[PATCH QEMU for-4.7] main loop: Big hammer to fix logfile disk DoS in Xen setups

[Wei Liu <wei.liu2@citrix.com>]

From: Ian Jackson <ian.jackson@eu.citrix.com>

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>

Set the default to 0 so that it won't affect non-xen installation. The
limit will be set by Xen toolstack.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
---
 main-loop.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/main-loop.c b/main-loop.c
index 3997043..aa32f5b 100644
--- a/main-loop.c
+++ b/main-loop.c
@@ -164,6 +164,50 @@ int qemu_init_main_loop(Error **errp)
     return 0;
 }
 
+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) : 0;
+    }
+    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);
+}
+
 static int max_priority;
 
 #ifndef _WIN32
@@ -216,6 +260,8 @@ static int os_host_main_loop_wait(int64_t timeout)
     int ret;
     static int spin_counter;
 
+    check_cve_2014_3672_xen();
+
     glib_pollfds_fill(&timeout);
 
     /* If the I/O thread is very busy or we are incorrectly busy waiting in
@@ -407,6 +453,8 @@ static int os_host_main_loop_wait(int64_t timeout)
     fd_set rfds, wfds, xfds;
     int nfds;
 
+    check_cve_2014_3672_xen();
+
     /* XXX: need to suppress polling by better using win32 events */
     ret = 0;
     for (pe = first_polling_entry; pe != NULL; pe = pe->next) {
-- 
2.1.4


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