[PATCH] mtd: mtdpart: fix uninitialized erasesize on MTDPART_OFS_RETAIN error path

[PATCH] mtd: mtdpart: fix uninitialized erasesize on MTDPART_OFS_RETAIN error path

[Nikolay Ivchenko]

When parsing partition layouts, if a partition requested with
MTDPART_OFS_RETAIN runs out of space, the allocator jumps directly
to 'out_register' to preserve partition numbering.

However, this jump bypasses child->erasesize initialization, leaving
it at zero. When add_mtd_device() is later called on this child, the
registration fails and triggers a WARN_ON() due to the zero ->erasesize.

Fix this by zeroing out child->part.offset and child->part.size, and
initializing child->erasesize to parent->erasesize. This is the exact
same pattern already used just a few lines below in the "out of reach"
error check (child->part.offset >= parent_size) to safely register a
disabled partition.

Reported-by: syzbot+3ae80219c633aca5431c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3ae80219c633aca5431c
Signed-off-by: Nikolay Ivchenko <nivchenko.dev@gmail.com>
---
 drivers/mtd/mtdpart.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/mtd/mtdpart.c b/drivers/mtd/mtdpart.c
index 795a94e6b482..7f23f8a1b59c 100644
--- a/drivers/mtd/mtdpart.c
+++ b/drivers/mtd/mtdpart.c
@@ -118,6 +118,9 @@ static struct mtd_info *allocate_partition(struct mtd_info *parent,
 				part->name, parent_size - child->part.offset,
 				child->part.size);
 			/* register to preserve ordering */
+			child->part.offset = 0;
+			child->part.size = 0;
+			child->erasesize = parent->erasesize;
 			goto out_register;
 		}
 	}
-- 
2.43.0

Re: [PATCH] mtd: mtdpart: fix uninitialized erasesize on MTDPART_OFS_RETAIN error path

[Miquel Raynal]

On Sat, 20 Jun 2026 20:06:03 +0300, Nikolay Ivchenko wrote:
> When parsing partition layouts, if a partition requested with
> MTDPART_OFS_RETAIN runs out of space, the allocator jumps directly
> to 'out_register' to preserve partition numbering.
> 
> However, this jump bypasses child->erasesize initialization, leaving
> it at zero. When add_mtd_device() is later called on this child, the
> registration fails and triggers a WARN_ON() due to the zero ->erasesize.
> 
> [...]

Applied to mtd/fixes, thanks!

[1/1] mtd: mtdpart: fix uninitialized erasesize on MTDPART_OFS_RETAIN error path
      commit: ae9d8058f13238cad5f5d16f676da91187684581

Patche(s) should be available on mtd/linux.git and will be
part of the next PR (provided that no robot complains by then).

Kind regards,
Miquèl