Re: [PATCH 9/9] MODSIGN: Allow the "db" UEFI variable to be suppressed

[Josh Boyer]

On Mon, Nov 21, 2016 at 11:18 AM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
> On 16 November 2016 at 18:11, David Howells <dhowells@xxxxxxxxxx> wrote:
>> From: Josh Boyer <jwboyer@xxxxxxxxxxxxxxxxx>
>>
>> If a user tells shim to not use the certs/hashes in the UEFI db variable
>> for verification purposes, shim will set a UEFI variable called
>> MokIgnoreDB.  Have the uefi import code look for this and ignore the db
>> variable if it is found.
>>
>
> Similar concern as in the previous patch: it appears to me that you
> can DoS a machine by setting MokIgnoreDB if, e.g., its modules are
> signed against a cert that resides in db, and shim/mokmanager are not
> being used.

If shim/mokmanager aren't used, then you can't actually modify
MokIgnoreDB.  Again, it requires physical access and a reboot into
mokmanager to actually take effect.

josh


>> Signed-off-by: Josh Boyer <jwboyer@xxxxxxxxxxxxxxxxx>
>> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
>> ---
>>
>>  certs/load_uefi.c |   44 ++++++++++++++++++++++++++++++++++----------
>>  1 file changed, 34 insertions(+), 10 deletions(-)
>>
>> diff --git a/certs/load_uefi.c b/certs/load_uefi.c
>> index b44e464c3ff4..3d8845986019 100644
>> --- a/certs/load_uefi.c
>> +++ b/certs/load_uefi.c
>> @@ -13,6 +13,26 @@ static __initdata efi_guid_t efi_cert_x509_sha256_guid = EFI_CERT_X509_SHA256_GU
>>  static __initdata efi_guid_t efi_cert_sha256_guid = EFI_CERT_SHA256_GUID;
>>
>>  /*
>> + * Look to see if a UEFI variable called MokIgnoreDB exists and return true if
>> + * it does.
>> + *
>> + * This UEFI variable is set by the shim if a user tells the shim to not use
>> + * the certs/hashes in the UEFI db variable for verification purposes.  If it
>> + * is set, we should ignore the db variable also and the true return indicates
>> + * this.
>> + */
>> +static __init bool uefi_check_ignore_db(void)
>> +{
>> +       efi_status_t status;
>> +       unsigned int db = 0;
>> +       unsigned long size = sizeof(db);
>> +       efi_guid_t guid = EFI_SHIM_LOCK_GUID;
>> +
>> +       status = efi.get_variable(L"MokIgnoreDB", &guid, NULL, &size, &db);
>> +       return status == EFI_SUCCESS;
>> +}
>> +
>> +/*
>>   * Get a certificate list blob from the named EFI variable.
>>   */
>>  static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
>> @@ -113,7 +133,9 @@ static __init efi_element_handler_t get_handler_for_dbx(const efi_guid_t *sig_ty
>>  }
>>
>>  /*
>> - * Load the certs contained in the UEFI databases
>> + * Load the certs contained in the UEFI databases into the secondary trusted
>> + * keyring and the UEFI blacklisted X.509 cert SHA256 hashes into the blacklist
>> + * keyring.
>>   */
>>  static int __init load_uefi_certs(void)
>>  {
>> @@ -129,15 +151,17 @@ static int __init load_uefi_certs(void)
>>         /* Get db, MokListRT, and dbx.  They might not exist, so it isn't
>>          * an error if we can't get them.
>>          */
>> -       db = get_cert_list(L"db", &secure_var, &dbsize);
>> -       if (!db) {
>> -               pr_err("MODSIGN: Couldn't get UEFI db list\n");
>> -       } else {
>> -               rc = parse_efi_signature_list("UEFI:db",
>> -                                             db, dbsize, get_handler_for_db);
>> -               if (rc)
>> -                       pr_err("Couldn't parse db signatures: %d\n", rc);
>> -               kfree(db);
>> +       if (!uefi_check_ignore_db()) {
>> +               db = get_cert_list(L"db", &secure_var, &dbsize);
>> +               if (!db) {
>> +                       pr_err("MODSIGN: Couldn't get UEFI db list\n");
>> +               } else {
>> +                       rc = parse_efi_signature_list("UEFI:db",
>> +                                                     db, dbsize, get_handler_for_db);
>> +                       if (rc)
>> +                               pr_err("Couldn't parse db signatures: %d\n", rc);
>> +                       kfree(db);
>> +               }
>>         }
>>
>>         mok = get_cert_list(L"MokListRT", &mok_var, &moksize);
>>
>> --
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