[PATCH 3/3] firmware: add external firmware PBL support

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Normally, barebox embds firmware into the binary referencing it, which
means that device tree blobs, RAM training code and e.g. TF-A for i.MX8M
end up in the prebootloader, while, e.g. Freescale FMan microcode ends
up in barebox proper. The only exception so far was barebox proper:
When only the PBL fits in on-chip SRAM, barebox proper is chainloaded
from the boot medium. To avoid TOCTOU attack, it's read fully into DRAM
after setup and then a SHA256 is calculated and compared against the
hash embedded in barebox PBL, which in a secure boot system would be
trusted by virtue of the PBL as a whole being verified beforehand by
the BootROM.

Reuse this mechanism to support arbitrary firmware, which is now termed
external firmware. Such firmware is placed beyond the piggydata (barebox
proper) and only offset and hash are included in the prebootloader
image. The new get_builtin_firmware_ext() is used to retrieve this
external firmware after integrity verification with SHA256.

This enables referencing firmware blobs from PBL that would bloat the
size of the PBL beyond what can fit into on-chip SRAM, e.g. very big
OP-TEE binaries. As users of get_builtin_firmware() didn't have to worry
about TOCTOU so far, we panic when a firmware verification fails to
ensure that we never load an OP-TEE that has been modified in-transit
in transit. We can't include the OP-TEE binary in barebox proper,
because we need to install it in EL3, but barebox proper on the i.MX8M
runs as BL33 in a lower exception level.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 arch/arm/lib/pbl.lds.S | 11 +++++++++++
 firmware/Makefile      | 37 ++++++++++++++++++++++++++++++++++++-
 include/firmware.h     | 31 +++++++++++++++++++++++++++++++
 3 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/arch/arm/lib/pbl.lds.S b/arch/arm/lib/pbl.lds.S
index e77b3220fcb0..87d9b297f0dc 100644
--- a/arch/arm/lib/pbl.lds.S
+++ b/arch/arm/lib/pbl.lds.S
@@ -62,6 +62,10 @@ SECTIONS
 	}
 	__shasum_end = .;
 
+	.pblext_shasum : {
+		KEEP(*(.pblext_shasum.*))
+	}
+
 	.rel_dyn_start : { *(.__rel_dyn_start) }
 #ifdef CONFIG_CPU_32
 	.rel.dyn : { *(.rel*) }
@@ -101,6 +105,13 @@ SECTIONS
 	}
 	__piggydata_end = .;
 
+	. = ALIGN(4);
+	__pblext_start = .;
+	.pblext : {
+		*(.pblext.*)
+	}
+	__pblext_end = .;
+
 	.image_end : { KEEP(*(.__image_end)) }
 
 	pbl_image_size =  . - BASE;
diff --git a/firmware/Makefile b/firmware/Makefile
index 87bd033f6e5c..eb8b59a3b1aa 100644
--- a/firmware/Makefile
+++ b/firmware/Makefile
@@ -51,16 +51,51 @@ filechk_fwbin = { \
 $(obj)/%.gen.S: FORCE
 	$(call filechk,fwbin)
 
+FWEXTNAME    = $(patsubst $(obj)/%.extgen.S,%,$@)
+FWEXTSTR     = $(subst /,_,$(subst .,_,$(subst -,_,$(FWEXTNAME))))
+
+filechk_fwextbin = { \
+	echo "/* Generated by $(src)/Makefile */"		;\
+	echo "    .section .pblext.$(FWEXTSTR)",\"a\"		;\
+	echo "    .p2align $(ASM_ALIGN)"			;\
+	echo ".global _fwext_$(FWEXTSTR)_start"			;\
+	echo "_fwext_$(FWEXTSTR)_start:"			;\
+	echo "    .incbin \"$(fwdir)/$(FWEXTNAME)\""		;\
+	echo ".global _fwext_$(FWEXTSTR)_end"			;\
+	echo "_fwext_$(FWEXTSTR)_end:"				;\
+	echo "    .section .pblext_shasum.$(FWEXTSTR), \"a\""	;\
+	echo "    .p2align $(ASM_ALIGN)"			;\
+	echo ".global _fwext_$(FWEXTSTR)_sha_start"		;\
+	echo "_fwext_$(FWEXTSTR)_sha_start:"			;\
+	echo "    .incbin \"$(objdir)/firmware/$(FWEXTNAME).sha.bin\""	;\
+	echo ".global _fwext_$(FWEXTSTR)_sha_end"		;\
+	echo "_fwext_$(FWEXTSTR)_sha_end:"			;\
+}
+
+$(obj)/%.extgen.S: $(obj)/%.sha.bin FORCE
+	$(call filechk,fwextbin)
+
+$(obj)/%.sha.bin: $(obj)/%.sum FORCE
+	$(call if_changed,sha256bin)
+
+$(obj)/%.sum: $(obj)/% FORCE
+	$(call if_changed,sha256sum)
+
+clean-files += *.sha.bin *.sum
+
 # The .o files depend on the binaries directly; the .S files don't.
 $(patsubst %,$(obj)/%.gen.o, $(obj-pbl-y)): $(obj)/%.gen.o: $(fwdir)/%
 
 # The same for pbl:
 $(patsubst %,$(obj)/%.gen.pbl.o, $(obj-pbl-y)): $(obj)/%.gen.pbl.o: $(fwdir)/%
+$(patsubst %,$(obj)/%.extgen.pbl.o, $(pbl-y)): $(obj)/%.extgen.pbl.o: $(fwdir)/%
 
-obj-pbl-y			 += $(patsubst %,%.gen.o, $(fw-external-y))
+pbl-y := $(addsuffix .extgen.o, $(fw-external-y))
 
 targets := $(patsubst $(obj)/%,%, \
                                 $(shell find $(obj) -name \*.gen.S 2>/dev/null))
+targets += $(patsubst $(obj)/%,%, \
+                                $(shell find $(obj) -name \*.extgen.S 2>/dev/null))
 
 # just to build a built-in.o. Otherwise compilation fails when no
 # firmware is built.
diff --git a/include/firmware.h b/include/firmware.h
index 2583342230ae..b96726bd4416 100644
--- a/include/firmware.h
+++ b/include/firmware.h
@@ -6,8 +6,11 @@
 #ifndef FIRMWARE_H
 #define FIRMWARE_H
 
+#include <pbl.h>
 #include <types.h>
 #include <driver.h>
+#include <debug_ll.h>
+#include <linux/kernel.h>
 
 struct firmware_handler {
 	char *id; /* unique identifier for this firmware device */
@@ -57,6 +60,16 @@ static inline void firmware_set_searchpath(const char *path)
 
 void firmwaremgr_list_handlers(void);
 
+static inline void firmware_ext_verify(const void *data_start, size_t data_size,
+				       const void *hash_start, size_t hash_size)
+{
+	if (pbl_barebox_verify(data_start, data_size,
+			       hash_start, hash_size) != 0) {
+		putc_ll('!');
+		panic("hash mismatch, refusing to decompress");
+	}
+}
+
 #define get_builtin_firmware(name, start, size) \
 	{							\
 		extern char _fw_##name##_start[];		\
@@ -65,4 +78,22 @@ void firmwaremgr_list_handlers(void);
 		*size = _fw_##name##_end - _fw_##name##_start;	\
 	}
 
+#define get_builtin_firmware_ext(name, base, start, size)		\
+	{								\
+		extern char _fwext_##name##_start[];			\
+		extern char _fwext_##name##_end[];			\
+		extern char _fwext_##name##_sha_start[];		\
+		extern char _fwext_##name##_sha_end[];			\
+		long offset = (long)bl33 - (long)_text;			\
+		*start = (typeof(*start)) _fwext_##name##_start;	\
+		*size = _fwext_##name##_end - _fwext_##name##_start;	\
+		*start += offset;					\
+		firmware_ext_verify(					\
+			*start, *size,					\
+			_fwext_##name##_sha_start,			\
+			_fwext_##name##_sha_end -			\
+			_fwext_##name##_sha_start			\
+		);							\
+	}
+
 #endif /* FIRMWARE_H */
-- 
2.30.2