[Xen-devel] [PATCH v2 1/2] x86/hvm: Always do SMAP check when updating runstate_guest(v)

[Feng Wu <feng.wu@xxxxxxxxx>]

In the current implementation, we honor the guest's CPL and AC
to determain whether do the SMAP check or not for runstate_guest(v).
However, this doesn't work. The VMCS feild is invalid when we try
to get geust's SS by hvm_get_segment_register(), since the
right VMCS has not beed loaded for the current VCPU.

In this patch, we always do the SMAP check when updating
runstate_guest(v) for the guest when SMAP is enabled by it.

Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
---
 xen/arch/x86/domain.c        | 15 ++++++++++++---
 xen/arch/x86/mm/guest_walk.c | 41 ++++++++++++++++++++++++++++-------------
 xen/include/asm-x86/domain.h | 15 ++++++++++++++-
 3 files changed, 54 insertions(+), 17 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index e896210..b0c8810 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1349,22 +1349,31 @@ static void paravirt_ctxt_switch_to(struct vcpu *v)
 }
 
 /* Update per-VCPU guest runstate shared memory area (if registered). */
-bool_t update_runstate_area(const struct vcpu *v)
+bool_t update_runstate_area(struct vcpu *v)
 {
+    bool_t rc;
+
     if ( guest_handle_is_null(runstate_guest(v)) )
         return 1;
 
+    v->arch.smap_check_policy = SMAP_CHECK_ENABLED;
+
     if ( has_32bit_shinfo(v->domain) )
     {
         struct compat_vcpu_runstate_info info;
 
         XLAT_vcpu_runstate_info(&info, &v->runstate);
         __copy_to_guest(v->runstate_guest.compat, &info, 1);
-        return 1;
+        rc = 1;
+        goto out;
     }
 
-    return __copy_to_guest(runstate_guest(v), &v->runstate, 1) !=
+    rc = __copy_to_guest(runstate_guest(v), &v->runstate, 1) !=
            sizeof(v->runstate);
+
+out:
+    v->arch.smap_check_policy = SMAP_CHECK_HONOR_CPL_AC;
+    return rc;
 }
 
 static void _update_runstate_area(struct vcpu *v)
diff --git a/xen/arch/x86/mm/guest_walk.c b/xen/arch/x86/mm/guest_walk.c
index bb38fda..1afa7fd 100644
--- a/xen/arch/x86/mm/guest_walk.c
+++ b/xen/arch/x86/mm/guest_walk.c
@@ -164,25 +164,40 @@ guest_walk_tables(struct vcpu *v, struct p2m_domain *p2m,
         struct segment_register seg;
         const struct cpu_user_regs *regs = guest_cpu_user_regs();
 
-        hvm_get_segment_register(v, x86_seg_ss, &seg);
-
         /* SMEP: kernel-mode instruction fetches from user-mode mappings
          * should fault.  Unlike NX or invalid bits, we're looking for _all_
          * entries in the walk to have _PAGE_USER set, so we need to do the
          * whole walk as if it were a user-mode one and then invert the 
answer. */
         smep =  hvm_smep_enabled(v) && (pfec & PFEC_insn_fetch);
 
-        /*
-         * SMAP: kernel-mode data accesses from user-mode mappings should fault
-         * A fault is considered as a SMAP violation if the following
-         * conditions come true:
-         *   - X86_CR4_SMAP is set in CR4
-         *   - A user page is accessed
-         *   - CPL = 3 or X86_EFLAGS_AC is clear
-         *   - Page fault in kernel mode
-         */
-        smap = hvm_smap_enabled(v) &&
-               ((seg.attr.fields.dpl == 3) || !(regs->eflags & X86_EFLAGS_AC));
+        switch ( v->arch.smap_check_policy )
+        {
+        case SMAP_CHECK_HONOR_CPL_AC:
+            hvm_get_segment_register(v, x86_seg_ss, &seg);
+
+            /*
+             * SMAP: kernel-mode data accesses from user-mode mappings
+             * should fault.
+             * A fault is considered as a SMAP violation if the following
+             * conditions come true:
+             *   - X86_CR4_SMAP is set in CR4
+             *   - A user page is accessed
+             *   - CPL = 3 or X86_EFLAGS_AC is clear
+             *   - Page fault in kernel mode
+             */
+            smap = hvm_smap_enabled(v) &&
+                   ((seg.attr.fields.dpl == 3) ||
+                   !(regs->eflags & X86_EFLAGS_AC));
+            break;
+        case SMAP_CHECK_ENABLED:
+            smap = hvm_smap_enabled(v);
+            break;
+        case SMAP_CHECK_DISABLED:
+            break;
+        default:
+            printk(XENLOG_INFO "Invalid SMAP check type!\n");
+            break;
+        }
     }
 
     if ( smep || smap )
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index abf55fb..d7cac4f 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -446,13 +446,26 @@ struct arch_vcpu
 
     /* A secondary copy of the vcpu time info. */
     XEN_GUEST_HANDLE(vcpu_time_info_t) time_info_guest;
+
+    /*
+     * The SMAP check policy when updating runstate_guest(v) and the
+     * secondary system time.
+     *     SMAP_CHECK_HONOR_CPL_AC - honor the guest's CPL and AC
+     *     SMAP_CHECK_ENABLED      - enable the check
+     *     SMAP_CHECK_DISABLED     - disable the check
+     */
+    uint8_t smap_check_policy;
 } __cacheline_aligned;
 
+#define SMAP_CHECK_HONOR_CPL_AC  0
+#define SMAP_CHECK_ENABLED       1
+#define SMAP_CHECK_DISABLED      2
+
 /* Shorthands to improve code legibility. */
 #define hvm_vmx         hvm_vcpu.u.vmx
 #define hvm_svm         hvm_vcpu.u.svm
 
-bool_t update_runstate_area(const struct vcpu *);
+bool_t update_runstate_area(struct vcpu *);
 bool_t update_secondary_system_time(const struct vcpu *,
                                     struct vcpu_time_info *);
 
-- 
1.8.3.1


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