[PATCH] image-fit: Check nesting depth in fdt_check_no_at()

[Anton Ivanov]

From: Binarly Vulnerability Research <vr at binarly.io>

The fdt_check_no_at(), located in boot/image-fit.c, is a helper
that guards against libfdt's unit-address ambiguity; it scans the
FIT tree and rejects any node whose name contains an '@':

	static int fdt_check_no_at(const void *fit, int parent)
	{
		const char *name;
		int node;
		int ret;

		name = fdt_get_name(fit, parent, NULL);
		if (!name || strchr(name, '@'))
			return -EADDRNOTAVAIL;

		fdt_for_each_subnode(node, fit, parent) {
			ret = fdt_check_no_at(fit, node);
			if (ret)
				return ret;
		}

		return 0;
	}

The function is recursive: for every subnode, it calls itself, and
its termination condition is "no more subnodes".

It is called from the fit_check_format() (boot/image-fit.c):

	int fit_check_format(const void *fit, ulong size)
	{
		int ret;

		/* A FIT image must be a valid FDT */
		ret = fdt_check_header(fit);
		if (ret) {
			log_debug("Wrong FIT format: not a flattened device tree (err=%d)\n",
				ret);
			return -ENOEXEC;
		}

		if (CONFIG_IS_ENABLED(FIT_FULL_CHECK)) {
			/*
			* If we are not given the size, make do with calculating it.
			* This is not as secure, so we should consider a flag to
			* control this.
			*/
			if (size == IMAGE_SIZE_INVAL)
				size = fdt_totalsize(fit);
			ret = fdt_check_full(fit, size);
			if (ret)
				ret = -EINVAL;

			/*
			* U-Boot stopped using unit addressed in 2017. Since libfdt
			* can match nodes ignoring any unit address, signature
			* verification can see the wrong node if one is inserted with
			* the same name as a valid node but with a unit address
			* attached. Protect against this by disallowing unit addresses.
			*/
			if (!ret && CONFIG_IS_ENABLED(FIT_SIGNATURE)) {
				ret = fdt_check_no_at(fit, 0);
				...

The only guard executed prior to fdt_check_no_at() that validates
the nesting depth of the FIT tree is in fdt_check_full():

	case FDT_BEGIN_NODE:
		depth++;
		if (depth > INT_MAX)
			return -FDT_ERR_BADSTRUCTURE;

It is only checked that the depth does not exceed INT_MAX, which
means it is theoretically possible to create a FIT image with a
depth of 2147483647 (if the device has enough RAM to support it).
Each call to fdt_check_no_at() consumes 16 bytes of stack space
(one 'const char *' and two int local variables, and a return
address), while introducing a nested entry in the FIT requires 12
bytes (FDT_BEGIN_NODE -- 4 bytes, node name -- at least 4 bytes,
and FDT_END_NODE -- 4 bytes). The stack is therefore guaranteed to
be exhausted on any RAM size available.

Fix: Validate the nesting depth of the FIT tree.

Signed-off-by: Binarly Vulnerability Research <vr at binarly.io>
---
 boot/image-fit.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/boot/image-fit.c b/boot/image-fit.c
index b0fcaf6e17f..0e225ecd0c2 100644
--- a/boot/image-fit.c
+++ b/boot/image-fit.c
@@ -41,6 +41,7 @@ DECLARE_GLOBAL_DATA_PTR;
 #include <bootm.h>
 #include <image.h>
 #include <bootstage.h>
+#include <fdt_region.h>
 #include <upl.h>
 #include <u-boot/crc.h>
 
@@ -1643,20 +1644,24 @@ int fit_image_check_comp(const void *fit, int noffset, uint8_t comp)
  *
  * @fit: FIT to check
  * @parent: Parent node to check
- * Return: 0 if OK, -EADDRNOTAVAIL is a node has a name containing '@'
+ * @depth: Current recursion depth
+ * Return: 0 if OK, or error value
  */
-static int fdt_check_no_at(const void *fit, int parent)
+static int fdt_check_no_at(const void *fit, int parent, int depth)
 {
 	const char *name;
 	int node;
 	int ret;
 
+	if (depth >= FDT_MAX_DEPTH)
+		return -FDT_ERR_BADSTRUCTURE;
+
 	name = fdt_get_name(fit, parent, NULL);
 	if (!name || strchr(name, '@'))
 		return -EADDRNOTAVAIL;
 
 	fdt_for_each_subnode(node, fit, parent) {
-		ret = fdt_check_no_at(fit, node);
+		ret = fdt_check_no_at(fit, node, depth + 1);
 		if (ret)
 			return ret;
 	}
@@ -1696,7 +1701,7 @@ int fit_check_format(const void *fit, ulong size)
 		 * attached. Protect against this by disallowing unit addresses.
 		 */
 		if (!ret && CONFIG_IS_ENABLED(FIT_SIGNATURE)) {
-			ret = fdt_check_no_at(fit, 0);
+			ret = fdt_check_no_at(fit, 0, 0);
 
 			if (ret) {
 				log_debug("FIT check error %d\n", ret);
-- 
2.53.0