Re: [Xen-devel] xl network-attach SEGV in 4.2 and 4.1

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Ian Campbell writes ("Re: [Xen-devel] xl network-attach SEGV in 4.2 and 4.1"):
> On Wed, 2013-04-17 at 03:49 +0100, Marek Marczykowski wrote:
> > Hi all,
> > When "device/vif" directory exists but is empty l!=NULL, but nb==0, so
> > l[nb-1] is invalid. Add missing check.

Thanks for pointing this out.  The root cause is the expectation that
libxl__xs_directory won't return a non-null list but set *nb to 0.

I found some occurrences of this bug in (xen-unstable) staging.
See patch below, which should probably go into 4.1 and 4.2.

> > Signed-Off-by: Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > 
> > diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
> > index 5783cd2..9e06a7d 100644
> > --- a/tools/libxl/libxl.c
> > +++ b/tools/libxl/libxl.c
> > @@ -2569,7 +2569,8 @@ void libxl__device_nic_add(libxl__egc *egc, uint32_t
> > domid,
> >              goto out_free;
> >          }
> >          if (!(l = libxl__xs_directory(gc, XBT_NULL,
> > -                                     libxl__sprintf(gc, "%s/device/vif",
> > dompath), &nb))) {
> > +                                     libxl__sprintf(gc, "%s/device/vif",
> > dompath), &nb)) ||
> > +                nb == 0) {

I think this is the right way to fix this in the stable trees.  I
looked at the backport of "5420f2650 libxl: Set vfb and vkb devid if
not done so by the caller" and it's nontrivial, so although it's a
bugfix I'd rather leave it.

Ian.


commit d1f52920ee96c91665813fb875c0c76e5eb7a312
Author: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Date:   Thu Apr 18 16:27:46 2013 +0100

    libxl: Avoid realloc(,0) when libxl__xs_directory returns empty list
    
    If the named path is a leaf node, libxl__xs_directory can succeed,
    returning non-null, but set *nb to 0.
    
    In three places in libxl this may result in a zero size argument being
    passed to malloc() or realloc(), which is not adviseable.
    
    Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 0f936c0..1c04d37 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -1840,7 +1840,7 @@ libxl_device_vtpm *libxl_device_vtpm_list(libxl_ctx *ctx, 
uint32_t domid, int *n
 
     fe_path = libxl__sprintf(gc, "%s/device/vtpm", libxl__xs_get_dompath(gc, 
domid));
     dir = libxl__xs_directory(gc, XBT_NULL, fe_path, &ndirs);
-    if(dir) {
+    if (dir && ndirs) {
        vtpms = malloc(sizeof(*vtpms) * ndirs);
        libxl_device_vtpm* vtpm;
        libxl_device_vtpm* end = vtpms + ndirs;
@@ -2314,7 +2314,7 @@ static int libxl__append_disk_list_of_type(libxl__gc *gc,
     be_path = libxl__sprintf(gc, "%s/backend/%s/%d",
                              libxl__xs_get_dompath(gc, 0), type, domid);
     dir = libxl__xs_directory(gc, XBT_NULL, be_path, &n);
-    if (dir) {
+    if (dir && n) {
         libxl_device_disk *tmp;
         tmp = realloc(*disks, sizeof (libxl_device_disk) * (*ndisks + n));
         if (tmp == NULL)
@@ -3006,7 +3006,7 @@ static int libxl__append_nic_list_of_type(libxl__gc *gc,
     be_path = libxl__sprintf(gc, "%s/backend/%s/%d",
                              libxl__xs_get_dompath(gc, 0), type, domid);
     dir = libxl__xs_directory(gc, XBT_NULL, be_path, &n);
-    if (dir) {
+    if (dir && n) {
         libxl_device_nic *tmp;
         tmp = realloc(*nics, sizeof (libxl_device_nic) * (*nnics + n));
         if (tmp == NULL)

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