[PATCH v2] ext4: fix the logical block counter overflow in indirect migration

[Yichong Chen]

update_tind_extent_range() advances lb->curr_block, an ext4_lblk_t, by
max_entries * max_entries for every empty triple-indirect slot.  One
triple-indirect block spans max_entries^3 logical blocks, which exceeds
2^32 as soon as the block size is 8K or larger (16384^3 = 2^42 with 64K
blocks), so the counter wraps while that block is walked.

A wrapped counter makes the migration store a block number that is 2^32
blocks away from the one the pointer block describes.  Two ranges can then
end up with the same ee_block, which trips
BUG_ON(newext->ee_block == nearex->ee_block) in ext4_ext_insert_extent(),
and without that collision the data is still moved to the wrong logical
block while the migration reports success.

Keep the counter in 64 bit so that it cannot wrap, and refuse the
migration with -EOPNOTSUPP once a data block would land on EXT_MAX_BLOCKS
or beyond: an extent may not cover that block, because
ext4_valid_extent() rejects a wrapping ee_block + ee_len, and only a
corrupt block map has data there.

Fixes: c14c6fd5c56a ("ext4: Add EXT4_IOC_MIGRATE ioctl")
Signed-off-by: Yichong Chen <chenyichong@xxxxxxxxxxxxx>
---

Notes:
    v2: refuse EXT_MAX_BLOCKS (0xffffffff) as well, so the bound is >= instead
        of >.  An extent may not cover that block: ext4_valid_extent() rejects a
        wrapping ee_block + ee_len, so storing it would leave the migrated inode
        corrupt.

 fs/ext4/migrate.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/migrate.c b/fs/ext4/migrate.c
index e06d847033a1..2ce587043945 100644
--- a/fs/ext4/migrate.c
+++ b/fs/ext4/migrate.c
@@ -14,7 +14,7 @@
  * represented by a single extent
  */
 struct migrate_struct {
-	ext4_lblk_t first_block, last_block, curr_block;
+	u64 first_block, last_block, curr_block;
 	ext4_fsblk_t first_pblock, last_pblock;
 };
 
@@ -65,6 +65,10 @@ static int update_extent_range(handle_t *handle, struct inode *inode,
 			       ext4_fsblk_t pblock, struct migrate_struct *lb)
 {
 	int retval;
+
+	if (lb->curr_block >= EXT_MAX_BLOCKS)
+		return -EOPNOTSUPP;
+
 	/*
 	 * See if we can add on to the existing range (if it exists)
 	 */
-- 
2.51.0