libxl: adjust PoD target by memory fudge, too

PoD guests need to balloon at least as far as required by PoD, or risk
crashing.  Currently they don't necessarily know what the right value
is, because our memory accounting is (at the very least) confusing.

Apply the memory limit fudge factor to the in-hypervisor PoD memory
target, too.  This will increase the size of the guest's PoD cache by
the fudge factor LIBXL_MAXMEM_CONSTANT (currently 1Mby).  This ensures
that even with a slightly-off balloon driver, the guest will be
stable even under memory pressure.

There are two call sites of xc_domain_set_pod_target that need fixing:

The one in libxl_set_memory_target is straightforward.

The one in xc_hvm_build_x86.c:setup_guest is more awkward.  Simply
setting the PoD target differently does not work because the various
amounts of memory during domain construction no longer match up.
Instead, we adjust the guest memory target in xenstore (but only for
PoD guests).

This introduces a 1Mby discrepancy between the balloon target of a PoD
guest at boot, and the target set by an apparently-equivalent `xl
mem-set' (or similar) later.  This approach is low-risk for a security
fix but we need to fix this up properly in xen.git#staging and
probably also in stable trees.

This is XSA-153.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
(cherry picked from commit 56fb5fd62320eb40a7517206f9706aa9188d6f7b)
(cherry picked from commit 423d2cd814e8460d5ea8bd191a770f3c48b3947c)

Conflicts:
	tools/libxl/libxl_dom.c
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 24c3f6e7cc208796dd28949177bb37224a17ff8f..e2a5be35f7c1b898a810c2741b096d90e2df91c3 100644 (file)
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -4132,7 +4132,7 @@ retry_transaction:
 
     new_target_memkb -= videoram;
     rc = xc_domain_set_pod_target(ctx->xch, domid,
-            new_target_memkb / 4, NULL, NULL, NULL);
+            (new_target_memkb + LIBXL_MAXMEM_CONSTANT) / 4, NULL, NULL, NULL);
     if (rc != 0) {
         LIBXL__LOG_ERRNO(ctx, LIBXL__LOG_ERROR,
                 "xc_domain_set_pod_target domid=%d, memkb=%d "

diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c
index 69e6088a529e28b321aa009537faa7a74d6efa5e..52bc01ac0cf75a12d348da408be8f2bde2d2c0d3 100644 (file)
--- a/tools/libxl/libxl_dom.c
+++ b/tools/libxl/libxl_dom.c
@@ -270,6 +270,7 @@ int libxl__build_post(libxl__gc *gc, uint32_t domid,
     xs_transaction_t t;
     char **ents, **hvm_ents;
     int i, rc;
+    int64_t mem_target_fudge;
 
     rc = libxl_domain_sched_params_set(CTX, domid, &info->sched_params);
     if (rc)
@@ -286,11 +287,17 @@ int libxl__build_post(libxl__gc *gc, uint32_t domid,
     if (info->cpuid != NULL)
         libxl_cpuid_set(ctx, domid, info->cpuid);
 
+    mem_target_fudge =
+        (info->type == LIBXL_DOMAIN_TYPE_HVM &&
+         info->max_memkb > info->target_memkb)
+        ? LIBXL_MAXMEM_CONSTANT : 0;
+
     ents = libxl__calloc(gc, 12 + (info->max_vcpus * 2) + 2, sizeof(char *));
     ents[0] = "memory/static-max";
     ents[1] = GCSPRINTF("%"PRId64, info->max_memkb);
     ents[2] = "memory/target";
-    ents[3] = GCSPRINTF("%"PRId64, info->target_memkb - info->video_memkb);
+    ents[3] = GCSPRINTF("%"PRId64, info->target_memkb - info->video_memkb
+                        - mem_target_fudge);
     ents[4] = "memory/videoram";
     ents[5] = GCSPRINTF("%"PRId64, info->video_memkb);
     ents[6] = "domid";