9p: only copy valid iattrs in 9P2000.L setattr implementation

The 9P2000.L setattr method v9fs_vfs_setattr_dotl() copies struct iattr
values without checking whether they are valid causing unitialized
values to be copied. The 9P2000 setattr method v9fs_vfs_setattr() method
gets this right. Check whether struct iattr fields are valid first
before copying in v9fs_vfs_setattr_dotl() too and make sure that all
other fields are set to 0 apart from {g,u}id which should be set to
INVALID_{G,U}ID. This ensure that they can be safely sent over the wire
or printed for debugging later on.

Link: https://lkml.kernel.org/r/20211129114434.3637938-1-brauner@kernel.org
Link: https://lkml.kernel.org/r/000000000000a0d53f05d1c72a4c%40google.com
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: Dominique Martinet <asmadeus@codewreck.org>
Cc: stable@kernel.org
Cc: v9fs-developer@lists.sourceforge.net
Reported-by: syzbot+dfac92a50024b54acaa4@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
[Dominique: do not set a/mtime with just ATTR_A/MTIME as discussed]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

fs/9p/vfs_inode_dotl.c

@@ -551,7 +551,10 @@ int v9fs_vfs_setattr_dotl(struct user_namespace *mnt_userns,
 {
 	int retval, use_dentry = 0;
 	struct p9_fid *fid = NULL;
-	struct p9_iattr_dotl p9attr;
+	struct p9_iattr_dotl p9attr = {
+		.uid = INVALID_UID,
+		.gid = INVALID_GID,
+	};
 	struct inode *inode = d_inode(dentry);
 
 	p9_debug(P9_DEBUG_VFS, "\n");
@@ -561,14 +564,22 @@ int v9fs_vfs_setattr_dotl(struct user_namespace *mnt_userns,
 		return retval;
 
 	p9attr.valid = v9fs_mapped_iattr_valid(iattr->ia_valid);
+	if (iattr->ia_valid & ATTR_MODE)
 		p9attr.mode = iattr->ia_mode;
+	if (iattr->ia_valid & ATTR_UID)
 		p9attr.uid = iattr->ia_uid;
+	if (iattr->ia_valid & ATTR_GID)
 		p9attr.gid = iattr->ia_gid;
+	if (iattr->ia_valid & ATTR_SIZE)
 		p9attr.size = iattr->ia_size;
+	if (iattr->ia_valid & ATTR_ATIME_SET) {
 		p9attr.atime_sec = iattr->ia_atime.tv_sec;
 		p9attr.atime_nsec = iattr->ia_atime.tv_nsec;
+	}
+	if (iattr->ia_valid & ATTR_MTIME_SET) {
 		p9attr.mtime_sec = iattr->ia_mtime.tv_sec;
 		p9attr.mtime_nsec = iattr->ia_mtime.tv_nsec;
+	}
 
 	if (iattr->ia_valid & ATTR_FILE) {
 		fid = iattr->ia_file->private_data;