Subject: Re: [PATCH v2 1/2] efi: add secure boot get helper

From: Chester Lin <[email-masked]>
Date: Thu, 15 Oct 2020 20:21:22 +0800

Hi Ard and Mimi,

On Wed, Oct 14, 2020 at 07:56:17AM -0400, Mimi Zohar wrote:
> On Wed, 2020-10-14 at 13:00 +0200, Ard Biesheuvel wrote:
> > Hello Chester,
> > 
> > Thanks for looking into this.
> > 
> > Some comments below.
> > 
> > On Wed, 14 Oct 2020 at 12:41, Chester Lin <[email-masked]> wrote:
> > >
> > > Separate the get_sb_mode() from arch/x86 and treat it as a common function
> > > [rename to efi_get_secureboot_mode] so all EFI-based architectures can
> > > reuse the same logic.
> > >
> > > Signed-off-by: Chester Lin <[email-masked]>
> > > ---
> > >  arch/x86/kernel/ima_arch.c | 47 ++------------------------------------
> > >  drivers/firmware/efi/efi.c | 43 ++++++++++++++++++++++++++++++++++
> > >  include/linux/efi.h        |  5 ++++
> > >  3 files changed, 50 insertions(+), 45 deletions(-)
> > >
> > > diff --git a/arch/x86/kernel/ima_arch.c b/arch/x86/kernel/ima_arch.c
> > > index 7dfb1e808928..ed4623ecda6e 100644
> > > --- a/arch/x86/kernel/ima_arch.c
> > > +++ b/arch/x86/kernel/ima_arch.c
> > > @@ -8,49 +8,6 @@
> > >
> > >  extern struct boot_params boot_params;
> > >
> > > -static enum efi_secureboot_mode get_sb_mode(void)
> > > -{
> > > -       efi_guid_t efi_variable_guid = EFI_GLOBAL_VARIABLE_GUID;
> > > -       efi_status_t status;
> > > -       unsigned long size;
> > > -       u8 secboot, setupmode;
> > > -
> > > -       size = sizeof(secboot);
> > > -
> > > -       if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE)) {
> > > -               pr_info("ima: secureboot mode unknown, no efi\n");
> > > -               return efi_secureboot_mode_unknown;
> > > -       }
> > > -
> > > -       /* Get variable contents into buffer */
> > > -       status = efi.get_variable(L"SecureBoot", &efi_variable_guid,
> > > -                                 NULL, &size, &secboot);
> > > -       if (status == EFI_NOT_FOUND) {
> > > -               pr_info("ima: secureboot mode disabled\n");
> > > -               return efi_secureboot_mode_disabled;
> > > -       }
> > > -
> > > -       if (status != EFI_SUCCESS) {
> > > -               pr_info("ima: secureboot mode unknown\n");
> > > -               return efi_secureboot_mode_unknown;
> > > -       }
> > > -
> > > -       size = sizeof(setupmode);
> > > -       status = efi.get_variable(L"SetupMode", &efi_variable_guid,
> > > -                                 NULL, &size, &setupmode);
> > > -
> > > -       if (status != EFI_SUCCESS)      /* ignore unknown SetupMode */
> > > -               setupmode = 0;
> > > -
> > > -       if (secboot == 0 || setupmode == 1) {
> > > -               pr_info("ima: secureboot mode disabled\n");
> > > -               return efi_secureboot_mode_disabled;
> > > -       }
> > > -
> > > -       pr_info("ima: secureboot mode enabled\n");
> > > -       return efi_secureboot_mode_enabled;
> > > -}
> > > -
> > >  bool arch_ima_get_secureboot(void)
> > >  {
> > >         static enum efi_secureboot_mode sb_mode;
> > > @@ -60,7 +17,7 @@ bool arch_ima_get_secureboot(void)
> > >                 sb_mode = boot_params.secure_boot;
> > >
> > >                 if (sb_mode == efi_secureboot_mode_unset)
> > > -                       sb_mode = get_sb_mode();
> > > +                       sb_mode = efi_get_secureboot_mode();
> > >                 initialized = true;
> > >         }
> > >
> > > @@ -70,7 +27,7 @@ bool arch_ima_get_secureboot(void)
> > >                 return false;
> > >  }
> > >
> > > -/* secureboot arch rules */
> > > +/* secure and trusted boot arch rules */
> > >  static const char * const sb_arch_rules[] = {
> > >  #if !IS_ENABLED(CONFIG_KEXEC_SIG)
> > >         "appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig",
> > > diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> > > index 5e5480a0a32d..68ffa6a069c0 100644
> > > --- a/drivers/firmware/efi/efi.c
> > > +++ b/drivers/firmware/efi/efi.c
> > > @@ -1022,3 +1022,46 @@ static int __init register_update_efi_random_seed(void)
> > >  }
> > >  late_initcall(register_update_efi_random_seed);
> > >  #endif
> > > +
> > > +enum efi_secureboot_mode efi_get_secureboot_mode(void)
> > > +{
> > > +       efi_guid_t efi_variable_guid = EFI_GLOBAL_VARIABLE_GUID;
> > > +       efi_status_t status;
> > > +       unsigned long size;
> > > +       u8 secboot, setupmode;
> > > +
> > > +       size = sizeof(secboot);
> > > +
> > > +       if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE)) {
> > > +               pr_info("ima: secureboot mode unknown, no efi\n");
> > 
> > These prints don't belong here anymore.
> > 
> > Also, it would be useful if we could reuse this logic in the EFI stub
> > as well, which is built as a separate executable, and does not provide
> > efi.get_variable().
> > 
> > So, you could you please break this up into
> > 
> > static inline
> > enum efi_secureboot_mode efi_get_secureboot_mode(efi_get_variable_t *get_var)
> > {
> > }
> > 
> > placed into include/linux/efi.h, which encapsulates the core logic,
> > but using get_var(), and without the prints.
> > 
> > Then, we could put something like
> > 
> > bool efi_ima_get_secureboot(void)
> > {
> > }
> > 
> > in drivers/firmware/efi/efi.c (guarded by #ifdef CONFIG_IMA_xxx),
> > which performs the
> > efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE)) check, calls
> > efi_get_secureboot_mode(efi.get_variable), and implements the logic.
> > 
> > And actually, if the logic is identical between x86 and arm64, I
> > wonder if it wouldn't be better to put the whole thing into
> > 
> > drivers/firmware/efi/efi-ima.c
> > 
> > or
> > 
> > security/integrity/ima/ima-efi.c
> > 
> > with the only difference being the boot_params->secure_boot access for
> > x86, which we can factor out to a static inline helper.
> > 
> > Mimi, any thoughts here?
> 
> Sounds good.  Keeping as much IMA code in the IMA directory makes
> sense.   The IMA Makefile would then include ima-efi.c based on an EFI
> Kconfig option.
> 
> thanks,
> 
> Mimi

Thanks for your suggestions. I will include them in v3.

Regards,
Chester

> > 
> > 
> > 
> > > +               return efi_secureboot_mode_unknown;
> > > +       }
> > > +
> > > +       /* Get variable contents into buffer */
> > > +       status = efi.get_variable(L"SecureBoot", &efi_variable_guid,
> > > +                                 NULL, &size, &secboot);
> > > +       if (status == EFI_NOT_FOUND) {
> > > +               pr_info("ima: secureboot mode disabled\n");
> > > +               return efi_secureboot_mode_disabled;
> > > +       }
> > > +
> > > +       if (status != EFI_SUCCESS) {
> > > +               pr_info("ima: secureboot mode unknown\n");
> > > +               return efi_secureboot_mode_unknown;
> > > +       }
> > > +
> > > +       size = sizeof(setupmode);
> > > +       status = efi.get_variable(L"SetupMode", &efi_variable_guid,
> > > +                                 NULL, &size, &setupmode);
> > > +
> > > +       if (status != EFI_SUCCESS)      /* ignore unknown SetupMode */
> > > +               setupmode = 0;
> > > +
> > > +       if (secboot == 0 || setupmode == 1) {
> > > +               pr_info("ima: secureboot mode disabled\n");
> > > +               return efi_secureboot_mode_disabled;
> > > +       }
> > > +
> > > +       pr_info("ima: secureboot mode enabled\n");
> > > +       return efi_secureboot_mode_enabled;
> > > +}
> > > diff --git a/include/linux/efi.h b/include/linux/efi.h
> > > index d7c0e73af2b9..a73e5ae04672 100644
> > > --- a/include/linux/efi.h
> > > +++ b/include/linux/efi.h
> > > @@ -1076,8 +1076,13 @@ static inline int efi_runtime_map_copy(void *buf, size_t bufsz)
> > >
> > >  #ifdef CONFIG_EFI
> > >  extern bool efi_runtime_disabled(void);
> > > +extern enum efi_secureboot_mode efi_get_secureboot_mode(void);
> > >  #else
> > >  static inline bool efi_runtime_disabled(void) { return true; }
> > > +static inline enum efi_secureboot_mode efi_get_secureboot_mode(void)
> > > +{
> > > +       return efi_secureboot_mode_disabled;
> > > +}
> > >  #endif
> > >
> > >  extern void efi_call_virt_check_flags(unsigned long flags, const char *call);
> > > --
> > > 2.26.1
> > >
> 
> 

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