libext2fs: always refuse to open a file system with a zero s_desc_sizemaint

Commit 42c11edd0863 ("ext2fs_open[2](), return an error if s_desc_size
is too large") added a check for an insanely large s_desc_size to
prevent some failures triggered by fuzz testing.  However, it would
allow e2fsck to fall back to recover the file system by using the
backup superblocks by having e2fsck pass the flag
EXT2_FLAG_IGNORE_SB_ERRORS.  But by allowing an s_desc_Size of zero,
it's possible that e2fsck will die with a division of zero error.
With this fix, e2fsck will now print an error message and exit
instead.

https://github.com/tytso/e2fsprogs/issues/183

Fixes: 42c11edd0863 ("ext2fs_open[2](), return an error if s_desc_size is too large")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

5 files changed, 22 insertions, 5 deletions

diff --git a/lib/ext2fs/openfs.c b/lib/ext2fs/openfs.c
index fd56a9acd..eb44d5864 100644
--- a/lib/ext2fs/openfs.c
+++ b/lib/ext2fs/openfs.c
@@ -330,13 +330,14 @@ retry:
 	}
 
 	/* Enforce the block group descriptor size */
-	if (!(flags & EXT2_FLAG_IGNORE_SB_ERRORS) &&
-	    ext2fs_has_feature_64bit(fs->super)) {
+	if (ext2fs_has_feature_64bit(fs->super)) {
 		unsigned desc_size = fs->super->s_desc_size;
 
-		if ((desc_size < EXT2_MIN_DESC_SIZE_64BIT) ||
-		    (desc_size > EXT2_MAX_DESC_SIZE) ||
-		    (desc_size & (desc_size - 1)) != 0) {
+		if (desc_size == 0 ||
+		    (!(flags & EXT2_FLAG_IGNORE_SB_ERRORS) &&
+		     ((desc_size > EXT2_MAX_DESC_SIZE) ||
+		      (desc_size < EXT2_MIN_DESC_SIZE_64BIT) ||
+		      (desc_size & (desc_size - 1)) != 0))) {
 			retval = EXT2_ET_BAD_DESC_SIZE;
 			goto cleanup;
 		}
diff --git a/tests/f_desc_size_zero/expect.1 b/tests/f_desc_size_zero/expect.1
new file mode 100644
index 000000000..acb752a75
--- /dev/null
+++ b/tests/f_desc_size_zero/expect.1
@@ -0,0 +1,13 @@
+../e2fsck/e2fsck: Block group descriptor size incorrect while trying to open test.img
+../e2fsck/e2fsck: Trying to load superblock despite errors...
+../e2fsck/e2fsck: Block group descriptor size incorrect while trying to open test.img
+
+The superblock could not be read or does not describe a valid ext2/ext3/ext4
+filesystem.  If the device is valid and it really contains an ext2/ext3/ext4
+filesystem (and not swap or ufs or something else), then the superblock
+is corrupt, and you might try running e2fsck with an alternate superblock:
+    e2fsck -b 8193 <device>
+ or
+    e2fsck -b 32768 <device>
+
+Exit status is 8
diff --git a/tests/f_desc_size_zero/image.gz b/tests/f_desc_size_zero/image.gz
new file mode 100644
index 000000000..4e43c0c61
--- /dev/null
+++ b/tests/f_desc_size_zero/image.gz
diff --git a/tests/f_desc_size_zero/name b/tests/f_desc_size_zero/name
new file mode 100644
index 000000000..e77bb11c2
--- /dev/null
+++ b/tests/f_desc_size_zero/name
@@ -0,0 +1 @@
+zero s_desc_size
diff --git a/tests/f_desc_size_zero/script b/tests/f_desc_size_zero/script
new file mode 100644
index 000000000..8ab2b9c61
--- /dev/null
+++ b/tests/f_desc_size_zero/script
@@ -0,0 +1,2 @@
+ONE_PASS_ONLY="true"
+. $cmd_dir/run_e2fsck