[Xen-devel] [PATCH for-4.5 v11 5/9] xen/arm: Allow hypervisor access to mem_access protected pages

[Tamas K Lengyel <tklengyel@xxxxxxxxxxxxx>]

The guestcopy helpers use the MMU to verify that the given guest has read/write
access to a given page during hypercalls. As we may have custom mem_access
permissions set on these pages, we do a software-based type checking in case
the MMU based approach failed, but only if access_in_use is set.

These memory accesses are not forwarded to the mem_event listener. Accesses
performed by the hypervisor are currently not part of the mem_access scheme.
This is consistent behaviour with the x86 side as well.

Signed-off-by: Tamas K Lengyel <tklengyel@xxxxxxxxxxxxx>
---
 xen/arch/arm/guestcopy.c | 120 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 117 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/guestcopy.c b/xen/arch/arm/guestcopy.c
index 0173597..b0727b1 100644
--- a/xen/arch/arm/guestcopy.c
+++ b/xen/arch/arm/guestcopy.c
@@ -6,6 +6,111 @@
 
 #include <asm/mm.h>
 #include <asm/guest_access.h>
+#include <asm/p2m.h>
+
+/*
+ * If mem_access is in use it might have been the reason why get_page_from_gva
+ * failed to fetch the page, as it uses the MMU for the permission checking.
+ * Only in these cases we do a software-based type check and fetch the page if
+ * we indeed found a conflicting mem_access setting.
+ */
+static int check_type_get_page(vaddr_t gva, unsigned long flag,
+                               struct page_info** page)
+{
+    long rc;
+    paddr_t ipa;
+    unsigned long maddr;
+    xenmem_access_t xma;
+    p2m_type_t t;
+
+    rc = gva_to_ipa(gva, &ipa);
+    if ( rc < 0 )
+        return rc;
+
+    /*
+     * We do this first as this is faster in the default case when no
+     * permission is set on the page.
+     */
+    rc = p2m_get_mem_access(current->domain, paddr_to_pfn(ipa), &xma);
+    if ( rc < 0 )
+        return rc;
+
+    /* Let's check if mem_access limited the access. */
+    switch ( xma )
+    {
+    default:
+    case XENMEM_access_rwx:
+    case XENMEM_access_rw:
+        return -EFAULT;
+    case XENMEM_access_n2rwx:
+    case XENMEM_access_n:
+    case XENMEM_access_x:
+        break;
+    case XENMEM_access_wx:
+    case XENMEM_access_w:
+        if ( flag == GV2M_READ )
+            break;
+        else return -EFAULT;
+    case XENMEM_access_rx2rw:
+    case XENMEM_access_rx:
+    case XENMEM_access_r:
+        if ( flag == GV2M_WRITE )
+            break;
+        else return -EFAULT;
+    }
+
+    /*
+     * We had a mem_access permission limiting the access, but the page type
+     * could also be limiting, so we need to check that as well.
+     */
+    maddr = p2m_lookup(current->domain, ipa, &t);
+    if ( maddr == INVALID_PADDR )
+        return -EFAULT;
+
+    /*
+     * All page types are readable so we only have to check the
+     * type if writing.
+     */
+    if ( flag == GV2M_WRITE )
+    {
+        switch ( t )
+        {
+        case p2m_ram_rw:
+        case p2m_iommu_map_rw:
+        case p2m_map_foreign:
+        case p2m_grant_map_rw:
+        case p2m_mmio_direct:
+            /* Base type allows writing, we are good to get the page. */
+            break;
+        default:
+            return -EFAULT;
+        }
+    }
+
+    *page = mfn_to_page(maddr >> PAGE_SHIFT);
+    ASSERT(*page);
+
+    if ( unlikely(!get_page(*page, current->domain)) )
+    {
+        *page = NULL;
+        return -EFAULT;
+    }
+
+    return 0;
+}
+
+/*
+ * If mem_access is not in use, we have a fault. If mem_access is in use, do 
the
+ * software-based type checking.
+ */
+static inline
+int check_mem_access(vaddr_t gva, unsigned long flag, struct page_info **page)
+{
+    if( !current->domain->arch.p2m.access_in_use )
+        return -EFAULT;
+
+    return check_type_get_page(gva, flag, page);
+}
 
 static unsigned long raw_copy_to_guest_helper(void *to, const void *from,
                                               unsigned len, int flush_dcache)
@@ -21,7 +126,10 @@ static unsigned long raw_copy_to_guest_helper(void *to, 
const void *from,
 
         page = get_page_from_gva(current->domain, (vaddr_t) to, GV2M_WRITE);
         if ( page == NULL )
-            return len;
+        {
+            if ( check_mem_access((vaddr_t) to, GV2M_WRITE, &page) < 0 )
+                return len;
+        }
 
         p = __map_domain_page(page);
         p += offset;
@@ -68,7 +176,10 @@ unsigned long raw_clear_guest(void *to, unsigned len)
 
         page = get_page_from_gva(current->domain, (vaddr_t) to, GV2M_WRITE);
         if ( page == NULL )
-            return len;
+        {
+            if ( check_mem_access((vaddr_t) to, GV2M_WRITE, &page) < 0 )
+                return len;
+        }
 
         p = __map_domain_page(page);
         p += offset;
@@ -100,7 +211,10 @@ unsigned long raw_copy_from_guest(void *to, const void 
__user *from, unsigned le
 
         page = get_page_from_gva(current->domain, (vaddr_t) from, GV2M_READ);
         if ( page == NULL )
-            return len;
+        {
+            if ( check_mem_access((vaddr_t) from, GV2M_READ, &page) < 0 )
+                return len;
+        }
 
         p = __map_domain_page(page);
         p += ((vaddr_t)from & (~PAGE_MASK));
-- 
2.1.0


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