[Pkg-xen-changes] [xen] 69/70: libxl: adjust PoD target by memory fudge, too

[Ian James Campbell]

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ijc pushed a commit to branch feature/bug805508
in repository xen.

commit 5b32745c45b186c5fe716ccbf5d4a13272de2ee0
Author: Ian Jackson <ian.jackson at eu.citrix.com>
Date:   Wed Oct 21 16:18:30 2015 +0100

    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 at eu.citrix.com>
    (cherry picked from commit 56fb5fd62320eb40a7517206f9706aa9188d6f7b)
    
    Patch-Name: CVE-2015-7972.diff
---
 tools/libxl/libxl.c     | 2 +-
 tools/libxl/libxl_dom.c | 9 ++++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index d38d0c7..1366177 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -4815,7 +4815,7 @@ retry_transaction:
     }
 
     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 b514377..8019f4e 100644
--- a/tools/libxl/libxl_dom.c
+++ b/tools/libxl/libxl_dom.c
@@ -486,6 +486,7 @@ int libxl__build_post(libxl__gc *gc, uint32_t domid,
     xs_transaction_t t;
     char **ents;
     int i, rc;
+    int64_t mem_target_fudge;
 
     if (info->num_vnuma_nodes && !info->num_vcpu_soft_affinity) {
         rc = set_vnuma_affinity(gc, domid, info);
@@ -518,11 +519,17 @@ int libxl__build_post(libxl__gc *gc, uint32_t domid,
         }
     }
 
+    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";

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