[Xen-changelog] [xen stable-4.8] xen/arm: p2m: Avoid aliasing guest physical frame

[patchbot@xxxxxxx]

commit ae9ec06784d401b9448c9d705a2b47a063f640a4
Author:     Julien Grall <julien.grall@xxxxxxx>
AuthorDate: Mon Nov 4 15:27:45 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Nov 4 15:27:45 2019 +0100

    xen/arm: p2m: Avoid aliasing guest physical frame
    
    The P2M helpers implementation is quite lax and will end up to ignore
    the unused top bits of a guest physical frame.
    
    This effectively means that p2m_set_entry() will create a mapping for a
    different frame (it is always equal to gfn & (mask unused bits)). Yet
    p2m->max_mapped_gfn will be updated using the original frame.
    
    At the moment, p2m_get_entry() and p2m_resolve_translation_fault()
    assume that p2m_get_root_pointer() will always return a non-NULL pointer
    when the GFN is smaller than p2m->max_mapped_gfn.
    
    Unfortunately, because of the aliasing described above, it would be
    possible to set p2m->max_mapped_gfn high enough so it covers frame that
    would lead p2m_get_root_pointer() to return NULL.
    
    As we don't sanity check the guest physical frame provided by a guest, a
    malicious guest could craft a series of hypercalls that will hit the
    BUG_ON() and therefore DoS Xen.
    
    To prevent aliasing, the function p2m_get_root_pointer() is now reworked
    to return NULL If any of the unused top bits are not zero. The caller
    can then decide what's the appropriate action to do. Since the two paths
    (i.e. P2M_ROOT_PAGES == 1 and P2M_ROOT_PAGES != 1) are now very
    similarly, take the opportunity to consolidate them making the code a
    bit simpler.
    
    With this change, p2m_get_entry() will not try to insert a mapping as
    the root pointer is invalid.
    
    Note that root_table is now switch to unsigned long as unsigned int is
    not enough to hold part of a GFN.
    
    This is part of XSA-301.
    
    Reported-by: Julien Grall <Julien.Grall@xxxxxxx>
    Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
    master commit: 88aaf40eeff771c546ad3bbb02000171648a89f7
    master date: 2019-10-31 16:17:33 +0100
---
 xen/arch/arm/p2m.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 29034b0a84..cf4be60987 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -218,21 +218,14 @@ static void p2m_flush_tlb_sync(struct p2m_domain *p2m)
 static lpae_t *p2m_get_root_pointer(struct p2m_domain *p2m,
                                     gfn_t gfn)
 {
-    unsigned int root_table;
-
-    if ( P2M_ROOT_PAGES == 1 )
-        return __map_domain_page(p2m->root);
+    unsigned long root_table;
 
     /*
-     * Concatenated root-level tables. The table number will be the
-     * offset at the previous level. It is not possible to
-     * concatenate a level-0 root.
+     * While the root table index is the offset from the previous level,
+     * we can't use (P2M_ROOT_LEVEL - 1) because the root level might be
+     * 0. Yet we still want to check if all the unused bits are zeroed.
      */
-    ASSERT(P2M_ROOT_LEVEL > 0);
-
-    root_table = gfn_x(gfn) >> (level_orders[P2M_ROOT_LEVEL - 1]);
-    root_table &= LPAE_ENTRY_MASK;
-
+    root_table = gfn_x(gfn) >> (level_orders[P2M_ROOT_LEVEL] + LPAE_SHIFT);
     if ( root_table >= P2M_ROOT_PAGES )
         return NULL;
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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