[xen stable-4.11] x86/ept: fix missing IOMMU flush in atomic_write_ept_entry

[patchbot@xxxxxxx]

commit 76d369d33179a5f8e5f6607f3917db9ab8c22968
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Thu Feb 18 13:02:04 2021 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Feb 18 13:02:04 2021 +0100

    x86/ept: fix missing IOMMU flush in atomic_write_ept_entry
    
    Backport of XSA-321 missed a flush in atomic_write_ept_entry when
    level was different than 0. Such omission will undermine the fix for
    XSA-321, because page table entries cached in the IOMMU can get out
    of sync and contain stale entries.
    
    Fix this by slightly re-arranging the code to prevent the early return
    when level is different that 0. Note that the early return is just an
    optimization because foreign entries cannot have level > 0.
    
    This is XSA-366.
    
    Reported-by: M. Vefa Bicakci <m.v.b@xxxxxxxxxx>
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/mm/p2m-ept.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/xen/arch/x86/mm/p2m-ept.c b/xen/arch/x86/mm/p2m-ept.c
index 02071752e8..e0fec73e3f 100644
--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -53,12 +53,7 @@ static int atomic_write_ept_entry(ept_entry_t *entryptr, 
ept_entry_t new,
     bool_t check_foreign = (new.mfn != entryptr->mfn ||
                             new.sa_p2mt != entryptr->sa_p2mt);
 
-    if ( level )
-    {
-        ASSERT(!is_epte_superpage(&new) || !p2m_is_foreign(new.sa_p2mt));
-        write_atomic(&entryptr->epte, new.epte);
-        return 0;
-    }
+    ASSERT(!level || !is_epte_superpage(&new) || !p2m_is_foreign(new.sa_p2mt));
 
     if ( unlikely(p2m_is_foreign(new.sa_p2mt)) )
     {
--
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