Linux kernel SMB client: concurrent writes during O_TRUNC silently lose data

In the Linux kernel, the following vulnerability has been resolved:

smb: client: fix data corruption with concurrent writes and O_TRUNC

cifs_do_truncate() flushes dirty pages with filemap_write_and_wait() and truncates the file on the server, but in the old code both operations ran without holding i_rwsem or invalidate_lock. A concurrent buffered write via netfs_perform_write() -- which only needs i_rwsem shared -- could dirty new pages after the flush but before the local truncation, and those pages would be silently discarded by cifs_setsize() -> truncate_pagecache().

Fix by acquiring inode_lock (exclusive i_rwsem) and filemap_invalidate_lock at the top of cifs_do_truncate(), so the entire flush-truncate-resize sequence is atomic with respect to:

• buffered writes (blocked by exclusive i_rwsem, since netfs_start_io_write takes i_rwsem shared),
• read page faults (blocked by exclusive invalidate_lock, since filemap_fault takes it shared),
• writeback collection (blocked by netfs_wb_begin/netfs_wb_end around the server truncate and local resize, since netfs_writepages also acquires the wb lock).