"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.

Additionally, XSA-299 introduced a mode whereby if a page was known to
have been only partially validated, _put_page_type() would be called
with PTF_partial_set, indicating that if the page had been
de-validated by someone else, the type count should be left alone.

Unfortunately, this change did not account for the required accounting
for linear page table uses and entries; in the case that a previously
partially-devalidated pagetable was fully-devalidated by someone else,
the linear_pt_counts are not updated.

This could happen in one of two places:

1. In the case a partially-devalidated page was re-validated by
someone else

2. During domain tear-down, when pages are force-invalidated while
leaving the type count intact.

The second could be ignored, since at that point the pages can no
longer be abused; but the first requires handling.  Note however that
this would not be a security issue: having the counts be too high is
overly strict (i.e., will prevent a page from being used in a way
which is perfectly safe), but shouldn't cause any other issues.

Fix this by adjusting the linear counts when a page loses validation,
regardless of whether the de-validation completed or was only partial.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
Release exception justification: This is a fix for a bug.

CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Jan Beulich <jbeulich@suse.com>
CC: Juergen Gross <jgross@suse.com>
---
 xen/arch/x86/mm.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index bd8182f40f..7d4dd80a85 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2780,14 +2780,17 @@ static int _put_final_page_type(struct page_info *page, unsigned long type,
 {
     int rc = free_page_type(page, type, preemptible);
 
+    if ( ptpg && PGT_type_equal(type, ptpg->u.inuse.type_info) &&
+         (type & PGT_validated) && rc != -EINTR )
+    {
+        /* Any time we begin de-validation of a page, adjust linear counts */
+        dec_linear_uses(page);
+        dec_linear_entries(ptpg);
+    }
+
     /* No need for atomic update of type_info here: noone else updates it. */
     if ( rc == 0 )
     {
-        if ( ptpg && PGT_type_equal(type, ptpg->u.inuse.type_info) )
-        {
-            dec_linear_uses(page);
-            dec_linear_entries(ptpg);
-        }
         ASSERT(!page->linear_pt_count || page_get_owner(page)->is_dying);
         set_tlbflush_timestamp(page);
         smp_wmb();
-- 
2.24.0


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On 22.11.19 19:52, George Dunlap wrote:
> "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.
> 
> Additionally, XSA-299 introduced a mode whereby if a page was known to
> have been only partially validated, _put_page_type() would be called
> with PTF_partial_set, indicating that if the page had been
> de-validated by someone else, the type count should be left alone.
> 
> Unfortunately, this change did not account for the required accounting
> for linear page table uses and entries; in the case that a previously
> partially-devalidated pagetable was fully-devalidated by someone else,
> the linear_pt_counts are not updated.
> 
> This could happen in one of two places:
> 
> 1. In the case a partially-devalidated page was re-validated by
> someone else
> 
> 2. During domain tear-down, when pages are force-invalidated while
> leaving the type count intact.
> 
> The second could be ignored, since at that point the pages can no
> longer be abused; but the first requires handling.  Note however that
> this would not be a security issue: having the counts be too high is
> overly strict (i.e., will prevent a page from being used in a way
> which is perfectly safe), but shouldn't cause any other issues.
> 
> Fix this by adjusting the linear counts when a page loses validation,
> regardless of whether the de-validation completed or was only partial.
> 
> Signed-off-by: George Dunlap <george.dunlap@citrix.com>
> Reviewed-by: Jan Beulich <jbeulich@suse.com>

Release-acked-by: Juergen Gross <jgross@suse.com>


Juergen

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