[Xen-devel] [PATCH 1/2] x86/mm: Avoid NULL dereference when checking altp2m's for shareability

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Coverity identifies that __get_gfn_type_access() unconditionally writes to its
type parameter under a number of circumstances.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Tim Deegan <tim@xxxxxxx>
CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
CC: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>

There is a second complaint that ap2ma and p2ma are used before initialisation
in the following line, although that is harder to reason about.  I think the
code is OK...
---
 xen/arch/x86/mm/mem_sharing.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index 47e0820..14952ce 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -870,6 +870,7 @@ int mem_sharing_nominate_page(struct domain *d,
         unsigned int i;
         struct p2m_domain *ap2m;
         mfn_t amfn;
+        p2m_type_t ap2mt;
         p2m_access_t ap2ma;
 
         altp2m_list_lock(d);
@@ -880,7 +881,7 @@ int mem_sharing_nominate_page(struct domain *d,
             if ( !ap2m )
                 continue;
 
-            amfn = get_gfn_type_access(ap2m, gfn, NULL, &ap2ma, 0, NULL);
+            amfn = get_gfn_type_access(ap2m, gfn, &ap2mt, &ap2ma, 0, NULL);
             if ( mfn_valid(amfn) && (mfn_x(amfn) != mfn_x(mfn) || ap2ma != 
p2ma) )
             {
                 altp2m_list_unlock(d);
-- 
2.1.4


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