ntfs: serialize resident iomap reads with mrec_lock

Summary

In the Linux kernel, the following vulnerability has been resolved: ntfs: serialize resident iomap reads with mrec_lock ntfs_read_iomap_begin_resident() walks the MFT record through ntfs_attr_lookup() -> ntfs_attr_find() without taking ni->mrec_lock, while ntfs_attr_record_resize(), ntfs_make_room_for_attr() and ntfs_resident_attr_record_add() memmove() the same base_ni->mrec buffer under that lock. map_mft_record() only takes a reference and does not serialize, so the reader can observe torn attribute length and offset fields while a writer is relocating the records. KCSAN reports the race between the mmap read fault path and both link() and unlink(): BUG: KCSAN: data-race in ntfs_attr_find / ntfs_attr_record_resize write to 0xffff888100af1018 of 4 bytes by task 96 on cpu 1: ntfs_attr_record_resize+0xd2/0x130 ntfs_attr_record_rm+0xad/0x530 ntfs_delete+0x224/0x640 ntfs_unlink+0x14d/0x280 vfs_unlink+0x157/0x520 read to 0xffff888100af1018 of 4 bytes by task 95 on cpu 0: ntfs_attr_find+0x104/0x5b0 ntfs_attr_lookup+0x39c/0x10c0 ntfs_read_iomap_begin_resident+0xc6/0x230 ntfs_read_iomap_begin+0x5d/0xa0 iomap_iter+0x2e2/0x6e0 iomap_read_folio+0x147/0x2a0 ntfs_read_folio+0x108/0x170 filemap_read_folio+0x35/0x100 filemap_fault+0x993/0x1000 value changed: 0x00000250 -> 0x000001f0 The address is mrec + 0x18, i.e. mft_record.bytes_in_use, and the change is the 96 bytes of one $FILE_NAME attribute being removed. Keep base_ni->mrec_lock from the resident read iomap lookup through iomap_end(). This protects both the attribute walk and the subsequent copy from iomap->inline_data, which points into the MFT record. The non-resident path is left alone: ntfs_lookup() already holds the directory inode's mrec_lock when it reads an index folio through read_mapping_folio(), and taking the lock in the shared wrapper deadlocks there with recursive locking on mrec_lock. The comment above the read_mapping_folio() call in fs/ntfs/dir.c notes the same hazard. The seek path uses the same lookup helper but does not dereference iomap->inline_data. Release the lock before returning from that path, whereas the regular read path records base_ni in iomap->private and releases the lock from its iomap_end() callback. Tested with a reproducer that faults in a 16-byte resident file while another thread runs link()/unlink() on it. Before: 40 KCSAN reports in about one second. After: no reports in 180 seconds over 206,090 read iterations and 423,540 link/unlink cycles. A PROVE_LOCKING build shows no lockdep splat with the same reproducer running for 60 seconds.