md/bitmap: fix GPF in write_page caused by resize race

A General Protection Fault occurs in write_page() during array resize:
RIP: 0010:write_page+0x22b/0x3c0 [md_mod]

This is a use-after-free race between bitmap_daemon_work() and
__bitmap_resize(). The daemon iterates over `bitmap->storage.filemap`
without locking, while the resize path frees that storage via
md_bitmap_file_unmap(). `quiesce()` does not stop the md thread,
allowing concurrent access to freed pages.

Fix by holding `mddev->bitmap_info.mutex` during the bitmap update.

Link: https://lore.kernel.org/linux-raid/[email protected]
Closes: https://lore.kernel.org/linux-raid/CAMGffE=Mbfp=7xD_hYxXk1PAaCZNSEAVeQGKGy7YF9f2S4=NEA@mail.gmail.com/T/#u
Cc: [email protected]
Fixes: d60b479 ("md/bitmap: add bitmap_resize function to allow bitmap resizing.")
Signed-off-by: Jack Wang <[email protected]>
Signed-off-by: Yu Kuai <[email protected]>

Author: jinpuwang

drivers/md/md-bitmap.c

@@ -2453,6 +2453,7 @@ static int __bitmap_resize(struct bitmap *bitmap, sector_t blocks,
 		memcpy(page_address(store.sb_page),
 		       page_address(bitmap->storage.sb_page),
 		       sizeof(bitmap_super_t));
+	mutex_lock(&bitmap->mddev->bitmap_info.mutex);
 	spin_lock_irq(&bitmap->counts.lock);
 	md_bitmap_file_unmap(&bitmap->storage);
 	bitmap->storage = store;
@@ -2560,7 +2561,7 @@ static int __bitmap_resize(struct bitmap *bitmap, sector_t blocks,
 			set_page_attr(bitmap, i, BITMAP_PAGE_DIRTY);
 	}
 	spin_unlock_irq(&bitmap->counts.lock);
-
+	mutex_unlock(&bitmap->mddev->bitmap_info.mutex);
 	if (!init) {
 		__bitmap_unplug(bitmap);
 		bitmap->mddev->pers->quiesce(bitmap->mddev, 0);