> From: Ilias Apalodimas <[email protected]> > Date: Mon, 3 Jan 2022 14:07:37 +0200 > > U-Boot, in some occasions, injects a 'kaslr-seed' property on the /chosen > node. That would be problematic in case we want to measure the DTB we > install in the config table, since it would change across reboots. > > The Linux kernel EFI-stub completely ignores it and only relies on > EFI_RNG_PROTOCOL for it's own randomness needs (i.e the randomization > of the physical placement of the kernel). In fact it (blindly) overwrites > the existing seed if the protocol is installed. However it still uses it > for randomizing it's virtual placement. > So let's get rid of it in the presence of the RNG protocol. > > It's worth noting that TPMs also provide an RNG. So if we tweak our > EFI_RNG_PROTOCOL slightly and install the protocol when a TPM device > is present the 'kaslr-seed' property will always be removed, allowing > us to reliably measure our DTB. > > Acked-by: Ard Biesheuvel <[email protected]> > Signed-off-by: Ilias Apalodimas <[email protected]> > --- > changes since v2: > - Mark proposed a better commit message description > changes since v1: > - Only removing the property if EFI_RNG_PROTOCOL is installed, since some > OS'es rely on kaslr-seed > - Only display an error message if the kaslr-seed entry was found but not > removed > cmd/bootefi.c | 2 ++ > include/efi_loader.h | 2 ++ > lib/efi_loader/efi_dt_fixup.c | 33 +++++++++++++++++++++++++++++++++ > 3 files changed, 37 insertions(+)
Reviewed-by: Mark Kettenis <[email protected]> > diff --git a/cmd/bootefi.c b/cmd/bootefi.c > index d77d3b6e943d..57f13ce701ec 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_try_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..1fe003db69e0 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_try_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..d3923e5dba1b 100644 > --- a/lib/efi_loader/efi_dt_fixup.c > +++ b/lib/efi_loader/efi_dt_fixup.c > @@ -8,6 +8,7 @@ > #include <common.h> > #include <efi_dt_fixup.h> > #include <efi_loader.h> > +#include <efi_rng.h> > #include <fdtdec.h> > #include <mapmem.h> > > @@ -40,6 +41,38 @@ static void efi_reserve_memory(u64 addr, u64 size, bool > nomap) > addr, size); > } > > +/** > + * efi_try_purge_kaslr_seed() - Remove unused kaslr-seed > + * > + * Kernel's EFI STUB only relies on EFI_RNG_PROTOCOL for randomization > + * and completely ignores the kaslr-seed for its own randomness needs > + * (i.e the randomization of the physical placement of the kernel). > + * 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_try_purge_kaslr_seed(void *fdt) > +{ > + const efi_guid_t efi_guid_rng_protocol = EFI_RNG_PROTOCOL_GUID; > + struct efi_handler *handler; > + efi_status_t ret; > + int nodeoff = 0; > + int err = 0; > + > + ret = efi_search_protocol(efi_root, &efi_guid_rng_protocol, &handler); > + if (ret != EFI_SUCCESS) > + return; > + > + nodeoff = fdt_path_offset(fdt, "/chosen"); > + if (nodeoff < 0) > + return; > + > + err = fdt_delprop(fdt, nodeoff, "kaslr-seed"); > + if (err < 0 && err != -FDT_ERR_NOTFOUND) > + log_err("Error deleting kaslr-seed\n"); > +} > + > /** > * efi_carve_out_dt_rsv() - Carve out DT reserved memory ranges > * > -- > 2.30.2 > >
