libext2fs: avoid buffer overflow if s_first_meta_bg is too big

If s_first_meta_bg is greater than the of number block group
descriptor blocks, then reading or writing the block group descriptors
will end up overruning the memory buffer allocated for the
descriptors.  Fix this by limiting first_meta_bg to no more than
fs->desc_blocks.  This doesn't correct the bad s_first_meta_bg value,
but it avoids causing the e2fsprogs userspace programs from
potentially crashing.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/lib/ext2fs/closefs.c b/lib/ext2fs/closefs.c
index 4599eef..1f99113 100644 (file)
--- a/lib/ext2fs/closefs.c
+++ b/lib/ext2fs/closefs.c
@@ -344,9 +344,11 @@ errcode_t ext2fs_flush2(ext2_filsys fs, int flags)
         * superblocks and group descriptors.
         */
        group_ptr = (char *) group_shadow;
-       if (fs->super->s_feature_incompat & EXT2_FEATURE_INCOMPAT_META_BG)
+       if (fs->super->s_feature_incompat & EXT2_FEATURE_INCOMPAT_META_BG) {
                old_desc_blocks = fs->super->s_first_meta_bg;
-       else
+               if (old_desc_blocks > fs->super->s_first_meta_bg)
+                       old_desc_blocks = fs->desc_blocks;
+       } else
                old_desc_blocks = fs->desc_blocks;
 
        ext2fs_numeric_progress_init(fs, &progress, NULL,

diff --git a/lib/ext2fs/openfs.c b/lib/ext2fs/openfs.c
index a1a3517..ba501e6 100644 (file)
--- a/lib/ext2fs/openfs.c
+++ b/lib/ext2fs/openfs.c
@@ -378,9 +378,11 @@ errcode_t ext2fs_open2(const char *name, const char *io_options,
 #ifdef WORDS_BIGENDIAN
        groups_per_block = EXT2_DESC_PER_BLOCK(fs->super);
 #endif
-       if (fs->super->s_feature_incompat & EXT2_FEATURE_INCOMPAT_META_BG)
+       if (fs->super->s_feature_incompat & EXT2_FEATURE_INCOMPAT_META_BG) {
                first_meta_bg = fs->super->s_first_meta_bg;
-       else
+               if (first_meta_bg > fs->desc_blocks)
+                       first_meta_bg = fs->desc_blocks;
+       } else
                first_meta_bg = fs->desc_blocks;
        if (first_meta_bg) {
                retval = io_channel_read_blk(fs->io, group_block +