[Pkg-xen-changes] [xen] 28/70: x86: guard against undue super page PTE creation

[Ian James Campbell]

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ijc pushed a commit to branch feature/bug805508
in repository xen.

commit e648e1dc3d782595779c7e3a8e6d92f0e102b48e
Author: Jan Beulich <jbeulich at suse.com>
Date:   Thu Oct 29 13:48:09 2015 +0100

    x86: guard against undue super page PTE creation
    
    When optional super page support got added (commit bd1cd81d64 "x86: PV
    support for hugepages"), two adjustments were missed: mod_l2_entry()
    needs to consider the PSE and RW bits when deciding whether to use the
    fast path, and the PSE bit must not be removed from L2_DISALLOW_MASK
    unconditionally.
    
    This is CVE-2015-7835 / XSA-148.
    
    Reported-by: "栾尚聪(好风)" <shangcong.lsc at alibaba-inc.com>
    Signed-off-by: Jan Beulich <jbeulich at suse.com>
    Reviewed-by: Tim Deegan <tim at xen.org>
    master commit: fe360c90ea13f309ef78810f1a2b92f2ae3b30b8
    master date: 2015-10-29 13:35:07 +0100
    
    (cherry picked from commit 2d094bd87072e26ac29b07917d31fcbf13892288)
    
    Patch-Name: CVE-2015-7835.diff
---
 xen/arch/x86/mm.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 202ff76..fc65982 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -160,7 +160,10 @@ static void put_superpage(unsigned long mfn);
 static uint32_t base_disallow_mask;
 /* Global bit is allowed to be set on L1 PTEs. Intended for user mappings. */
 #define L1_DISALLOW_MASK ((base_disallow_mask | _PAGE_GNTTAB) & ~_PAGE_GLOBAL)
-#define L2_DISALLOW_MASK (base_disallow_mask & ~_PAGE_PSE)
+
+#define L2_DISALLOW_MASK (unlikely(opt_allow_superpage) \
+                          ? base_disallow_mask & ~_PAGE_PSE \
+                          : base_disallow_mask)
 
 #define l3_disallow_mask(d) (!is_pv_32bit_domain(d) ? \
                              base_disallow_mask : 0xFFFFF198U)
@@ -1839,7 +1842,10 @@ static int mod_l2_entry(l2_pgentry_t *pl2e,
         }
 
         /* Fast path for identical mapping and presence. */
-        if ( !l2e_has_changed(ol2e, nl2e, _PAGE_PRESENT) )
+        if ( !l2e_has_changed(ol2e, nl2e,
+                              unlikely(opt_allow_superpage)
+                              ? _PAGE_PSE | _PAGE_RW | _PAGE_PRESENT
+                              : _PAGE_PRESENT) )
         {
             adjust_guest_l2e(nl2e, d);
             if ( UPDATE_ENTRY(l2, pl2e, ol2e, nl2e, pfn, vcpu, preserve_ad) )

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