[coreboot-gerrit] New patch to review for coreboot: util/cbfstool: Improve heuristic for cbfs header pointer protection

[Patrick Georgi (pgeorgi@google.com)]

Patrick Georgi (pgeorgi at google.com) just uploaded a new patch set to gerrit, which you can find at https://review.coreboot.org/13672

-gerrit

commit a54672ba73da6d834dbe3ee88de0353b39d89cb2
Author: Patrick Georgi <pgeorgi at chromium.org>
Date:   Wed Feb 10 18:07:52 2016 +0100

    util/cbfstool: Improve heuristic for cbfs header pointer protection
    
    cbfstool has a routine to deal with old images that may encourage it to
    overwrite the master header. That routine is triggered for
    "cbfstool add-master-header" prepared images even though these are not
    at risk, and - worse - destroys the chain structure (through a negative
    file length), so avoid touching such images.
    
    Change-Id: I9d0bbe3e6300b9b9f3e50347737d1850f83ddad8
    Signed-off-by: Patrick Georgi <pgeorgi at chromium.org>
---
 util/cbfstool/cbfs_image.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/util/cbfstool/cbfs_image.c b/util/cbfstool/cbfs_image.c
index 314ea57..65d7f7c 100644
--- a/util/cbfstool/cbfs_image.c
+++ b/util/cbfstool/cbfs_image.c
@@ -110,6 +110,9 @@ static int cbfs_fix_legacy_size(struct cbfs_image *image, char *hdr_loc)
 	// A bug in old cbfstool may produce extra few bytes (by alignment) and
 	// cause cbfstool to overwrite things after free space -- which is
 	// usually CBFS header on x86. We need to workaround that.
+	// Except when the last file is smaller than the header alignment,
+	// since that's a pretty good clue that the last file exists to guard
+	// the header pointer (and the master header is in a file, too).
 
 	struct cbfs_file *entry, *first = NULL, *last = NULL;
 	for (first = entry = cbfs_find_first_entry(image);
@@ -118,7 +121,8 @@ static int cbfs_fix_legacy_size(struct cbfs_image *image, char *hdr_loc)
 		last = entry;
 	}
 	if ((char *)first < (char *)hdr_loc &&
-	    (char *)entry > (char *)hdr_loc) {
+	    (char *)entry > (char *)hdr_loc &&
+	    (ntohl(last->len) > image->header.align)) {
 		WARN("CBFS image was created with old cbfstool with size bug. "
 		     "Fixing size in last entry...\n");
 		last->len = htonl(ntohl(last->len) - image->header.align);