References: bsc#981264 CVE-2014-3672 XSA-180

From 7490dab5c1a01b1623e9d87bdc653cb4f963dd8a Mon Sep 17 00:00:00 2001
From: Ian Jackson <ian.jackson@eu.citrix.com>
Date: Thu, 19 May 2016 19:38:35 +0100
Subject: [PATCH] main loop: Big hammer to fix logfile disk DoS in Xen setups

Each time round the main loop, we now fstat stderr.  If it is too big,
we dup2 /dev/null onto it.  This is not a very pretty patch but it is
very simple, easy to see that it's correct, and has a low risk of
collateral damage.

The limit is 1Mby by default but can be adjusted by setting a new
environment variable.

This fixes CVE-2014-3672.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Tested-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
---
 vl.c |   46 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

Index: xen-4.7.0-testing/tools/qemu-xen-traditional-dir-remote/vl.c
===================================================================
--- xen-4.7.0-testing.orig/tools/qemu-xen-traditional-dir-remote/vl.c
+++ xen-4.7.0-testing/tools/qemu-xen-traditional-dir-remote/vl.c
@@ -3752,6 +3752,50 @@ static void host_main_loop_wait(int *tim
 }
 #endif
 
+static void check_cve_2014_3672_xen(void)
+{
+    static unsigned long limit = ~0UL;
+    const int fd = 2;
+    struct stat stab;
+
+    if (limit == ~0UL) {
+        const char *s = getenv("XEN_QEMU_CONSOLE_LIMIT");
+        /* XEN_QEMU_CONSOLE_LIMIT=0 means no limit */
+        limit = s ? strtoul(s,0,0) : 1*1024*1024;
+    }
+    if (limit == 0)
+        return;
+
+    int r = fstat(fd, &stab);
+    if (r) {
+        perror("fstat stderr (for CVE-2014-3672 check)");
+        exit(-1);
+    }
+    if (!S_ISREG(stab.st_mode))
+        return;
+    if (stab.st_size <= limit)
+        return;
+
+    /* oh dear */
+    fprintf(stderr,"\r\n"
+            "Closing stderr due to CVE-2014-3672 limit. "
+            " Set XEN_QEMU_CONSOLE_LIMIT to number of bytes to override,"
+            " or 0 for no limit.\n");
+    fflush(stderr);
+
+    int nfd = open("/dev/null", O_WRONLY);
+    if (nfd < 0) {
+        perror("open /dev/null (for CVE-2014-3672 check)");
+        exit(-1);
+    }
+    r = dup2(nfd, fd);
+    if (r != fd) {
+        perror("dup2 /dev/null (for CVE-2014-3672 check)");
+        exit(-1);
+    }
+    close(nfd);
+}
+
 void main_loop_wait(int timeout)
 {
     IOHandlerRecord *ioh;
@@ -3763,6 +3807,8 @@ void main_loop_wait(int timeout)
 
     host_main_loop_wait(&timeout);
 
+    check_cve_2014_3672_xen();
+
     /* poll any events */
     /* XXX: separate device handlers from system ones */
     nfds = -1;