[PATCH] smb: client: fix data corruption with concurrent writes and O_TRUNC

[Paulo Alcantara <pc@manguebit.org>]

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).

Fixes: 110fee6b9bb5 ("smb: client: fix missing timestamp updates with O_TRUNC")
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: stable@vger.kernel.org
---
 fs/smb/client/file.c | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/fs/smb/client/file.c b/fs/smb/client/file.c
index 100acc76e9be..61f9c6ccc6be 100644
--- a/fs/smb/client/file.c
+++ b/fs/smb/client/file.c
@@ -999,42 +999,50 @@ static int cifs_do_truncate(const unsigned int xid, struct dentry *dentry)
 	struct cifs_tcon *tcon;
 	int rc;
 
-	rc = filemap_write_and_wait(inode->i_mapping);
-	if (is_interrupt_error(rc))
+	rc = inode_lock_killable(inode);
+	if (rc)
 		return -ERESTARTSYS;
+
+	filemap_invalidate_lock(inode->i_mapping);
+
+	rc = filemap_write_and_wait(inode->i_mapping);
+	if (is_interrupt_error(rc)) {
+		rc = -ERESTARTSYS;
+		goto out;
+	}
 	mapping_set_error(inode->i_mapping, rc);
 
 	cfile = find_writable_file(cinode, FIND_FSUID_ONLY);
 	rc = cifs_file_flush(xid, inode, cfile);
 	if (!rc) {
 		if (cfile) {
+			struct netfs_inode *ictx = netfs_inode(inode);
+
 			tcon = tlink_tcon(cfile->tlink);
 			server = tcon->ses->server;
+			netfs_wb_begin(ictx, false);
 			rc = server->ops->set_file_size(xid, tcon,
 							cfile, 0, false);
 			if (!rc) {
-				inode_lock(inode);
-				filemap_invalidate_lock(inode->i_mapping);
 				netfs_resize_file(&cinode->netfs, 0, true);
 				cifs_setsize(inode, 0);
-				filemap_invalidate_unlock(inode->i_mapping);
-				inode_unlock(inode);
 				cifs_invalidate_cache(inode, 0);
 			}
+			netfs_wb_end(ictx);
 		} else {
 			/*
 			 * No cached handle; evict stale pages so they can't
 			 * be served after the file is later extended; let
 			 * the server's O_TRUNC open response set the i_size
 			 */
-			inode_lock(inode);
-			filemap_invalidate_lock(inode->i_mapping);
 			truncate_inode_pages(inode->i_mapping, 0);
-			filemap_invalidate_unlock(inode->i_mapping);
-			inode_unlock(inode);
 			cifs_invalidate_cache(inode, 0);
 		}
 	}
+
+out:
+	filemap_invalidate_unlock(inode->i_mapping);
+	inode_unlock(inode);
 	if (cfile)
 		cifsFileInfo_put(cfile);
 	return rc;
-- 
2.55.0