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Date:   Fri, 3 May 2019 16:58:34 +0800
From:   joeyli <jlee@suse.com>
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     Peter Jones <pjones@redhat.com>,
        Matthew Garrett <mjg59@google.com>,
        "Lee, Chun-Yi" <joeyli.kernel@gmail.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        David Howells <dhowells@redhat.com>,
        James Morris <jmorris@namei.org>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        Josh Boyer <jwboyer@fedoraproject.org>,
        Nayna Jain <nayna@linux.ibm.com>,
        linux-efi <linux-efi@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2 v3] efi: print appropriate status message when
 loading certificates
Message-ID: <20190503085834.GQ11486@linux-l9pv.suse>
References: <20190502040441.30372-1-jlee@suse.com>
 <20190502040441.30372-2-jlee@suse.com>
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 <20190503071819.GN11486@linux-l9pv.suse>
 <CAKv+Gu9CdWYMELxBz9WqaB4BSRbRx81iR-x4P+2OANAcfLUhUQ@mail.gmail.com>
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On Fri, May 03, 2019 at 10:07:59AM +0200, Ard Biesheuvel wrote:
> On Fri, 3 May 2019 at 09:18, joeyli <jlee@suse.com> 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@gmail.com> 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@namei.org>
> > > > Cc: Serge E. Hallyn" <serge@hallyn.com>
> > > > Cc: David Howells <dhowells@redhat.com>
> > > > Cc: Nayna Jain <nayna@linux.ibm.com>
> > > > Cc: Josh Boyer <jwboyer@fedoraproject.org>
> > > > Cc: Mimi Zohar <zohar@linux.ibm.com>
> > > > Signed-off-by: "Lee, Chun-Yi" <jlee@suse.com>
> > > > ---
> > > >  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.
>

OK. I will remove the debug message.
 
> > > >         }
> > > > +       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?
>

I agree. When secure boot enabled, kernel can not be booted without db. 
 
> And what about the converse case: if secure boot is not enabled, why
> should we trust the contents of db?
>

The db and dbx are authenticated variables that it protected by KEK.
So it can be trusted even secure boot is disabled. Unless manufacturer
or user's KEK is leaked. 
 
> > > - 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?
>

Without shim, the signed EFI stub can still be booted by EFI boot manager.
But the MokListRT will not be created for runtime. So MOK signed kernel
module can not be verified. (or IMA can not verify MOK signed kernel image
for kexec...) 
 
> > > - 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.

Here is a simple summary:

When secure boot is enabled:
    - db/dbx: Can be trusted because they are authenticated variables.
              (unless end user doesn't want to trust db/dbx)
    - MokListRT:
	- with shim: MokListRT will be created. It can be trusted.
	- without shim: MokListRT will not be created.
			MOK protected kernel module or file can not be
			verified. 

When secure boot is disabled:
    - db/dbx: Can be trusted because they are authenticated variables.
	      (unless end user doesn't want to trust db/dbx)
    - MokListRT:
        - with shim: MokListRT will be created. But it can not be trusted.
                     MOK protected kernel module or file can not be
                     verified.
        - without shim: MokListRT will not be created.
                        MOK protected kernel module or file can not be
                        verified.
 
Thanks a lot!
Joey Lee