var = (struct efi_var_entry *)
> > > > - ALIGN((uintptr_t)data + var->length, 8);
> > > > + for (var = buf->var; var < last_var;
> > > > + var = (struct efi_var_entry *)
> > > > + ALIGN((uintptr_t)data + var->length, 8)) {
> > > > +
> > > > + data = var->name + u16_strlen(var->name) + 1;
> > > > +
> > > > + /*
> > > > + * Secure boot related and non-volatile variables shall only be
> > > > + * restored from U-Boot's preseed.
> > > > + */
> > > > + if (!safe &&
> > > > + (efi_auth_var_get_type(var->name, &var->guid) !=
> > > > + EFI_AUTH_VAR_NONE ||
> > > > + !(var->attr & EFI_VARIABLE_NON_VOLATILE)))
> > > > + continue;
> > > > + if (!var->length)
> > > > + continue;
> > > > + ret = efi_var_mem_ins(var->name, &var->guid, var->attr,
> > > > + var->length, data, 0, NULL,
> > > > + var->time);
> > > > + if (ret != EFI_SUCCESS)
> > > > + log_err("Failed to set EFI variable %ls\n", var->name);
> > > > }
> > > > return EFI_SUCCESS;
> > > > }
> > > > @@ -213,7 +222,7 @@ efi_status_t efi_var_from_file(void)
> > > > log_err("Failed to load EFI variables\n");
> > > > goto error;
> > > > }
> > > > - if (buf->length != len || efi_var_restore(buf) != EFI_SUCCESS)
> > > > + if (buf->length != len || efi_var_restore(buf, false) != EFI_SUCCESS)
> > > > log_err("Invalid EFI variables file\n");
> > > > error:
> > > > free(buf);
> > > > diff --git a/lib/efi_loader/efi_variable.c b/lib/efi_loader/efi_variable.c
> > > > index ba0874e9e7..a7d305ffbc 100644
> > > > --- a/lib/efi_loader/efi_variable.c
> > > > +++ b/lib/efi_loader/efi_variable.c
> > > > @@ -426,7 +426,7 @@ efi_status_t efi_init_variables(void)
> > > >
> > > > if (IS_ENABLED(CONFIG_EFI_VARIABLES_PRESEED)) {
> > > > ret = efi_var_restore((struct efi_var_file *)
> > > > - __efi_var_file_begin);
> > > > + __efi_var_file_begin, true);
> > > > if (ret != EFI_SUCCESS)
> > > > log_err("Invalid EFI variable seed\n");
> > > > }
> > > > --
> > > > 2.30.2
> > > >
> >
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