[Xen-devel] [PATCH] xen/arm: p2m: Free the p2m entry after flushing the IOMMU TLBs

[Julien Grall <julien.grall@xxxxxxx>]

When freeing a p2m entry, all the sub-tree behind it will also be freed.
This may include intermediate page-tables or any l3 entry requiring to
drop a reference (e.g for foreign pages). As soon as pages are freed,
they may be re-used by Xen or another domain. Therefore it is necessary
to flush *all* the TLBs beforehand.

While CPU TLBs will be flushed before freeing the pages, this is not
the case for IOMMU TLBs. This can be solved by moving the IOMMU TLBs
flush earlier in the code.

This wasn't considered as a security issue as device passthrough on Arm
is not security supported.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---

Cc: olekstysh@xxxxxxxxx
Cc: oleksandr_tyshchenko@xxxxxxxx

    I discovered it while looking at the code, so I don't have any
    reproducer of the issue. There is a small windows where page could
    be reallocated to Xen or another domain but still present in the
    IOMMU TLBs.

    This patch only address the case where the flush succeed. In the
    unlikely case where it does not succeed, then we will still free the
    pages. The IOMMU helper will crash domain, but the device may still
    not be quiescent. So there are a potentially issues do DMA on wrong
    things.

    At the moment, none of the Arm IOMMUs drivers (including the IPMMU
    one under review) are return an error here. Note that flush may
    still fail (see timeout), but is ignored. This is not great as it
    means a device may DMA into something that does not belong to the
    domain. So we probably want to return an error here.

    Even if an error is returned, there are still potential issues
    (see above). The fix is not entirely trivial, we would need to keep
    the page around until the a device is quiescent or the IOMMU is
    reset. This mostly likely means until the domain is fully destroyed.

    One of the solution would be to:
       1) Have a pool of memory for each domain p2m page-tables. So the
       domain can only touch itself
       2) Defer foreign mapping removal

    1) should also solve the case where the P2M is trying to shatter
    everything and therefore hog the memory. Note that today we don't
    free empty page-tables.

    2) I always felt trying to remove the foreign page reference in the
    p2m code was wrong. This is done because we currently allow the
    guest to remove any mapping. So we need to protect ourself against a
    rogue guest. We could try to restrict what the guest can do on the
    p2m.
---
 xen/arch/arm/p2m.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 3c8287a048..963cd1d600 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -1048,14 +1048,6 @@ static int __p2m_set_entry(struct p2m_domain *p2m,
         p2m->lowest_mapped_gfn = gfn_min(p2m->lowest_mapped_gfn, sgfn);
     }
 
-    /*
-     * Free the entry only if the original pte was valid and the base
-     * is different (to avoid freeing when permission is changed).
-     */
-    if ( p2m_is_valid(orig_pte) &&
-         !mfn_eq(lpae_get_mfn(*entry), lpae_get_mfn(orig_pte)) )
-        p2m_free_entry(p2m, orig_pte, level);
-
     if ( has_iommu_pt(p2m->domain) &&
          (lpae_is_valid(orig_pte) || lpae_is_valid(*entry)) )
     {
@@ -1072,6 +1064,14 @@ static int __p2m_set_entry(struct p2m_domain *p2m,
     else
         rc = 0;
 
+    /*
+     * Free the entry only if the original pte was valid and the base
+     * is different (to avoid freeing when permission is changed).
+     */
+    if ( p2m_is_valid(orig_pte) &&
+         !mfn_eq(lpae_get_mfn(*entry), lpae_get_mfn(orig_pte)) )
+        p2m_free_entry(p2m, orig_pte, level);
+
 out:
     unmap_domain_page(table);
 
-- 
2.11.0


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