[xen stable-4.16] xen/page_alloc: Harden assign_pages()

[patchbot@xxxxxxx]

commit 143501861d48e1bfef495849fd68584baac05849
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Mon Nov 22 11:11:05 2021 +0000
Commit:     Ian Jackson <iwj@xxxxxxxxxxxxxx>
CommitDate: Mon Nov 22 11:11:05 2021 +0000

    xen/page_alloc: Harden assign_pages()
    
    domain_tot_pages() and d->max_pages are 32-bit values. While the order
    should always be quite small, it would still be possible to overflow
    if domain_tot_pages() is near to (2^32 - 1).
    
    As this code may be called by a guest via XENMEM_increase_reservation
    and XENMEM_populate_physmap, we want to make sure the guest is not going
    to be able to allocate more than it is allowed.
    
    Rework the allocation check to avoid any possible overflow. While the
    check domain_tot_pages() < d->max_pages should technically not be
    necessary, it is probably best to have it to catch any possible
    inconsistencies in the future.
    
    This is CVE-2021-28706 / part of XSA-385.
    
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/common/grant_table.c |  7 ++++---
 xen/common/page_alloc.c  | 33 +++++++++++++++++++++++++++------
 2 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 8b322b51c0..012a74455b 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -2341,7 +2341,8 @@ gnttab_transfer(
          * pages when it is dying.
          */
         if ( unlikely(e->is_dying) ||
-             unlikely(domain_tot_pages(e) >= e->max_pages) )
+             unlikely(domain_tot_pages(e) >= e->max_pages) ||
+             unlikely(!(e->tot_pages + 1)) )
         {
             spin_unlock(&e->page_alloc_lock);
 
@@ -2350,8 +2351,8 @@ gnttab_transfer(
                          e->domain_id);
             else
                 gdprintk(XENLOG_INFO,
-                         "Transferee d%d has no headroom (tot %u, max %u)\n",
-                         e->domain_id, domain_tot_pages(e), e->max_pages);
+                         "Transferee %pd has no headroom (tot %u, max %u, ex 
%u)\n",
+                         e, domain_tot_pages(e), e->max_pages, e->extra_pages);
 
             gop.status = GNTST_general_error;
             goto unlock_and_copyback;
diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 5801358b4b..d0baaa2ecd 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -2310,20 +2310,41 @@ int assign_pages(
     }
     else if ( !(memflags & MEMF_no_refcount) )
     {
-        unsigned int tot_pages = domain_tot_pages(d) + nr;
+        unsigned int tot_pages = domain_tot_pages(d);
 
         if ( unlikely(tot_pages > d->max_pages) )
         {
-            gprintk(XENLOG_INFO, "Over-allocation for domain %u: "
-                    "%u > %u\n", d->domain_id, tot_pages, d->max_pages);
+            gprintk(XENLOG_INFO, "Inconsistent allocation for %pd: %u > %u\n",
+                    d, tot_pages, d->max_pages);
+            rc = -EPERM;
+            goto out;
+        }
+
+        if ( unlikely(nr > d->max_pages - tot_pages) )
+        {
+            gprintk(XENLOG_INFO, "Over-allocation for %pd: %Lu > %u\n",
+                    d, tot_pages + 0ull + nr, d->max_pages);
             rc = -E2BIG;
             goto out;
         }
     }
 
-    if ( !(memflags & MEMF_no_refcount) &&
-         unlikely(domain_adjust_tot_pages(d, nr) == nr) )
-        get_knownalive_domain(d);
+    if ( !(memflags & MEMF_no_refcount) )
+    {
+        if ( unlikely(d->tot_pages + nr < nr) )
+        {
+            gprintk(XENLOG_INFO,
+                    "Excess allocation for %pd: %Lu (%u extra)\n",
+                    d, d->tot_pages + 0ull + nr, d->extra_pages);
+            if ( pg[0].count_info & PGC_extra )
+                d->extra_pages -= nr;
+            rc = -E2BIG;
+            goto out;
+        }
+
+        if ( unlikely(domain_adjust_tot_pages(d, nr) == nr) )
+            get_knownalive_domain(d);
+    }
 
     for ( i = 0; i < nr; i++ )
     {
--
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