[Xen-changelog] [xen stable-4.8] x86/shadow: fix ref-counting error handling

[patchbot@xxxxxxx]

commit 68db69443f8bd281609ea642a49b065dcd275719
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Dec 12 14:45:02 2017 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Dec 12 14:45:02 2017 +0100

    x86/shadow: fix ref-counting error handling
    
    The old-Linux handling in shadow_set_l4e() mistakenly ORed together the
    results of sh_get_ref() and sh_pin(). As the latter failing is not a
    correctness problem, simply ignore its return value.
    
    In sh_set_toplevel_shadow() a failing sh_get_ref() must not be
    accompanied by installing the entry, despite the domain being crashed.
    
    This is XSA-250.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Tim Deegan <tim@xxxxxxx>
    master commit: 10be8001de7d87be1f0ccdda75cc70e922e56d03
    master date: 2017-12-12 14:29:45 +0100
---
 xen/arch/x86/mm/shadow/multi.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index 2881104..aca28eb 100644
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -910,7 +910,7 @@ static int shadow_set_l4e(struct domain *d,
                           shadow_l4e_t new_sl4e,
                           mfn_t sl4mfn)
 {
-    int flags = 0, ok;
+    int flags = 0;
     shadow_l4e_t old_sl4e;
     paddr_t paddr;
     ASSERT(sl4e != NULL);
@@ -925,15 +925,16 @@ static int shadow_set_l4e(struct domain *d,
     {
         /* About to install a new reference */
         mfn_t sl3mfn = shadow_l4e_get_mfn(new_sl4e);
-        ok = sh_get_ref(d, sl3mfn, paddr);
-        /* Are we pinning l3 shadows to handle wierd linux behaviour? */
-        if ( sh_type_is_pinnable(d, SH_type_l3_64_shadow) )
-            ok |= sh_pin(d, sl3mfn);
-        if ( !ok )
+
+        if ( !sh_get_ref(d, sl3mfn, paddr) )
         {
             domain_crash(d);
             return SHADOW_SET_ERROR;
         }
+
+        /* Are we pinning l3 shadows to handle weird Linux behaviour? */
+        if ( sh_type_is_pinnable(d, SH_type_l3_64_shadow) )
+            sh_pin(d, sl3mfn);
     }
 
     /* Write the new entry */
@@ -3914,14 +3915,15 @@ sh_set_toplevel_shadow(struct vcpu *v,
 
     /* Take a ref to this page: it will be released in sh_detach_old_tables()
      * or the next call to set_toplevel_shadow() */
-    if ( !sh_get_ref(d, smfn, 0) )
+    if ( sh_get_ref(d, smfn, 0) )
+        new_entry = pagetable_from_mfn(smfn);
+    else
     {
         SHADOW_ERROR("can't install %#lx as toplevel shadow\n", mfn_x(smfn));
         domain_crash(d);
+        new_entry = pagetable_null();
     }
 
-    new_entry = pagetable_from_mfn(smfn);
-
  install_new_entry:
     /* Done.  Install it */
     SHADOW_PRINTK("%u/%u [%u] gmfn %#"PRI_mfn" smfn %#"PRI_mfn"\n",
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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