Re: [PATCH 2/2 v3] efi: print appropriate status message when loading certificates

[Ard Biesheuvel]

On Fri, 3 May 2019 at 09:18, joeyli <jlee@xxxxxxxx> wrote:
>
> Hi Ard,
>
> On Thu, May 02, 2019 at 11:04:34AM +0200, Ard Biesheuvel wrote:
> > On Thu, 2 May 2019 at 06:04, Lee, Chun-Yi <joeyli.kernel@xxxxxxxxx> wrote:
> > >
> > > When loading certificates list from UEFI variable, the original error
> > > message direct shows the efi status code from UEFI firmware. It looks
> > > ugly:
> > >
> > > [    2.335031] Couldn't get size: 0x800000000000000e
> > > [    2.335032] Couldn't get UEFI MokListRT
> > > [    2.339985] Couldn't get size: 0x800000000000000e
> > > [    2.339987] Couldn't get UEFI dbx list
> > >
> > > So, this patch shows the status string instead of status code.
> > >
> > > On the other hand, the "Couldn't get UEFI" message doesn't need
> > > to be exposed when db/dbx/mok variable do not exist. So, this
> > > patch set the message level to debug.
> > >
> > > v3.
> > > - Print messages similar to db/mok when loading dbx hash to blacklist:
> > > [    1.500952] EFI: Blacklisting hash of an executable: UEFI:dbx
> > > [    1.501773] blacklist: Loaded blacklisting hash
> > > 'bin:80b4d96931bf0d02fd91a61e19d14f1da452e66db2408ca8604d411f92659f0a'
> > >
> > > - Setting messages for the existence of db/mok/dbx lists to debug level.
> > >
> > > v2.
> > > Setting the MODSIGN messages level to debug.
> > >
> > > Link:
> > > https://forums.opensuse.org/showthread.php/535324-MODSIGN-Couldn-t-get-UEFI-db-list?p=2897516#post2897516
> > > Cc: James Morris <jmorris@xxxxxxxxx>
> > > Cc: Serge E. Hallyn" <serge@xxxxxxxxxx>
> > > Cc: David Howells <dhowells@xxxxxxxxxx>
> > > Cc: Nayna Jain <nayna@xxxxxxxxxxxxx>
> > > Cc: Josh Boyer <jwboyer@xxxxxxxxxxxxxxxxx>
> > > Cc: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> > > Signed-off-by: "Lee, Chun-Yi" <jlee@xxxxxxxx>
> > > ---
> > >  certs/blacklist.c                             |  3 +-
> > >  security/integrity/platform_certs/load_uefi.c | 40 +++++++++++++++++++--------
> > >  2 files changed, 31 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/certs/blacklist.c b/certs/blacklist.c
> > > index 3a507b9e2568..f91437e39e44 100644
> > > --- a/certs/blacklist.c
> > > +++ b/certs/blacklist.c
> > > @@ -100,7 +100,8 @@ int mark_hash_blacklisted(const char *hash)
> > >         if (IS_ERR(key)) {
> > >                 pr_err("Problem blacklisting hash (%ld)\n", PTR_ERR(key));
> > >                 return PTR_ERR(key);
> > > -       }
> > > +       } else
> > > +               pr_notice("Loaded blacklisting hash '%s'\n", hash);
> > >         return 0;
> > >  }
> > >
> > > diff --git a/security/integrity/platform_certs/load_uefi.c b/security/integrity/platform_certs/load_uefi.c
> > > index 81b19c52832b..6b6996e5bc27 100644
> > > --- a/security/integrity/platform_certs/load_uefi.c
> > > +++ b/security/integrity/platform_certs/load_uefi.c
> > > @@ -1,5 +1,7 @@
> > >  // SPDX-License-Identifier: GPL-2.0
> > >
> > > +#define pr_fmt(fmt) "EFI: "fmt
> > > +
> > >  #include <linux/kernel.h>
> > >  #include <linux/sched.h>
> > >  #include <linux/cred.h>
> > > @@ -35,6 +37,18 @@ static __init bool uefi_check_ignore_db(void)
> > >         return status == EFI_SUCCESS;
> > >  }
> > >
> > > +static void str16_to_str(efi_char16_t *str16, char *str, int str_size)
> > > +{
> > > +       int i = 0;
> > > +
> > > +       while (str16[i] != '\0' && i < (str_size - 1)) {
> > > +               str[i] = str16[i];
> > > +               i++;
> > > +       }
> > > +
> > > +       str[i] = '\0';
> > > +}
> > > +
> > >  /*
> > >   * Get a certificate list blob from the named EFI variable.
> > >   */
> > > @@ -44,13 +58,20 @@ static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
> > >         efi_status_t status;
> > >         unsigned long lsize = 4;
> > >         unsigned long tmpdb[4];
> > > +       char namestr[16];
> > >         void *db;
> > >
> > > +       str16_to_str(name, namestr, ARRAY_SIZE(namestr));
> >
> > Please drop this (and the function above) - instead, just return NULL
> > if the variable is not found (without reporting an error).
> >
>
> This name string is for printing debug level message, not error message.
> This function already returns NULL when EFI_NOT_FOUND be returned by
> firmware.
>
> > >         status = efi.get_variable(name, guid, NULL, &lsize, &tmpdb);
> > >         if (status != EFI_BUFFER_TOO_SMALL) {
> > > -               pr_err("Couldn't get size: 0x%lx\n", status);
> > > +               if (status == EFI_NOT_FOUND)
> > > +                       pr_debug("UEFI %s list doesn't exist\n", namestr);
> > > +               else
> > > +                       pr_err("Couldn't get size for UEFI %s list: %s\n",
> > > +                               namestr, efi_status_to_str(status));
> > >                 return NULL;
>
> here returns NULL when EFI_NOT_FOUND. The message of existence is for
> debugging.
>

I understand that. But I don't think we need it.

> > >         }
> > > +       pr_debug("UEFI %s list exists\n", namestr);
> > >
> > >         db = kmalloc(lsize, GFP_KERNEL);
> > >         if (!db)
> > > @@ -59,7 +80,8 @@ static __init void *get_cert_list(efi_char16_t *name, efi_guid_t *guid,
> > >         status = efi.get_variable(name, guid, NULL, &lsize, db);
> > >         if (status != EFI_SUCCESS) {
> > >                 kfree(db);
> > > -               pr_err("Error reading db var: 0x%lx\n", status);
> > > +               pr_err("Error reading UEFI %s list: %s\n",
> > > +                       namestr, efi_status_to_str(status));
> > >                 return NULL;
> > >         }
> > >
> > > @@ -95,6 +117,7 @@ static __init void uefi_blacklist_hash(const char *source, const void *data,
> > >  static __init void uefi_blacklist_x509_tbs(const char *source,
> > >                                            const void *data, size_t len)
> > >  {
> > > +       pr_info("Blacklisting X.509 TBS hash: %s\n", source);
> > >         uefi_blacklist_hash(source, data, len, "tbs:", 4);
> > >  }
> > >
> > > @@ -104,6 +127,7 @@ static __init void uefi_blacklist_x509_tbs(const char *source,
> > >  static __init void uefi_blacklist_binary(const char *source,
> > >                                          const void *data, size_t len)
> > >  {
> > > +       pr_info("Blacklisting hash of an executable: %s\n", source);
> > >         uefi_blacklist_hash(source, data, len, "bin:", 4);
> > >  }
> > >
> >
> > These are separate changes - I don't have an opinion whether they are
> > appropriate or not, but they should be in a separate patch.
> >
>
> I will move the message of blacklising hash to other patch. Thanks!
>
> > > @@ -154,9 +178,7 @@ static int __init load_uefi_certs(void)
> > >          */
> > >         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 {
> > > +               if (db) {
> > >                         rc = parse_efi_signature_list("UEFI:db",
> > >                                         db, dbsize, get_handler_for_db);
> > >                         if (rc)
> > > @@ -167,9 +189,7 @@ static int __init load_uefi_certs(void)
> > >         }
> > >
> > >         mok = get_cert_list(L"MokListRT", &mok_var, &moksize);
> > > -       if (!mok) {
> > > -               pr_info("Couldn't get UEFI MokListRT\n");
> > > -       } else {
> > > +       if (mok) {
> > >                 rc = parse_efi_signature_list("UEFI:MokListRT",
> > >                                               mok, moksize, get_handler_for_db);
> > >                 if (rc)
> > > @@ -178,9 +198,7 @@ static int __init load_uefi_certs(void)
> > >         }
> > >
> > >         dbx = get_cert_list(L"dbx", &secure_var, &dbxsize);
> > > -       if (!dbx) {
> > > -               pr_info("Couldn't get UEFI dbx list\n");
> > > -       } else {
> > > +       if (dbx) {
> > >                 rc = parse_efi_signature_list("UEFI:dbx",
> > >                                               dbx, dbxsize,
> > >                                               get_handler_for_dbx);
> > > --
> > > 2.16.4
> > >
> >
> > I think we should consider carefully what it means if some of these
> > variables don't exist:
> > - if secure boot is enabled, db and dbx must exist, so if they don't,
> > something is wrong
>
> The existence of db/dbx is not related to secure boot. If manufacturer/user
> enrolled certificate/hash to db or dbx, then the variable will be created.
> If user didn't enroll anything to db/dbx, then variables will not show up.
>

Yes, but if secure boot is enabled and db is empty, how could you have
booted in the first place?

And what about the converse case: if secure boot is not enabled, why
should we trust the contents of db?

> > - secure boot might be enabled but we may be booting without shim.
>
> Shim always creates MokListRT no matter secure boot enabled or disabled.
>

That is not my point. What happens if you booted with secure boot
enabled but without the help of shim?

> > - secure boot might be disabled.
> >
>
> It's not about secure boot, db/dbx/MokListRT are always available at
> runtime if user was enrolled something to those list.
>

Yes, but again, depending on whether shim was involved, and/or whether
secure boot was enabled or not, the way we interpret these things may
be very different.

I want the reasoning to be sound before merging any patches that deal
with these variables.