[PATCH] Catch unsigned 32bit overflow when parsing flattened device tree offsets

[Anton Blanchard <anton-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>]

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We have a couple of checks of the form:

    if (offset+size > totalsize)
        die();

We need to check that offset+size doesn't overflow, otherwise the check
will pass, and we may access past totalsize.

Found with AFL.

Signed-off-by: Anton Blanchard <anton-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>
---

I've attached an example device tree, do we want to add binary blobs
to the test suite?

diff --git a/flattree.c b/flattree.c
index bd99fa2..ec14954 100644
--- a/flattree.c
+++ b/flattree.c
@@ -889,7 +889,7 @@ struct boot_info *dt_from_blob(const char *fname)
 
 	if (version >= 3) {
 		uint32_t size_str = fdt32_to_cpu(fdt->size_dt_strings);
-		if (off_str+size_str > totalsize)
+		if ((off_str+size_str < off_str) || (off_str+size_str > totalsize))
 			die("String table extends past total size\n");
 		inbuf_init(&strbuf, blob + off_str, blob + off_str + size_str);
 	} else {
@@ -898,7 +898,7 @@ struct boot_info *dt_from_blob(const char *fname)
 
 	if (version >= 17) {
 		size_dt = fdt32_to_cpu(fdt->size_dt_struct);
-		if (off_dt+size_dt > totalsize)
+		if ((off_dt+size_dt < off_dt) || (off_dt+size_dt > totalsize))
 			die("Structure block extends past total size\n");
 	}
 

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