[Xen-changelog] [xen master] x86/PV: check GDT/LDT limits during emulation

[patchbot@xxxxxxx]

commit 93021cbe880a8013691a48d0febef8ed7d3e3ebd
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Oct 31 16:08:16 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Oct 31 16:08:16 2019 +0100

    x86/PV: check GDT/LDT limits during emulation
    
    Accesses beyond the LDT limit originating from emulation would trigger
    the ASSERT() in pv_map_ldt_shadow_page(). On production builds such
    accesses would cause an attempt to promote the touched page (offset from
    the present LDT base address) to a segment descriptor one. If this
    happens to succeed, guest user mode would be able to elevate its
    privileges to that of the guest kernel. This is particularly easy when
    there's no LDT at all, in which case the LDT base stored internally to
    Xen is simply zero.
    
    Also adjust the ASSERT() that was triggering: It was off by one to
    begin with, and for production builds we also better use
    ASSERT_UNREACHABLE() instead with suitable recovery code afterwards.
    
    This is XSA-298.
    
    Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/pv/emul-gate-op.c | 10 ++++++++--
 xen/arch/x86/pv/emulate.c      |  9 ++++++++-
 xen/arch/x86/pv/mm.c           |  8 ++++++--
 3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/pv/emul-gate-op.c b/xen/arch/x86/pv/emul-gate-op.c
index 6dbf3c12a0..06fcbbce30 100644
--- a/xen/arch/x86/pv/emul-gate-op.c
+++ b/xen/arch/x86/pv/emul-gate-op.c
@@ -51,7 +51,13 @@ static int read_gate_descriptor(unsigned int gate_sel,
     const seg_desc_t *pdesc = gdt_ldt_desc_ptr(gate_sel);
 
     if ( (gate_sel < 4) ||
-         ((gate_sel >= FIRST_RESERVED_GDT_BYTE) && !(gate_sel & 4)) ||
+         /*
+          * We're interested in call gates only, which occupy a single
+          * seg_desc_t for 32-bit and a consecutive pair of them for 64-bit.
+          */
+         ((gate_sel >> 3) + !is_pv_32bit_vcpu(v) >=
+          (gate_sel & 4 ? v->arch.pv.ldt_ents
+                        : v->arch.pv.gdt_ents)) ||
          __get_user(desc, pdesc) )
         return 0;
 
@@ -70,7 +76,7 @@ static int read_gate_descriptor(unsigned int gate_sel,
     if ( !is_pv_32bit_vcpu(v) )
     {
         if ( (*ar & 0x1f00) != 0x0c00 ||
-             (gate_sel >= FIRST_RESERVED_GDT_BYTE - 8 && !(gate_sel & 4)) ||
+             /* Limit check done above already. */
              __get_user(desc, pdesc + 1) ||
              (desc.b & 0x1f00) )
             return 0;
diff --git a/xen/arch/x86/pv/emulate.c b/xen/arch/x86/pv/emulate.c
index 877dfda75e..c0b153e2c5 100644
--- a/xen/arch/x86/pv/emulate.c
+++ b/xen/arch/x86/pv/emulate.c
@@ -31,7 +31,14 @@ int pv_emul_read_descriptor(unsigned int sel, const struct 
vcpu *v,
 {
     seg_desc_t desc;
 
-    if ( sel < 4)
+    if ( sel < 4 ||
+         /*
+          * Don't apply the GDT limit here, as the selector may be a Xen
+          * provided one. __get_user() will fail (without taking further
+          * action) for ones falling in the gap between guest populated
+          * and Xen ones.
+          */
+         ((sel & 4) && (sel >> 3) >= v->arch.pv.ldt_ents) )
         desc.b = desc.a = 0;
     else if ( __get_user(desc, gdt_ldt_desc_ptr(sel)) )
         return 0;
diff --git a/xen/arch/x86/pv/mm.c b/xen/arch/x86/pv/mm.c
index f5ea00ca4e..2b0dadc8da 100644
--- a/xen/arch/x86/pv/mm.c
+++ b/xen/arch/x86/pv/mm.c
@@ -92,12 +92,16 @@ bool pv_map_ldt_shadow_page(unsigned int offset)
     BUG_ON(unlikely(in_irq()));
 
     /*
-     * Hardware limit checking should guarantee this property.  NB. This is
+     * Prior limit checking should guarantee this property.  NB. This is
      * safe as updates to the LDT can only be made by MMUEXT_SET_LDT to the
      * current vcpu, and vcpu_reset() will block until this vcpu has been
      * descheduled before continuing.
      */
-    ASSERT((offset >> 3) <= curr->arch.pv.ldt_ents);
+    if ( unlikely((offset >> 3) >= curr->arch.pv.ldt_ents) )
+    {
+        ASSERT_UNREACHABLE();
+        return false;
+    }
 
     if ( is_pv_32bit_domain(currd) )
         linear = (uint32_t)linear;
--
generated by git-patchbot for /home/xen/git/xen.git#master

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