[Xen-changelog] [xen stable-4.8] memory: properly check guest memory ranges in XENMEM_exchange handling

[patchbot@xxxxxxx]

commit 06403aa5f28bf697051de0435ef942f4c0d25849
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Apr 4 14:55:00 2017 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Apr 4 14:55:00 2017 +0200

    memory: properly check guest memory ranges in XENMEM_exchange handling
    
    The use of guest_handle_okay() here (as introduced by the XSA-29 fix)
    is insufficient here, guest_handle_subrange_okay() needs to be used
    instead.
    
    Note that the uses are okay in
    - XENMEM_add_to_physmap_batch handling due to the size field being only
      16 bits wide,
    - livepatch_list() due to the limit of 1024 enforced on the
      number-of-entries input (leaving aside the fact that this can be
      called by a privileged domain only anyway),
    - compat mode handling due to counts there being limited to 32 bits,
    - everywhere else due to guest arrays being accessed sequentially from
      index zero.
    
    This is CVE-2017-7228 / XSA-212.
    
    Reported-by: Jann Horn <jannh@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: 938fd2586eb081bcbd694f4c1f09ae6a263b0d90
    master date: 2017-04-04 14:47:46 +0200
---
 xen/common/memory.c                  | 20 ++++++++++++++++++--
 xen/include/asm-x86/x86_64/uaccess.h |  8 +++++---
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/xen/common/memory.c b/xen/common/memory.c
index 21797ca..17f9e1e 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -437,8 +437,8 @@ static long 
memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
         goto fail_early;
     }
 
-    if ( !guest_handle_okay(exch.in.extent_start, exch.in.nr_extents) ||
-         !guest_handle_okay(exch.out.extent_start, exch.out.nr_extents) )
+    if ( !guest_handle_subrange_okay(exch.in.extent_start, exch.nr_exchanged,
+                                     exch.in.nr_extents - 1) )
     {
         rc = -EFAULT;
         goto fail_early;
@@ -448,11 +448,27 @@ static long 
memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
     {
         in_chunk_order  = exch.out.extent_order - exch.in.extent_order;
         out_chunk_order = 0;
+
+        if ( !guest_handle_subrange_okay(exch.out.extent_start,
+                                         exch.nr_exchanged >> in_chunk_order,
+                                         exch.out.nr_extents - 1) )
+        {
+            rc = -EFAULT;
+            goto fail_early;
+        }
     }
     else
     {
         in_chunk_order  = 0;
         out_chunk_order = exch.in.extent_order - exch.out.extent_order;
+
+        if ( !guest_handle_subrange_okay(exch.out.extent_start,
+                                         exch.nr_exchanged << out_chunk_order,
+                                         exch.out.nr_extents - 1) )
+        {
+            rc = -EFAULT;
+            goto fail_early;
+        }
     }
 
     d = rcu_lock_domain_by_any_id(exch.in.domid);
diff --git a/xen/include/asm-x86/x86_64/uaccess.h 
b/xen/include/asm-x86/x86_64/uaccess.h
index 953abe7..4275e66 100644
--- a/xen/include/asm-x86/x86_64/uaccess.h
+++ b/xen/include/asm-x86/x86_64/uaccess.h
@@ -29,8 +29,9 @@ extern void *xlat_malloc(unsigned long *xlat_page_current, 
size_t size);
 /*
  * Valid if in +ve half of 48-bit address space, or above Xen-reserved area.
  * This is also valid for range checks (addr, addr+size). As long as the
- * start address is outside the Xen-reserved area then we will access a
- * non-canonical address (and thus fault) before ever reaching VIRT_START.
+ * start address is outside the Xen-reserved area, sequential accesses
+ * (starting at addr) will hit a non-canonical address (and thus fault)
+ * before ever reaching VIRT_START.
  */
 #define __addr_ok(addr) \
     (((unsigned long)(addr) < (1UL<<47)) || \
@@ -40,7 +41,8 @@ extern void *xlat_malloc(unsigned long *xlat_page_current, 
size_t size);
     (__addr_ok(addr) || is_compat_arg_xlat_range(addr, size))
 
 #define array_access_ok(addr, count, size) \
-    (access_ok(addr, (count)*(size)))
+    (likely(((count) ?: 0UL) < (~0UL / (size))) && \
+     access_ok(addr, (count) * (size)))
 
 #define __compat_addr_ok(d, addr) \
     ((unsigned long)(addr) < HYPERVISOR_COMPAT_VIRT_START(d))
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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