ext4: prevent right-shifting extents beyond EXT_MAX_BLOCKS

During the "insert range" fallocate operation, extents starting at the
range offset are shifted "right" (to a higher file offset) by the range
length.  But, as shown by syzbot, it's not validated that this doesn't
cause extents to be shifted beyond EXT_MAX_BLOCKS.  In that case
->ee_block can wrap around, corrupting the extent tree.

Fix it by returning an error if the space between the end of the last
extent and EXT4_MAX_BLOCKS is smaller than the range being inserted.

This bug can be reproduced by running the following commands when the
current directory is on an ext4 filesystem with a 4k block size:

        fallocate -l 8192 file
        fallocate --keep-size -o 0xfffffffe000 -l 4096 -n file
        fallocate --insert-range -l 8192 file

Then after unmounting the filesystem, e2fsck reports corruption.

Reported-by: syzbot+06c885be0edcdaeab40c@syzkaller.appspotmail.com
Fixes: 331573febb6a ("ext4: Add support FALLOC_FL_INSERT_RANGE for fallocate")
Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/extents.c

@@ -5329,8 +5329,9 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
 	stop = le32_to_cpu(extent->ee_block);
 
        /*
-	 * In case of left shift, Don't start shifting extents until we make
-	 * sure the hole is big enough to accommodate the shift.
+	* For left shifts, make sure the hole on the left is big enough to
+	* accommodate the shift.  For right shifts, make sure the last extent
+	* won't be shifted beyond EXT_MAX_BLOCKS.
 	*/
 	if (SHIFT == SHIFT_LEFT) {
 		path = ext4_find_extent(inode, start - 1, &path,
@@ -5350,9 +5351,14 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
 
 		if ((start == ex_start && shift > ex_start) ||
 		    (shift > start - ex_end)) {
-			ext4_ext_drop_refs(path);
-			kfree(path);
-			return -EINVAL;
+			ret = -EINVAL;
+			goto out;
+		}
+	} else {
+		if (shift > EXT_MAX_BLOCKS -
+		    (stop + ext4_ext_get_actual_len(extent))) {
+			ret = -EINVAL;
+			goto out;
 		}
 	}