jffs2: return -ERANGE when xattr buffer is too small

BugLink: https://bugs.launchpad.net/bugs/1801900

When a file have multiple xattrs and the passed buffer is
smaller than the required size, jffs2_listxattr() should
return -ERANGE instead of continue, else Oops may occur
due to memory corruption.

Also remove the unnecessary check ("rc < 0"), because
xhandle->list(...) will not return an error number.

Spotted by generic/377 in xfstests-dev.

NB: The problem had been fixed by commit 764a5c6b ("xattr
handlers: Simplify list operation") in v4.5-rc1, but the
modification in that commit may be too much because it modifies
all file-systems which implement xattr, so I create a single
patch for jffs2 to fix the problem.
Signed-off-by: Hou Tao <houtao1@huawei.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

fs/jffs2/xattr.c

@@ -1004,12 +1004,14 @@ ssize_t jffs2_listxattr(struct dentry *dentry, char *buffer, size_t size)
 			rc = xhandle->list(xhandle, dentry, buffer + len,
 					   size - len, xd->xname,
 					   xd->name_len);
+			if (rc > size - len) {
+				rc = -ERANGE;
+				goto out;
+			}
 		} else {
 			rc = xhandle->list(xhandle, dentry, NULL, 0,
 					   xd->xname, xd->name_len);
 		}
-		if (rc < 0)
-			goto out;
 		len += rc;
 	}
 	rc = len;