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

Commit 42c11ed ("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. #183 Fixes: 42c11ed ("ext2fs_open[2](), return an error if s_desc_size is too large") Signed-off-by: Theodore Ts'o <[email protected]>
tytso · Apr 4, 2024 · ef4e082 · ef4e082
1 parent 72e3062
commit ef4e082
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Showing 5 changed files with 22 additions and 5 deletions.
diff --git a/lib/ext2fs/openfs.c b/lib/ext2fs/openfs.c
@@ -330,13 +330,14 @@ errcode_t ext2fs_open2(const char *name, const char *io_options,
 	}
 
 	/* 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
@@ -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
diff --git a/tests/f_desc_size_zero/name 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
@@ -0,0 +1,2 @@
+ONE_PASS_ONLY="true"
+. $cmd_dir/run_e2fsck