[libvirt] [PATCH] uml: avoid crash on partial read

[Eric Blake]

Coverity detected a potential dereference of uninitialized memory
if recvfrom got cut short.

* src/uml/uml_driver.c (umlMonitorCommand): Validate complete read
prior to dereferencing res.
---

The patch borrows ideas from macvtap.c, the only other file in
libvirt that currently uses recvfrom.

I did not analyze whether this is a security hole, where a
malicious UDP packet could intentionally force the dereferencing
of uninitialized memory to misbehave in a controlled manner.

 src/uml/uml_driver.c |   14 ++++++++++++--
 1 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/uml/uml_driver.c b/src/uml/uml_driver.c
index bbea429..eec239f 100644
--- a/src/uml/uml_driver.c
+++ b/src/uml/uml_driver.c
@@ -733,14 +733,24 @@ static int umlMonitorCommand(virConnectPtr conn,
     }

     do {
+        ssize_t nbytes;
         addrlen = sizeof(addr);
-        if (recvfrom(priv->monitor, &res, sizeof res, 0,
-                     (struct sockaddr *)&addr, &addrlen) < 0) {
+        nbytes = recvfrom(priv->monitor, &res, sizeof res, 0,
+                          (struct sockaddr *)&addr, &addrlen) < 0;
+        if (nbytes < 0) {
+            if (errno == EAGAIN || errno == EINTR)
+                continue;
             virReportSystemError(errno,
                                  _("cannot read reply %s"),
                                  cmd);
             goto error;
         }
+        if (nbytes < sizeof res) {
+            virReportSystemError(errno,
+                                 _("incomplete reply %s"),
+                                 cmd);
+            goto error;
+        }

         if (VIR_REALLOC_N(retdata, retlen + res.length) < 0) {
             virReportOOMError();
-- 
1.6.6.1