libfdt: Make fdt_check_header() more thorough

Currently fdt_check_header() performs only some rudimentary checks, which
is not really what the name suggests.  This strengthens fdt_check_header()
to check as much about the blob as is possible from the header alone:  as
well as checking the magic number and version, it checks that the total
size is sane, and that all the sub-blocks within the blob lie within the
total size.

 * This broadens the meaning of FDT_ERR_TRUNCATED to cover all sorts of
   improperly terminated blocks as well as just a structure block without
   FDT_END.

 * This makes fdt_check_header() only succeed on "complete" blobs, not
   in-progress sequential write blobs.  The only reason this didn't fail
   before was that this function used to be called by many RO functions
   which are supposed to also work on incomplete SW blobs.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Simon Glass <sjg@chromium.org>

libfdt/fdt.c

@@ -79,9 +79,64 @@ int fdt_ro_probe_(const void *fdt)
 	return 0;
 }
 
+static int check_off_(uint32_t hdrsize, uint32_t totalsize, uint32_t off)
+{
+	return (off >= hdrsize) && (off <= totalsize);
+}
+
+static int check_block_(uint32_t hdrsize, uint32_t totalsize,
+			uint32_t base, uint32_t size)
+{
+	if (!check_off_(hdrsize, totalsize, base))
+		return 0; /* block start out of bounds */
+	if ((base + size) < base)
+		return 0; /* overflow */
+	if (!check_off_(hdrsize, totalsize, base + size))
+		return 0; /* block end out of bounds */
+	return 1;
+}
+
 int fdt_check_header(const void *fdt)
 {
-	return fdt_ro_probe_(fdt);
+	size_t hdrsize = FDT_V16_SIZE;
+
+	if (fdt_magic(fdt) != FDT_MAGIC)
+		return -FDT_ERR_BADMAGIC;
+	if ((fdt_version(fdt) < FDT_FIRST_SUPPORTED_VERSION)
+	    || (fdt_last_comp_version(fdt) > FDT_LAST_SUPPORTED_VERSION))
+		return -FDT_ERR_BADVERSION;
+	if (fdt_version(fdt) < fdt_last_comp_version(fdt))
+		return -FDT_ERR_BADVERSION;
+
+	if (fdt_version(fdt) >= 17)
+		hdrsize = FDT_V17_SIZE;
+
+	if ((fdt_totalsize(fdt) < hdrsize)
+	    || (fdt_totalsize(fdt) > INT_MAX))
+		return -FDT_ERR_TRUNCATED;
+
+	/* Bounds check memrsv block */
+	if (!check_off_(hdrsize, fdt_totalsize(fdt), fdt_off_mem_rsvmap(fdt)))
+		return -FDT_ERR_TRUNCATED;
+
+	/* Bounds check structure block */
+	if (fdt_version(fdt) < 17) {
+		if (!check_off_(hdrsize, fdt_totalsize(fdt),
+				fdt_off_dt_struct(fdt)))
+			return -FDT_ERR_TRUNCATED;
+	} else {
+		if (!check_block_(hdrsize, fdt_totalsize(fdt),
+				  fdt_off_dt_struct(fdt),
+				  fdt_size_dt_struct(fdt)))
+			return -FDT_ERR_TRUNCATED;
+	}
+
+	/* Bounds check strings block */
+	if (!check_block_(hdrsize, fdt_totalsize(fdt),
+			  fdt_off_dt_strings(fdt), fdt_size_dt_strings(fdt)))
+		return -FDT_ERR_TRUNCATED;
+
+	return 0;
 }
 
 const void *fdt_offset_ptr(const void *fdt, int offset, unsigned int len)

libfdt/libfdt.h

@@ -90,8 +90,9 @@
 
 /* Error codes: codes for bad device tree blobs */
 #define FDT_ERR_TRUNCATED	8
-	/* FDT_ERR_TRUNCATED: Structure block of the given device tree
-	 * ends without an FDT_END tag. */
+	/* FDT_ERR_TRUNCATED: FDT or a sub-block is improperly
+	 * terminated (overflows, goes outside allowed bounds, or
+	 * isn't properly terminated).  */
 #define FDT_ERR_BADMAGIC	9
 	/* FDT_ERR_BADMAGIC: Given "device tree" appears not to be a
 	 * device tree at all - it is missing the flattened device
@@ -244,18 +245,20 @@ fdt_set_hdr_(size_dt_struct);
 #undef fdt_set_hdr_
 
 /**
- * fdt_check_header - sanity check a device tree or possible device tree
+ * fdt_check_header - sanity check a device tree header
  * @fdt: pointer to data which might be a flattened device tree
  *
  * fdt_check_header() checks that the given buffer contains what
- * appears to be a flattened device tree with sane information in its
- * header.
+ * appears to be a flattened device tree, and that the header contains
+ * valid information (to the extent that can be determined from the
+ * header alone).
  *
  * returns:
  *     0, if the buffer appears to contain a valid device tree
  *     -FDT_ERR_BADMAGIC,
  *     -FDT_ERR_BADVERSION,
- *     -FDT_ERR_BADSTATE, standard meanings, as above
+ *     -FDT_ERR_BADSTATE,
+ *     -FDT_ERR_TRUNCATED, standard meanings, as above
  */
 int fdt_check_header(const void *fdt);
 

libfdt/libfdt_env.h

@@ -56,6 +56,7 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
+#include <limits.h>
 
 #ifdef __CHECKER__
 #define FDT_FORCE __attribute__((force))

tests/.gitignore

@@ -8,6 +8,7 @@ tmp.*
 /asm_tree_dump
 /boot-cpuid
 /char_literal
+/check_header
 /check_path
 /del_node
 /del_property

tests/Makefile.tests

@@ -26,7 +26,7 @@ LIB_TESTS_L = get_mem_rsv \
 	property_iterate \
 	subnode_iterate \
 	overlay overlay_bad_fixup \
-	check_path
+	check_path check_header
 LIB_TESTS = $(LIB_TESTS_L:%=$(TESTS_PREFIX)%)
 
 LIBTREE_TESTS_L = truncated_property

tests/check_header.c

@@ -0,0 +1,128 @@
+/*
+ * libfdt - Flat Device Tree manipulation
+ *	Testcase for fdt_check_header
+ * Copyright (C) 2018 David Gibson
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public License
+ * as published by the Free Software Foundation; either version 2.1 of
+ * the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <stdio.h>
+
+#include <libfdt.h>
+
+#include "tests.h"
+
+static void *dtdup(void *dt)
+{
+	size_t bufsize = fdt_totalsize(dt);
+	void *buf = xmalloc(bufsize);
+	fdt_move(dt, buf, bufsize);
+	return buf;
+}
+
+#define CHECK_MANGLE(exerr, code)					\
+	do {								\
+		void *fdt = dtdup(template);				\
+		{ code }						\
+		err = fdt_check_header(fdt);				\
+		verbose_printf("\"%s\" => %s\n", #code, fdt_strerror(err)); \
+		if (err != (exerr))					\
+			FAIL("fdt_check_header() didn't catch mangle %s", \
+			     #code);					\
+		free(fdt);						\
+	} while (0)
+
+int main(int argc, char *argv[])
+{
+	void *template;
+	int err;
+
+	test_init(argc, argv);
+	template = load_blob(argv[1]);
+
+	/* Check that the base dt is valid before mangling it */
+	err = fdt_check_header(template);
+	if (err != 0)
+		FAIL("Base tree fails: %s", fdt_strerror(err));
+
+	/* Check a no-op mangle doesn't break things */
+	CHECK_MANGLE(0, ; );
+
+	/* Mess up the magic number */
+	CHECK_MANGLE(-FDT_ERR_BADMAGIC,
+		fdt_set_magic(fdt, fdt_magic(fdt) ^ 0x1);
+	);
+	CHECK_MANGLE(-FDT_ERR_BADMAGIC,
+		fdt_set_magic(fdt, fdt_magic(fdt) ^ 0x80000000);
+	);
+
+	/* Mess up the version */
+	CHECK_MANGLE(-FDT_ERR_BADVERSION,
+		fdt_set_version(fdt, FDT_FIRST_SUPPORTED_VERSION - 1);
+		fdt_set_last_comp_version(fdt, FDT_FIRST_SUPPORTED_VERSION - 1);
+	);
+	CHECK_MANGLE(-FDT_ERR_BADVERSION,
+		fdt_set_version(fdt, FDT_LAST_SUPPORTED_VERSION + 1);
+		fdt_set_last_comp_version(fdt, FDT_LAST_SUPPORTED_VERSION + 1);
+	);
+	CHECK_MANGLE(-FDT_ERR_BADVERSION,
+		fdt_set_version(fdt, FDT_FIRST_SUPPORTED_VERSION);
+		fdt_set_last_comp_version(fdt, FDT_LAST_SUPPORTED_VERSION);
+	);
+
+	/* Out of bounds sizes */
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_totalsize(fdt, FDT_V1_SIZE - 1);
+	);
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		     fdt_set_totalsize(fdt, (uint32_t)INT_MAX + 1);
+	);
+
+	/* Truncate within various blocks */
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_totalsize(fdt, fdt_off_dt_struct(fdt) - 1);
+	);
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_totalsize(fdt, fdt_off_dt_strings(fdt) - 1);
+	);
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_totalsize(fdt, fdt_off_mem_rsvmap(fdt) - 1);
+	);
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_totalsize(fdt, fdt_off_dt_struct(fdt) + 1);
+	);
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_totalsize(fdt, fdt_off_dt_strings(fdt) + 1);
+	);
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_totalsize(fdt, fdt_off_mem_rsvmap(fdt) + 1);
+	);
+
+	/* Negative block sizes */
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_size_dt_struct(fdt, (uint32_t)-1);
+	);
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_size_dt_strings(fdt, (uint32_t)-1);
+	);
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		     fdt_set_size_dt_struct(fdt, (uint32_t)INT_MIN);
+	);
+	CHECK_MANGLE(-FDT_ERR_TRUNCATED,
+		fdt_set_size_dt_strings(fdt, (uint32_t)INT_MIN);
+	);
+
+	PASS();
+}

tests/run_tests.sh

@@ -441,6 +441,8 @@ libfdt_tests () {
     run_wrap_error_test $DTC nul-in-line-info2.dts
 
     run_wrap_error_test $DTC -I dtb -O dts -o /dev/null ovf_size_strings.dtb
+
+    run_test check_header test_tree1.dtb
 }
 
 dtc_tests () {