[PATCH] efi_loader: Get rid of kaslr-seed

[Mark Kettenis]

> From: Ard Biesheuvel <ardb at kernel.org>
> Date: Thu, 16 Dec 2021 15:54:55 +0100

Hi Ard, Ilias,

> On Thu, 16 Dec 2021 at 15:52, Ilias Apalodimas
> <ilias.apalodimas at linaro.org> wrote:
> >
> > Right now we unconditionally pass a 'kaslr-seed' property to the kernel
> > if the DTB we ended up in EFI includes the entry.  However the kernel
> > EFI stub completely ignores it and only relies on EFI_RNG_PROTOCOL.
> > So let's get rid of it unconditionally since it would mess up the
> > (upcoming) DTB TPM measuring as well.
> >
> > Signed-off-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> 
> Acked-by: Ard Biesheuvel <ardb at kernel.org>
> 
> Note that the EFI stub itself does not consume the DTB /chosen entry
> for its own randomness needs (i.e., the randomization of the physical
> placement of the kernel, which also affects low order virtual
> placement [bits 16-20]), and will blindly overwrite the seed with
> whatever the EFI_RNG_PROTOCOL returns.

But it will only do that if EFI_RNG_PROTOCOL is implemented and
sucessfully returns some random data.  Otherwise "kaslr-seed" will
survive as-is.  At least that is how I read the code in
drivers/firmware/efi/libstub/fdt.c:update_fdt().

And this is good.  On Apple M1 systems, the Apple bootloader actually
provides a block of entropy the the kernel in their version of the
device tree.  The m1n1 bootloader uses this entropy to populate the
kaslr-seed property in the device tree it passes on.  And U-Boot is
used to provide an EFI implementation such that we can boot a wide
variety of OSes.  At this point we don't know yet whether the SoC
includes an RNG that we can use to implement EFI_RNG_PROTOCOL in
U-Boot.

So the effect of tis change is that a Linux kernel on this platform
will run without KASLR.  That doesn't seem acceptable to me.

> > ---
> >  cmd/bootefi.c                 |  2 ++
> >  include/efi_loader.h          |  2 ++
> >  lib/efi_loader/efi_dt_fixup.c | 22 ++++++++++++++++++++++
> >  3 files changed, 26 insertions(+)
> >
> > diff --git a/cmd/bootefi.c b/cmd/bootefi.c
> > index d77d3b6e943d..25f9bfce9b84 100644
> > --- a/cmd/bootefi.c
> > +++ b/cmd/bootefi.c
> > @@ -310,6 +310,8 @@ efi_status_t efi_install_fdt(void *fdt)
> >         /* Create memory reservations as indicated by the device tree */
> >         efi_carve_out_dt_rsv(fdt);
> >
> > +       efi_purge_kaslr_seed(fdt);
> > +
> >         /* Install device tree as UEFI table */
> >         ret = efi_install_configuration_table(&efi_guid_fdt, fdt);
> >         if (ret != EFI_SUCCESS) {
> > diff --git a/include/efi_loader.h b/include/efi_loader.h
> > index 9dd6c2033634..e560401ac54f 100644
> > --- a/include/efi_loader.h
> > +++ b/include/efi_loader.h
> > @@ -519,6 +519,8 @@ efi_status_t EFIAPI efi_convert_pointer(efi_uintn_t debug_disposition,
> >                                         void **address);
> >  /* Carve out DT reserved memory ranges */
> >  void efi_carve_out_dt_rsv(void *fdt);
> > +/* Purge unused kaslr-seed */
> > +void efi_purge_kaslr_seed(void *fdt);
> >  /* Called by bootefi to make console interface available */
> >  efi_status_t efi_console_register(void);
> >  /* Called by bootefi to make all disk storage accessible as EFI objects */
> > diff --git a/lib/efi_loader/efi_dt_fixup.c b/lib/efi_loader/efi_dt_fixup.c
> > index b6fe5d2e5a34..02f7de73872e 100644
> > --- a/lib/efi_loader/efi_dt_fixup.c
> > +++ b/lib/efi_loader/efi_dt_fixup.c
> > @@ -40,6 +40,28 @@ static void efi_reserve_memory(u64 addr, u64 size, bool nomap)
> >                         addr, size);
> >  }
> >
> > +/**
> > + * efi_remove_kaslr_seed() - Removed unused kaslr-seed
> > + *
> > + * Kernel's EFI STUB only relies on EFI_RNG_PROTOCOL for randomization
> > + * and completely ignores the kaslr-seed.  Weed it out from the DTB we
> > + * hand over, which would mess up our DTB TPM measurements as well.
> > + *
> > + * @fdt: Pointer to device tree
> > + */
> > +void efi_purge_kaslr_seed(void *fdt)
> > +{
> > +       int nodeoff = fdt_path_offset(fdt, "/chosen");
> > +       int err = 0;
> > +
> > +       if (nodeoff < 0)
> > +               return;
> > +
> > +       err = fdt_delprop(fdt, nodeoff, "kaslr-seed");
> > +       if (err < 0)
> > +               log_err("Error deleting kaslr-seed\n");
> > +}
> > +
> >  /**
> >   * efi_carve_out_dt_rsv() - Carve out DT reserved memory ranges
> >   *
> > --
> > 2.30.2
> >
>