From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754417AbbLCU4A (ORCPT ); Thu, 3 Dec 2015 15:56:00 -0500 Received: from mail-lb0-f171.google.com ([209.85.217.171]:35890 "EHLO mail-lb0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753951AbbLCUve (ORCPT ); Thu, 3 Dec 2015 15:51:34 -0500 From: Rasmus Villemoes To: Andrew Morton , Andy Shevchenko , Rasmus Villemoes , Kees Cook , Martin Kletzander Cc: Ingo Molnar , linux-kernel@vger.kernel.org Subject: [PATCH v3 03/14] lib/vsprintf.c: eliminate potential race in string() Date: Thu, 3 Dec 2015 21:51:02 +0100 Message-Id: <1449175873-1780-4-git-send-email-linux@rasmusvillemoes.dk> X-Mailer: git-send-email 2.6.1 In-Reply-To: <1449175873-1780-1-git-send-email-linux@rasmusvillemoes.dk> References: <1449175873-1780-1-git-send-email-linux@rasmusvillemoes.dk> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If the string corresponding to a %s specifier can change under us, we might end up copying a \0 byte to the output buffer. There might be callers who expect the output buffer to contain a genuine C string whose length is exactly the snprintf return value (assuming truncation hasn't happened or has been checked for). We can avoid this by only passing over the source string once, stopping the first time we meet a nul byte (or when we reach the given precision), and then letting widen_string() handle left/right space padding. As a small bonus, this code reuse also makes the generated code slightly smaller. Cc: Ingo Molnar Signed-off-by: Rasmus Villemoes --- lib/vsprintf.c | 28 +++++++++------------------- 1 file changed, 9 insertions(+), 19 deletions(-) diff --git a/lib/vsprintf.c b/lib/vsprintf.c index a021e6380404..63ca52366049 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -557,32 +557,22 @@ char *widen_string(char *buf, int n, char *end, struct printf_spec spec) static noinline_for_stack char *string(char *buf, char *end, const char *s, struct printf_spec spec) { - int len, i; + int len = 0; + size_t lim = spec.precision; if ((unsigned long)s < PAGE_SIZE) s = "(null)"; - len = strnlen(s, spec.precision); - - if (!(spec.flags & LEFT)) { - while (len < spec.field_width--) { - if (buf < end) - *buf = ' '; - ++buf; - } - } - for (i = 0; i < len; ++i) { - if (buf < end) - *buf = *s; - ++buf; ++s; - } - while (len < spec.field_width--) { + while (lim--) { + char c = *s++; + if (!c) + break; if (buf < end) - *buf = ' '; + *buf = c; ++buf; + ++len; } - - return buf; + return widen_string(buf, len, end, spec); } static noinline_for_stack -- 2.6.1