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_open2() 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>

diff --git a/lib/ext2fs/openfs.c b/lib/ext2fs/openfs.c
index ec2d6cb..3331452 100644 (file)
--- a/lib/ext2fs/openfs.c
+++ b/lib/ext2fs/openfs.c
@@ -435,7 +435,8 @@ errcode_t ext2fs_open2(const char *name, const char *io_options,
                gdp = (struct ext2_group_desc *) dest;
                for (j=0; j < groups_per_block*first_meta_bg; 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
                dest += fs->blocksize*first_meta_bg;
@@ -455,7 +456,8 @@ errcode_t ext2fs_open2(const char *name, const char *io_options,
                for (j=0; j < groups_per_block; j++) {
                        gdp = ext2fs_group_desc(fs, fs->group_desc,
                                                i * groups_per_block + j);
-                       ext2fs_swap_group_desc2(fs, gdp);
+                       if (gdp)
+                               ext2fs_swap_group_desc2(fs, gdp);
                }
 #endif
                dest += fs->blocksize;