[Xen-changelog] [xen master] x86/shadow: correct SH_LINEAR mapping detection in sh_guess_wrmap()

[patchbot@xxxxxxx]

commit d20daf4294adbdb9316850566013edb98db7bfbc
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu Nov 16 10:38:14 2017 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Nov 16 10:38:14 2017 +0100

    x86/shadow: correct SH_LINEAR mapping detection in sh_guess_wrmap()
    
    The fix for XSA-243 / CVE-2017-15592 (c/s bf2b4eadcf379) introduced a change
    in behaviour for sh_guest_wrmap(), where it had to cope with no shadow 
linear
    mapping being present.
    
    As the name suggests, guest_vtable is a mapping of the guests pagetable, not
    Xen's pagetable, meaning that it isn't the pagetable we need to check for 
the
    shadow linear slot in.
    
    The practical upshot is that a shadow HVM vcpu which switches into 4-level
    paging mode, with an L4 pagetable that contains a mapping which aliases 
Xen's
    SH_LINEAR_PT_VIRT_START will fool the safety check for whether a 
SHADOW_LINEAR
    mapping is present.  As the check passes (when it should have failed), Xen
    subsequently falls over the missing mapping with a pagefault such as:
    
        (XEN) Pagetable walk from ffff8140a0503880:
        (XEN)  L4[0x102] = 000000046c218063 ffffffffffffffff
        (XEN)  L3[0x102] = 000000046c218063 ffffffffffffffff
        (XEN)  L2[0x102] = 000000046c218063 ffffffffffffffff
        (XEN)  L1[0x103] = 0000000000000000 ffffffffffffffff
    
    This is part of XSA-243.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Tim Deegan <tim@xxxxxxx>
---
 xen/arch/x86/mm/shadow/multi.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index 9677256..3103e52 100644
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -4350,11 +4350,18 @@ static int sh_guess_wrmap(struct vcpu *v, unsigned long 
vaddr, mfn_t gmfn)
 
     /* Carefully look in the shadow linear map for the l1e we expect */
 #if SHADOW_PAGING_LEVELS >= 4
-    /* Is a shadow linear map is installed in the first place? */
-    sl4p  = v->arch.paging.shadow.guest_vtable;
-    sl4p += shadow_l4_table_offset(SH_LINEAR_PT_VIRT_START);
-    if ( !(shadow_l4e_get_flags(*sl4p) & _PAGE_PRESENT) )
-        return 0;
+    /*
+     * Non-external guests (i.e. PV) have a SHADOW_LINEAR mapping from the
+     * moment their shadows are created.  External guests (i.e. HVM) may not,
+     * but always have a regular linear mapping, which we can use to observe
+     * whether a SHADOW_LINEAR mapping is present.
+     */
+    if ( paging_mode_external(d) )
+    {
+        sl4p =  __linear_l4_table + l4_linear_offset(SH_LINEAR_PT_VIRT_START);
+        if ( !(shadow_l4e_get_flags(*sl4p) & _PAGE_PRESENT) )
+            return 0;
+    }
     sl4p = sh_linear_l4_table(v) + shadow_l4_linear_offset(vaddr);
     if ( !(shadow_l4e_get_flags(*sl4p) & _PAGE_PRESENT) )
         return 0;
--
generated by git-patchbot for /home/xen/git/xen.git#master

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