[Xen-changelog] [xen stable-4.8] x86/mm: Don't reset linear_pt_count on partial validation

[patchbot@xxxxxxx]

commit bafcd7fef85725b1c3a9e53873a631a8413418bf
Author:     George Dunlap <george.dunlap@xxxxxxxxxx>
AuthorDate: Wed Dec 11 15:58:29 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Dec 11 15:58:29 2019 +0100

    x86/mm: Don't reset linear_pt_count on partial validation
    
    "Linear pagetables" is a technique which involves either pointing a
    pagetable at itself, or to another pagetable the same or higher level.
    Xen has limited support for linear pagetables: A page may either point
    to itself, or point to another page of the same level (i.e., L2 to L2,
    L3 to L3, and so on).
    
    XSA-240 introduced an additional restriction that limited the "depth"
    of such chains by allowing pages to either *point to* other pages of
    the same level, or *be pointed to* by other pages of the same level,
    but not both.  To implement this, we keep track of the number of
    outstanding times a page points to or is pointed to another page
    table, to prevent both from happening at the same time.
    
    Unfortunately, the original commit introducing this reset this count
    when resuming validation of a partially-validated pagetable, dropping
    some "linear_pt_entry" counts.
    
    On debug builds on systems where guests used this feature, this might
    lead to crashes that look like this:
    
        Assertion 'oc > 0' failed at mm.c:874
    
    Worse, if an attacker could engineer such a situation to occur, they
    might be able to make loops or other abitrary chains of linear
    pagetables, leading to the denial-of-service situation outlined in
    XSA-240.
    
    This is XSA-309.
    
    Reported-by: Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>
    Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 7473efd12fb7a6548f5303f1f4c5cb521543a813
    master date: 2019-12-11 14:10:27 +0100
---
 xen/arch/x86/mm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index ed1e49bf6f..81b6c8da72 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3156,8 +3156,8 @@ static int __get_page_type(struct page_info *page, 
unsigned long type,
         {
             page->nr_validated_ptes = 0;
             page->partial_flags = 0;
+            page->linear_pt_count = 0;
         }
-        page->linear_pt_count = 0;
         rc = alloc_page_type(page, type, preemptible);
     }
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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