Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755135AbbLIJmX (ORCPT ); Wed, 9 Dec 2015 04:42:23 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:45616 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755118AbbLIJmS (ORCPT ); Wed, 9 Dec 2015 04:42:18 -0500 From: Luis Henriques To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@lists.ubuntu.com Cc: Mathias Krause , Greg Kroah-Hartman , Petr Mladek , Alex Elder , Joe Perches , Kay Sievers , Andrew Morton , Linus Torvalds , Kamal Mostafa , Luis Henriques Subject: [PATCH 3.16.y-ckt 125/126] printk: prevent userland from spoofing kernel messages Date: Wed, 9 Dec 2015 09:38:15 +0000 Message-Id: <1449653896-5236-126-git-send-email-luis.henriques@canonical.com> In-Reply-To: <1449653896-5236-1-git-send-email-luis.henriques@canonical.com> References: <1449653896-5236-1-git-send-email-luis.henriques@canonical.com> X-Extended-Stable: 3.16 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3272 Lines: 86 3.16.7-ckt21 -stable review patch. If anyone has any objections, please let me know. ------------------ From: Mathias Krause commit 3824657c522f19f85a76bd932821174a5557a382 upstream. The following statement of ABI/testing/dev-kmsg is not quite right: It is not possible to inject messages from userspace with the facility number LOG_KERN (0), to make sure that the origin of the messages can always be reliably determined. Userland actually can inject messages with a facility of 0 by abusing the fact that the facility is stored in a u8 data type. By using a facility which is a multiple of 256 the assignment of msg->facility in log_store() implicitly truncates it to 0, i.e. LOG_KERN, allowing users of /dev/kmsg to spoof kernel messages as shown below: The following call... # printf '<%d>Kernel panic - not syncing: beer empty\n' 0 >/dev/kmsg ...leads to the following log entry (dmesg -x | tail -n 1): user :emerg : [ 66.137758] Kernel panic - not syncing: beer empty However, this call... # printf '<%d>Kernel panic - not syncing: beer empty\n' 0x800 >/dev/kmsg ...leads to the slightly different log entry (note the kernel facility): kern :emerg : [ 74.177343] Kernel panic - not syncing: beer empty Fix that by limiting the user provided facility to 8 bit right from the beginning and catch the truncation early. Fixes: 7ff9554bb578 ("printk: convert byte-buffer to variable-length...") Signed-off-by: Mathias Krause Cc: Greg Kroah-Hartman Cc: Petr Mladek Cc: Alex Elder Cc: Joe Perches Cc: Kay Sievers Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds [ kamal: backport to 3.13-stable: retain local 'int i' ] Signed-off-by: Kamal Mostafa Signed-off-by: Luis Henriques --- kernel/printk/printk.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index f7f6f7e5ff8a..5fa3f3b3b338 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -259,6 +259,9 @@ static u32 clear_idx; #define PREFIX_MAX 32 #define LOG_LINE_MAX 1024 - PREFIX_MAX +#define LOG_LEVEL(v) ((v) & 0x07) +#define LOG_FACILITY(v) ((v) >> 3 & 0xff) + /* record buffer */ #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) #define LOG_ALIGN 4 @@ -547,12 +550,13 @@ static ssize_t devkmsg_writev(struct kiocb *iocb, const struct iovec *iv, line = buf; if (line[0] == '<') { char *endp = NULL; + unsigned int u; - i = simple_strtoul(line+1, &endp, 10); + u = simple_strtoul(line + 1, &endp, 10); if (endp && endp[0] == '>') { - level = i & 7; - if (i >> 3) - facility = i >> 3; + level = LOG_LEVEL(u); + if (LOG_FACILITY(u) != 0) + facility = LOG_FACILITY(u); endp++; len -= endp - line; line = endp; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/