[PATCH] fs/ntfs3: take run_lock when freeing offs_folio in ni_decompress_file

attr_wof_frame_info() allocates, caches and dereferences
ni->file.offs_folio under down_write(&ni->file.run_lock).
ni_decompress_file() frees the same folio with folio_put() without
taking run_lock. When a WOF externally-compressed file is opened for
write, ni_decompress_file() can drop the last reference while a
concurrent O_RDONLY reader is dereferencing the folio in
attr_wof_frame_info(), leading to a use-after-free:

 BUG: KASAN: use-after-free in attr_wof_frame_info (fs/ntfs3/attrib.c:1619)
 Read of size 4 at addr ffff8880120c9000 by task exploit
 Call Trace:
  attr_wof_frame_info (fs/ntfs3/attrib.c:1619)
  ni_read_frame (fs/ntfs3/frecord.c:2421)
  ni_read_folio_cmpr (fs/ntfs3/frecord.c:1916)
  ntfs_read_folio (fs/ntfs3/inode.c:648)
  read_pages (mm/readahead.c:181)
  page_cache_ra_unbounded (mm/readahead.c:292)
  force_page_cache_ra (mm/readahead.c:364)
  page_cache_sync_ra (mm/readahead.c:573)
  filemap_get_pages (mm/filemap.c:2688)
  filemap_read (mm/filemap.c:2806)
  generic_file_read_iter (mm/filemap.c:2994)
  ntfs_file_read_iter (fs/ntfs3/file.c:842)
  vfs_read (fs/read_write.c:574)
  __x64_sys_pread64 (fs/read_write.c:773)

Take run_lock around the folio_put(), as is done for every other
access to ni->file.offs_folio.

Fixes: 4342306f0f0d ("fs/ntfs3: Add file operations and implementation")
Reported-by: Xiang Mei <[email protected]>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <[email protected]>
---
 fs/ntfs3/frecord.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ntfs3/frecord.c b/fs/ntfs3/frecord.c
index 2b49bc077558..b11961a1f8d8 100644
--- a/fs/ntfs3/frecord.c
+++ b/fs/ntfs3/frecord.c
@@ -2148,10 +2148,13 @@ int ni_decompress_file(struct ntfs_inode *ni)
 
 	/* Clear cached flag. */
 	ni->ni_flags &= ~NI_FLAG_COMPRESSED_MASK;
+	/* offs_folio is accessed under run_lock in attr_wof_frame_info(). */
+	down_write(&ni->file.run_lock);
 	if (ni->file.offs_folio) {
 		folio_put(ni->file.offs_folio);
 		ni->file.offs_folio = NULL;
 	}
+	up_write(&ni->file.run_lock);
 	mapping->a_ops = &ntfs_aops;
 
 out:
-- 
2.43.0