Subject: Re: [PATCH 1/6] efistub: pass uefi secureboot flag via fdt params

From: Chester Lin <[email-masked]>
Date: Mon, 14 Sep 2020 16:05:22 +0800

Hi Ard,

On Fri, Sep 11, 2020 at 06:01:09PM +0300, Ard Biesheuvel wrote:
> On Fri, 4 Sep 2020 at 10:29, Chester Lin <[email-masked]> wrote:
> >
> > Add a new UEFI parameter: "linux,uefi-secure-boot" in fdt boot params
> > as other architectures have done in their own boot data. For example,
> > the boot_params->secure_boot in x86.
> >
> > Signed-off-by: Chester Lin <[email-masked]>
> 
> Why do we need this flag? Can't the OS simply check the variable directly?
> 

In fact, there's a difficulty to achieve this.

When linux kernel is booting on ARM, the runtime services are enabled later on.
It's done by arm_enable_runtime_services(), which is registered as an early_initcall.
Before it calls efi_native_runtime_setup(), all EFI runtime callbacks are still
NULL so calling efi.get_variable() will cause NULL pointer dereference.

There's a case that arch_ima_get_secureboot() can be called in early boot stage.
For example, when you try to set "ima_appraise=off" in kernel command line, it's
actually handled early:

[    0.000000] Kernel command line: BOOT_IMAGE=/boot/Image-5.9.0-rc3-9.gdd61cda-
vanilla root=UUID=a88bfb80-8abb-425c-a0f3-ad317465c28b splash=silent mitigations
=auto ignore_loglevel earlycon=pl011,mmio,0x9000000 console=ttyAMA0 ima_appraise=off
[    0.000000] ima: Secure boot enabled: ignoring ima_appraise=off boot parameter option
[    0.000000] Dentry cache hash table entries: 1048576 (order: 11, 8388608 bytes, linear)

However EFI services are remapped and enabled afterwards.

[    0.082286] rcu: Hierarchical SRCU implementation.
[    0.089592] Remapping and enabling EFI services.
[    0.097509] smp: Bringing up secondary CPUs ...

Another problem is that efi_rts_wq is created in subsys_initcall so we have to
wait for both EFI services mapping and the workqueue get initiated before calling
efi.get_variable() on ARM.

The only way I can think of is to put a flag via fdt params. May I have your
suggestions? I will appreciate if there's any better approach.

Thanks,
Chester

> > ---
> >  drivers/firmware/efi/libstub/fdt.c | 39 +++++++++++++++++++++++++++++-
> >  1 file changed, 38 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/firmware/efi/libstub/fdt.c b/drivers/firmware/efi/libstub/fdt.c
> > index 11ecf3c4640e..c9a341e4715f 100644
> > --- a/drivers/firmware/efi/libstub/fdt.c
> > +++ b/drivers/firmware/efi/libstub/fdt.c
> > @@ -136,6 +136,10 @@ static efi_status_t update_fdt(void *orig_fdt, unsigned long orig_fdt_size,
> >         if (status)
> >                 goto fdt_set_fail;
> >
> > +       status = fdt_setprop_var(fdt, node, "linux,uefi-secure-boot", fdt_val32);
> > +       if (status)
> > +               goto fdt_set_fail;
> > +
> >         if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
> >                 efi_status_t efi_status;
> >
> > @@ -199,6 +203,24 @@ static efi_status_t update_fdt_memmap(void *fdt, struct efi_boot_memmap *map)
> >         return EFI_SUCCESS;
> >  }
> >
> > +static efi_status_t update_fdt_secboot(void *fdt, u32 secboot)
> > +{
> > +       int node = fdt_path_offset(fdt, "/chosen");
> > +       u32 fdt_val32;
> > +       int err;
> > +
> > +       if (node < 0)
> > +               return EFI_LOAD_ERROR;
> > +
> > +       fdt_val32 = cpu_to_fdt32(secboot);
> > +
> > +       err = fdt_setprop_inplace_var(fdt, node, "linux,uefi-secure-boot", fdt_val32);
> > +       if (err)
> > +               return EFI_LOAD_ERROR;
> > +
> > +       return EFI_SUCCESS;
> > +}
> > +
> >  struct exit_boot_struct {
> >         efi_memory_desc_t       *runtime_map;
> >         int                     *runtime_entry_count;
> > @@ -208,6 +230,9 @@ struct exit_boot_struct {
> >  static efi_status_t exit_boot_func(struct efi_boot_memmap *map,
> >                                    void *priv)
> >  {
> > +       efi_status_t status;
> > +       enum efi_secureboot_mode secboot_status;
> > +       u32 secboot_var = 0;
> >         struct exit_boot_struct *p = priv;
> >         /*
> >          * Update the memory map with virtual addresses. The function will also
> > @@ -217,7 +242,19 @@ static efi_status_t exit_boot_func(struct efi_boot_memmap *map,
> >         efi_get_virtmap(*map->map, *map->map_size, *map->desc_size,
> >                         p->runtime_map, p->runtime_entry_count);
> >
> > -       return update_fdt_memmap(p->new_fdt_addr, map);
> > +       status = update_fdt_memmap(p->new_fdt_addr, map);
> > +
> > +       if (status != EFI_SUCCESS)
> > +               return status;
> > +
> > +       secboot_status = efi_get_secureboot();
> > +
> > +       if (secboot_status == efi_secureboot_mode_enabled)
> > +               secboot_var = 1;
> > +
> > +       status = update_fdt_secboot(p->new_fdt_addr, secboot_var);
> > +
> > +       return status;
> >  }
> >
> >  #ifndef MAX_FDT_SIZE
> > --
> > 2.26.1
> >
> 

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