[xen staging] xen/arm: mm: Access a PT entry before the table is unmapped

[patchbot@xxxxxxx]

commit 63b4c9bfb788ebfd35d0172f7e8e2e41ef948f70
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Sun Jun 7 16:51:54 2020 +0100
Commit:     Stefano Stabellini <sstabellini@xxxxxxxxxx>
CommitDate: Mon Jun 8 10:33:28 2020 -0700

    xen/arm: mm: Access a PT entry before the table is unmapped
    
    xen_pt_next_level() will retrieve the MFN from the entry right after the
    page-table has been unmapped.
    
    After calling xen_unmap_table(), there is no guarantee the mapping will
    still be valid. Depending on the implementation, this may result to a
    data abort in Xen.
    
    Re-order the code to retrieve the MFN before the table is unmapped.
    
    Fixes: 53abb9a1dcd9 ("xen/arm: mm: Rework Xen page-tables walk during 
update")
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
    Release-acked-by: Paul Durrant <paul@xxxxxxx>
---
 xen/arch/arm/mm.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 1b14f49345..9e2ff7c800 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1036,6 +1036,7 @@ static int xen_pt_next_level(bool read_only, unsigned int 
level,
 {
     lpae_t *entry;
     int ret;
+    mfn_t mfn;
 
     entry = *table + offset;
 
@@ -1053,8 +1054,10 @@ static int xen_pt_next_level(bool read_only, unsigned 
int level,
     if ( lpae_is_mapping(*entry, level) )
         return XEN_TABLE_SUPER_PAGE;
 
+    mfn = lpae_get_mfn(*entry);
+
     xen_unmap_table(*table);
-    *table = xen_map_table(lpae_get_mfn(*entry));
+    *table = xen_map_table(mfn);
 
     return XEN_TABLE_NORMAL_PAGE;
 }
--
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