From 20a8dbefbc0510430aa7744692221b843b657f62 Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso@mit.edu>
Date: Tue, 14 Jan 2020 10:58:10 -0500
Subject: [PATCH 02/46] libext2fs: fix crash in ext2fs_image_super_write() on
 Big Endian systems
Content-Type: text/plain

This is a similar fix as c9a8c53b17cc ("libext2fs: fix crash in
ext2fs_open2() on Big Endian systems").

Commit e6069a05: ("Teach ext2fs_open2() to honor the
EXT2_FLAG_SUPER_ONLY flag") changed how the function
ext2fs_group_desc() handled a request for a gdp pointer for a group
larger than the number of groups in the file system; it now returns
NULL, instead of returning a pointer beyond the end of the array.

Previously, the ext2fs_imager_super_write() function would swap all of
the block group descriptors in a block, even if they are beyond the
end of the file system.  This was OK, since we were not overrunning
the allocated memory, since it was rounded to a block boundary.  But
now that ext2fs_group_desc() would return NULL for those gdp, it would
cause ext2fs_open2(), when it was byte swapping the block group
descriptors on Big Endian systems, to dereference a null pointer and
crash.

This commit adds a NULL pointer check to avoid byte swapping those
block group descriptors in a bg descriptor block, but which are beyond
the end of the file system, to address this crash.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Anatoly Pugachev <matorola@gmail.com>
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
---
 lib/ext2fs/imager.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/ext2fs/imager.c b/lib/ext2fs/imager.c
index b3ede9a8..f8d67d86 100644
--- a/lib/ext2fs/imager.c
+++ b/lib/ext2fs/imager.c
@@ -249,10 +249,10 @@ errcode_t ext2fs_image_super_write(ext2_filsys fs, int fd,
 	 * if needed
 	 */
 	groups_per_block = EXT2_DESC_PER_BLOCK(fs->super);
-	gdp = (struct ext2_group_desc *) cp;
 	for (j=0; j < groups_per_block*fs->desc_blocks; j++) {
 		gdp = ext2fs_group_desc(fs, fs->group_desc, j);
-		ext2fs_swap_group_desc2(fs, gdp);
+		if (gdp)
+			ext2fs_swap_group_desc2(fs, gdp);
 	}
 #endif
 
@@ -261,10 +261,10 @@ errcode_t ext2fs_image_super_write(ext2_filsys fs, int fd,
 
 #ifdef WORDS_BIGENDIAN
 	groups_per_block = EXT2_DESC_PER_BLOCK(fs->super);
-	gdp = (struct ext2_group_desc *) cp;
 	for (j=0; j < groups_per_block*fs->desc_blocks; j++) {
 		gdp = ext2fs_group_desc(fs, fs->group_desc, j);
-		ext2fs_swap_group_desc2(fs, gdp);
+		if (gdp)
+			ext2fs_swap_group_desc2(fs, gdp);
 	}
 #endif
 
-- 
2.35.1