[Xen-changelog] [xen stable-4.6] x86/pod: prevent infinite loop when shattering large pages

[patchbot@xxxxxxx]

commit 37bb22be7d954adbbdc357d0898f05e1b2610328
Author:     Julien Grall <julien.grall@xxxxxxxxxx>
AuthorDate: Tue Nov 28 13:48:13 2017 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Nov 28 13:48:13 2017 +0100

    x86/pod: prevent infinite loop when shattering large pages
    
    When populating pages, the PoD may need to split large ones using
    p2m_set_entry and request the caller to retry (see ept_get_entry for
    instance).
    
    p2m_set_entry may fail to shatter if it is not possible to allocate
    memory for the new page table. However, the error is not propagated
    resulting to the callers to retry infinitely the PoD.
    
    Prevent the infinite loop by return false when it is not possible to
    shatter the large mapping.
    
    This is XSA-246.
    
    Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    master commit: a1c6c6768971ea387d7eba0803908ef0928b43ac
    master date: 2017-11-28 13:11:55 +0100
---
 xen/arch/x86/mm/p2m-pod.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
index a08cfef..519b80c 100644
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -1045,9 +1045,8 @@ p2m_pod_demand_populate(struct p2m_domain *p2m, unsigned 
long gfn,
          * NOTE: In a fine-grained p2m locking scenario this operation
          * may need to promote its locking from gfn->1g superpage
          */
-        p2m_set_entry(p2m, gfn_aligned, _mfn(INVALID_MFN), PAGE_ORDER_2M,
-                      p2m_populate_on_demand, p2m->default_access);
-        return 0;
+        return p2m_set_entry(p2m, gfn_aligned, _mfn(INVALID_MFN), 
PAGE_ORDER_2M,
+                             p2m_populate_on_demand, p2m->default_access);
     }
 
     /* Only reclaim if we're in actual need of more cache. */
@@ -1078,8 +1077,12 @@ p2m_pod_demand_populate(struct p2m_domain *p2m, unsigned 
long gfn,
 
     gfn_aligned = (gfn >> order) << order;
 
-    p2m_set_entry(p2m, gfn_aligned, mfn, order, p2m_ram_rw,
-                  p2m->default_access);
+    if ( p2m_set_entry(p2m, gfn_aligned, mfn, order, p2m_ram_rw,
+                       p2m->default_access) )
+    {
+        p2m_pod_cache_add(p2m, p, order);
+        goto out_fail;
+    }
 
     for( i = 0; i < (1UL << order); i++ )
     {
@@ -1124,13 +1127,18 @@ remap_and_retry:
     BUG_ON(order != PAGE_ORDER_2M);
     pod_unlock(p2m);
 
-    /* Remap this 2-meg region in singleton chunks */
-    /* NOTE: In a p2m fine-grained lock scenario this might
-     * need promoting the gfn lock from gfn->2M superpage */
+    /*
+     * Remap this 2-meg region in singleton chunks. See the comment on the
+     * 1G page splitting path above for why a single call suffices.
+     *
+     * NOTE: In a p2m fine-grained lock scenario this might
+     * need promoting the gfn lock from gfn->2M superpage.
+     */
     gfn_aligned = (gfn>>order)<<order;
-    for(i=0; i<(1<<order); i++)
-        p2m_set_entry(p2m, gfn_aligned + i, _mfn(INVALID_MFN), PAGE_ORDER_4K,
-                      p2m_populate_on_demand, p2m->default_access);
+    if ( p2m_set_entry(p2m, gfn_aligned, _mfn(INVALID_MFN), PAGE_ORDER_4K,
+                       p2m_populate_on_demand, p2m->default_access) )
+        return -1;
+
     if ( tb_init_done )
     {
         struct {
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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