[PATCH 1/2] libfdt: prevent integer overflow in fdt_next_tag

[PATCH 1/2] libfdt: prevent integer overflow in fdt_next_tag

[Tadeusz Struk]

Since fdt_next_tag() in a public API function all input parameters,
including the fdt blob should not be trusted. It is possible to forge
a blob with invalid property length that will cause integer overflow
during offset calculation. To prevent that validate the property length
read from the blob before doing calculations.

Signed-off-by: Tadeusz Struk <tadeusz.struk-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
---
 libfdt/fdt.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/libfdt/fdt.c b/libfdt/fdt.c
index 90a39e8..c3e112a 100644
--- a/libfdt/fdt.c
+++ b/libfdt/fdt.c
@@ -186,11 +186,17 @@ uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
 
 	case FDT_PROP:
 		lenp = fdt_offset_ptr(fdt, offset, sizeof(*lenp));
-		if (!can_assume(VALID_DTB) && !lenp)
+		if (!can_assume(VALID_DTB) &&
+		    (!lenp || (INT_MAX <= fdt32_to_cpu(*lenp))))
 			return FDT_END; /* premature end */
+
 		/* skip-name offset, length and value */
 		offset += sizeof(struct fdt_property) - FDT_TAGSIZE
 			+ fdt32_to_cpu(*lenp);
+
+		if (offset < 0)
+			return FDT_END; /* premature end */
+
 		if (!can_assume(LATEST) &&
 		    fdt_version(fdt) < 0x10 && fdt32_to_cpu(*lenp) >= 8 &&
 		    ((offset - fdt32_to_cpu(*lenp)) % 8) != 0)
-- 
2.37.3

[PATCH 2/2] libfdt: tests: add get_next_tag_invalid_prop_len

[Tadeusz Struk]

Add a new test get_next_tag_invalid_prop_len, which covers
fdt_next_tag() when it is passed a corrupted blob, with
invalid property len values.

Signed-off-by: Tadeusz Struk <tadeusz.struk-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
---
 tests/.gitignore                      |  1 +
 tests/Makefile.tests                  |  2 +-
 tests/get_next_tag_invalid_prop_len.c | 59 +++++++++++++++++++++++++++
 tests/meson.build                     |  1 +
 tests/run_tests.sh                    |  1 +
 5 files changed, 63 insertions(+), 1 deletion(-)
 create mode 100644 tests/get_next_tag_invalid_prop_len.c

diff --git a/tests/.gitignore b/tests/.gitignore
index 03bdde2..3376ed9 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -74,3 +74,4 @@ tmp.*
 /truncated_memrsv
 /utilfdt_test
 /value-labels
+/get_next_tag_invalid_prop_len
diff --git a/tests/Makefile.tests b/tests/Makefile.tests
index 2d36c5d..2c5b4c9 100644
--- a/tests/Makefile.tests
+++ b/tests/Makefile.tests
@@ -4,7 +4,7 @@ LIB_TESTS_L = get_mem_rsv \
 	get_path supernode_atdepth_offset parent_offset \
 	node_offset_by_prop_value node_offset_by_phandle \
 	node_check_compatible node_offset_by_compatible \
-	get_alias \
+	get_alias get_next_tag_invalid_prop_len \
 	char_literal \
 	sized_cells \
 	notfound \
diff --git a/tests/get_next_tag_invalid_prop_len.c b/tests/get_next_tag_invalid_prop_len.c
new file mode 100644
index 0000000..23de8c9
--- /dev/null
+++ b/tests/get_next_tag_invalid_prop_len.c
@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+/*
+ * libfdt - Flat Device Tree manipulation
+ *	Testcase for fdt_next_tag()
+ */
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdint.h>
+
+#include <libfdt.h>
+#include "tests.h"
+#include "testdata.h"
+
+int main(int argc, char *argv[])
+{
+	struct fdt_header *hdr;
+	struct fdt_property *prp;
+	void *fdt;
+	int size, nextoff;
+	uint32_t tag;
+
+	test_init(argc, argv);
+	size = sizeof(*hdr) + sizeof(*prp) + 256;
+	fdt = calloc(1, size);
+	if (!fdt)
+		FAIL("Can't allocate memory");
+
+	hdr = fdt;
+	prp = (struct fdt_property *)(((char *) fdt) + sizeof(*hdr));
+	fdt_set_magic(fdt, FDT_MAGIC);
+	fdt_set_totalsize(fdt, size);
+	fdt_set_version(fdt, 0x10);
+	prp->tag = cpu_to_fdt32(FDT_PROP);
+	prp->len = cpu_to_fdt32(256);
+	prp->nameoff = 0;
+
+	tag = fdt_next_tag(fdt, sizeof(*hdr), &nextoff);
+	if (tag != FDT_PROP)
+		FAIL("Invalid tag %X", tag);
+
+	if (nextoff != size)
+		FAIL("Invalid next_offset");
+
+	/* int overflow case */
+	prp->len = cpu_to_fdt32(0xFFFFFFFA);
+	tag = fdt_next_tag(fdt, sizeof(*hdr), &nextoff);
+	if (tag != FDT_END)
+		FAIL("Invalid tag, expected premature end");
+
+	/* negative offset case */
+	prp->len = cpu_to_fdt32(0x7FFFFFFA);
+	tag = fdt_next_tag(fdt, sizeof(*hdr), &nextoff);
+	if (tag != FDT_END)
+		FAIL("Invalid tag, expected premature end");
+
+	free(fdt);
+	PASS();
+}
diff --git a/tests/meson.build b/tests/meson.build
index 4ac154a..29a42dd 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -47,6 +47,7 @@ tests = [
   'get_path',
   'get_phandle',
   'get_prop_offset',
+  'get_next_tag_invalid_prop_len',
   'getprop',
   'incbin',
   'integer-expressions',
diff --git a/tests/run_tests.sh b/tests/run_tests.sh
index 244df8a..397b9cf 100755
--- a/tests/run_tests.sh
+++ b/tests/run_tests.sh
@@ -346,6 +346,7 @@ tree1_tests () {
     run_test get_prop_offset $TREE
     run_test get_phandle $TREE
     run_test get_path $TREE
+    run_test get_next_tag_invalid_prop_len $TREE #TREE not really needed
     run_test supernode_atdepth_offset $TREE
     run_test parent_offset $TREE
     run_test node_offset_by_prop_value $TREE
-- 
2.37.3

Re: [PATCH 1/2] libfdt: prevent integer overflow in fdt_next_tag

[Rob Herring]

On Thu, Sep 29, 2022 at 6:55 PM Tadeusz Struk <tadeusz.struk-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
>
> Since fdt_next_tag() in a public API function all input parameters,
> including the fdt blob should not be trusted. It is possible to forge
> a blob with invalid property length that will cause integer overflow
> during offset calculation. To prevent that validate the property length

comma: ...that, validate...

> read from the blob before doing calculations.
>
> Signed-off-by: Tadeusz Struk <tadeusz.struk-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> ---
>  libfdt/fdt.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/libfdt/fdt.c b/libfdt/fdt.c
> index 90a39e8..c3e112a 100644
> --- a/libfdt/fdt.c
> +++ b/libfdt/fdt.c
> @@ -186,11 +186,17 @@ uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)
>
>         case FDT_PROP:
>                 lenp = fdt_offset_ptr(fdt, offset, sizeof(*lenp));
> -               if (!can_assume(VALID_DTB) && !lenp)
> +               if (!can_assume(VALID_DTB) &&
> +                   (!lenp || (INT_MAX <= fdt32_to_cpu(*lenp))))

We now have fdt32_to_cpu(*lenp) 4 times. Stick it in a local var.

>                         return FDT_END; /* premature end */
> +
>                 /* skip-name offset, length and value */
>                 offset += sizeof(struct fdt_property) - FDT_TAGSIZE
>                         + fdt32_to_cpu(*lenp);
> +
> +               if (offset < 0)

Needs a can_assume(VALID_DTB) check.

> +                       return FDT_END; /* premature end */
> +
>                 if (!can_assume(LATEST) &&
>                     fdt_version(fdt) < 0x10 && fdt32_to_cpu(*lenp) >= 8 &&
>                     ((offset - fdt32_to_cpu(*lenp)) % 8) != 0)
> --
> 2.37.3
>