[Xen-changelog] [xen stable-4.4] x86: guard against undue super page PTE creation

[patchbot@xxxxxxx]

commit a6646a52fd7aad00c6d50668f7ae362288ec9a98
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Oct 29 14:07:01 2015 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Oct 29 14:07:01 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: "æ ¾å°?è?ª(好é£?)" <shangcong.lsc@xxxxxxxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Tim Deegan <tim@xxxxxxx>
    master commit: fe360c90ea13f309ef78810f1a2b92f2ae3b30b8
    master date: 2015-10-29 13:35:07 +0100
---
 xen/arch/x86/mm.c |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index f4987f9..dfae2e7 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -163,7 +163,10 @@ static void put_superpage(unsigned long mfn);

 static uint32_t base_disallow_mask;
 #define L1_DISALLOW_MASK (base_disallow_mask | _PAGE_GNTTAB)
-#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_32on64_domain(d) ?  \
                              base_disallow_mask :       \
@@ -1786,7 +1789,10 @@ 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) )
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.4

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