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

[Mimi Zohar]

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 <clin@xxxxxxxx> 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 <clin@xxxxxxxx>
> > ---
> >  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
> 
> 
> 
> > +               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
> >