efi_loader: Get rid of kaslr-seed if EFI_RNG_PROTOCOL is installed

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 configuration 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 <ardb@kernel.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/lib/efi_loader/efi_dt_fixup.c b/lib/efi_loader/efi_dt_fixup.c
index b6fe5d2..d3923e5 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>
 
@@ -41,6 +42,38 @@ static void efi_reserve_memory(u64 addr, u64 size, bool nomap)
 }
 
 /**
+ * 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
  *
  * The mem_rsv entries of the FDT are added to the memory map. Any failures are