[Xen-devel] [RFC PATCH v2 8/8] arm/mem_access: Walk the guest's pt in software

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

In this commit, we make use of the gpt walk functionality introduced in the
previous commits. If mem_access is active, hardware-based gva to ipa
translation might fail, as gva_to_ipa uses the guest's translation tables,
access to which might be restricted by the active VTTBR. To side-step potential
translation errors in the function p2m_mem_access_check_and_get_page due to
restricted memory (e.g. to the guest's page tables themselves), we walk the
guest's page tables in software.

Signed-off-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
---
Cc: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
Cc: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
---
v2: Check the returned access rights after walking the guest's page tables in
    the function p2m_mem_access_check_and_get_page.
---
 xen/arch/arm/mem_access.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/mem_access.c b/xen/arch/arm/mem_access.c
index 04b1506b00..0d3a3ff58b 100644
--- a/xen/arch/arm/mem_access.c
+++ b/xen/arch/arm/mem_access.c
@@ -101,6 +101,7 @@ p2m_mem_access_check_and_get_page(vaddr_t gva, unsigned 
long flag,
                                   const struct vcpu *v)
 {
     long rc;
+    unsigned int perm_ro;
     paddr_t ipa;
     gfn_t gfn;
     mfn_t mfn;
@@ -110,8 +111,25 @@ p2m_mem_access_check_and_get_page(vaddr_t gva, unsigned 
long flag,
     struct p2m_domain *p2m = &v->domain->arch.p2m;
 
     rc = gva_to_ipa(gva, &ipa, flag);
+
+    /*
+     * In case mem_access is active, hardware-based gva_to_ipa translation
+     * might fail. Since gva_to_ipa uses the guest's translation tables, access
+     * to which might be restricted by the active VTTBR, we perform a gva to
+     * ipa translation in software.
+     */
     if ( rc < 0 )
-        goto err;
+    {
+        if ( p2m_walk_gpt(p2m, gva, &ipa, &perm_ro) < 0 )
+            /*
+             * The software gva to ipa translation can still fail, e.g., if the
+             * gva is not mapped.
+             */
+            goto err;
+
+        if ( ((flag & GV2M_WRITE) == GV2M_WRITE) && perm_ro )
+            goto err;
+    }
 
     gfn = _gfn(paddr_to_pfn(ipa));
 
-- 
2.12.2


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