[Xen-devel] [PATCH v9 5/7] x86: Enable Supervisor Mode Access Prevention (SMAP) for Xen

[Feng Wu <feng.wu@xxxxxxxxx>]

Supervisor Mode Access Prevention (SMAP) is a new security
feature disclosed by Intel, please refer to the following
document:

http://software.intel.com/sites/default/files/319433-014.pdf

If CR4.SMAP = 1, supervisor-mode data accesses are not allowed
to linear addresses that are accessible in user mode. If CPL < 3,
SMAP protections are disabled if EFLAGS.AC = 1. If CPL = 3, SMAP
applies to all supervisor-mode data accesses (these are implicit
supervisor accesses) regardless of the value of EFLAGS.AC.

This patch enables SMAP in Xen to prevent Xen hypervisor from
accessing pv guest data, whose translation paging-structure
entries' U/S flags are all set.

Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>
---
 docs/misc/xen-command-line.markdown |  7 +++++
 xen/arch/x86/setup.c                | 20 +++++++++++++
 xen/arch/x86/traps.c                | 58 +++++++++++++++++++++++++++----------
 xen/include/asm-x86/cpufeature.h    |  1 +
 xen/include/asm-x86/domain.h        |  6 ++--
 5 files changed, 75 insertions(+), 17 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown 
b/docs/misc/xen-command-line.markdown
index 7dc938b..a7ac53d 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -912,6 +912,13 @@ Set the serial transmit buffer size.
 
 Flag to enable Supervisor Mode Execution Protection
 
+### smap
+> `= <boolean>`
+
+> Default: `true`
+
+Flag to enable Supervisor Mode Access Prevention
+
 ### snb\_igd\_quirk
 > `= <boolean>`
 
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 2e30701..d8598a3 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -61,6 +61,10 @@ integer_param("maxcpus", max_cpus);
 static bool_t __initdata disable_smep;
 invbool_param("smep", disable_smep);
 
+/* smap: Enable/disable Supervisor Mode Access Prevention (default on). */
+static bool_t __initdata disable_smap;
+invbool_param("smap", disable_smap);
+
 /* **** Linux config option: propagated to domain0. */
 /* "acpi=off":    Sisables both ACPI table parsing and interpreter. */
 /* "acpi=force":  Override the disable blacklist.                   */
@@ -1280,6 +1284,11 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     if ( cpu_has_smep )
         set_in_cr4(X86_CR4_SMEP);
 
+    if ( disable_smap )
+        setup_clear_cpu_cap(X86_FEATURE_SMAP);
+    if ( cpu_has_smap )
+        set_in_cr4(X86_CR4_SMAP);
+
     if ( cpu_has_fsgsbase )
         set_in_cr4(X86_CR4_FSGSBASE);
 
@@ -1386,6 +1395,14 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                initrdidx);
 
     /*
+     * Temporarily clear SMAP in CR4 to allow user-accesses in 
construct_dom0().
+     * This saves a large number of corner cases interactions with
+     * copy_from_user().
+     */
+    if ( cpu_has_smap )
+        write_cr4(read_cr4() & ~X86_CR4_SMAP);
+
+    /*
      * We're going to setup domain0 using the module(s) that we stashed safely
      * above our heap. The second module, if present, is an initrd ramdisk.
      */
@@ -1395,6 +1412,9 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                         bootstrap_map, cmdline) != 0)
         panic("Could not set up DOM0 guest OS");
 
+    if ( cpu_has_smap )
+        write_cr4(read_cr4() | X86_CR4_SMAP);
+
     /* Scrub RAM that is still free and so may go to an unprivileged domain. */
     scrub_heap_pages();
 
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index ba7d387..61e25d9 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1194,11 +1194,12 @@ static int handle_gdt_ldt_mapping_fault(
 enum pf_type {
     real_fault,
     smep_fault,
+    smap_fault,
     spurious_fault
 };
 
 static enum pf_type __page_fault_type(
-    unsigned long addr, unsigned int error_code)
+    unsigned long addr, const struct cpu_user_regs *regs)
 {
     unsigned long mfn, cr3 = read_cr3();
     l4_pgentry_t l4e, *l4t;
@@ -1206,6 +1207,7 @@ static enum pf_type __page_fault_type(
     l2_pgentry_t l2e, *l2t;
     l1_pgentry_t l1e, *l1t;
     unsigned int required_flags, disallowed_flags, page_user;
+    unsigned int error_code = regs->error_code;
 
     /*
      * We do not take spurious page faults in IRQ handlers as we do not
@@ -1274,19 +1276,37 @@ static enum pf_type __page_fault_type(
     page_user &= l1e_get_flags(l1e);
 
 leaf:
-    /*
-     * Supervisor Mode Execution Protection (SMEP):
-     * Disallow supervisor execution from user-accessible mappings
-     */
-    if ( (read_cr4() & X86_CR4_SMEP) && page_user &&
-         ((error_code & (PFEC_insn_fetch|PFEC_user_mode)) == PFEC_insn_fetch) )
-        return smep_fault;
+    if ( page_user )
+    {
+        unsigned long cr4 = read_cr4();
+        /*
+         * Supervisor Mode Execution Prevention (SMEP):
+         * Disallow supervisor execution from user-accessible mappings
+         */
+        if ( (cr4 & X86_CR4_SMEP) &&
+             ((error_code & (PFEC_insn_fetch|PFEC_user_mode)) == 
PFEC_insn_fetch) )
+            return smep_fault;
+
+        /*
+         * Supervisor Mode Access Prevention (SMAP):
+         * Disallow supervisor access user-accessible mappings
+         * A fault is considered as an SMAP violation if the following
+         * conditions are true:
+         *   - X86_CR4_SMAP is set in CR4
+         *   - A user page is being accessed
+         *   - CPL=3 or X86_EFLAGS_AC is clear
+         *   - Page fault in kernel mode
+         */
+        if ( (cr4 & X86_CR4_SMAP) && !(error_code & PFEC_user_mode) &&
+             (((regs->cs & 3) == 3) || !(regs->eflags & X86_EFLAGS_AC)) )
+            return smap_fault;
+    }
 
     return spurious_fault;
 }
 
 static enum pf_type spurious_page_fault(
-    unsigned long addr, unsigned int error_code)
+    unsigned long addr, const struct cpu_user_regs *regs)
 {
     unsigned long flags;
     enum pf_type pf_type;
@@ -1296,7 +1316,7 @@ static enum pf_type spurious_page_fault(
      * page tables from becoming invalid under our feet during the walk.
      */
     local_irq_save(flags);
-    pf_type = __page_fault_type(addr, error_code);
+    pf_type = __page_fault_type(addr, regs);
     local_irq_restore(flags);
 
     return pf_type;
@@ -1391,8 +1411,14 @@ void do_page_fault(struct cpu_user_regs *regs)
 
     if ( unlikely(!guest_mode(regs)) )
     {
-        pf_type = spurious_page_fault(addr, error_code);
-        BUG_ON(pf_type == smep_fault);
+        pf_type = spurious_page_fault(addr, regs);
+        if ( (pf_type == smep_fault) || (pf_type == smap_fault) )
+        {
+            console_start_sync();
+            printk("Xen SM%cP violation\n", (pf_type == smep_fault) ? 'E' : 
'A');
+            fatal_trap(TRAP_page_fault, regs);
+        }
+
         if ( pf_type != real_fault )
             return;
 
@@ -1418,10 +1444,12 @@ void do_page_fault(struct cpu_user_regs *regs)
 
     if ( unlikely(current->domain->arch.suppress_spurious_page_faults) )
     {
-        pf_type = spurious_page_fault(addr, error_code);
-        if ( pf_type == smep_fault )
+        pf_type = spurious_page_fault(addr, regs);
+        if ( (pf_type == smep_fault) || (pf_type == smap_fault))
         {
-            gdprintk(XENLOG_ERR, "Fatal SMEP fault\n");
+            printk(XENLOG_G_ERR "%pv fatal SM%cP violation\n",
+                   current, (pf_type == smep_fault) ? 'E' : 'A');
+
             domain_crash(current->domain);
         }
         if ( pf_type != real_fault )
diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index 20881c0..8014241 100644
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -190,6 +190,7 @@
 #define cpu_has_fsgsbase       boot_cpu_has(X86_FEATURE_FSGSBASE)
 
 #define cpu_has_smep            boot_cpu_has(X86_FEATURE_SMEP)
+#define cpu_has_smap            boot_cpu_has(X86_FEATURE_SMAP)
 #define cpu_has_fpu_sel         (!boot_cpu_has(X86_FEATURE_NO_FPU_SEL))
 
 #define cpu_has_ffxsr           ((boot_cpu_data.x86_vendor == X86_VENDOR_AMD) \
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index c5c266f..abf55fb 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -467,12 +467,14 @@ unsigned long pv_guest_cr4_fixup(const struct vcpu *, 
unsigned long guest_cr4);
     (((v)->arch.pv_vcpu.ctrlreg[4]                          \
       | (mmu_cr4_features                                   \
          & (X86_CR4_PGE | X86_CR4_PSE | X86_CR4_SMEP |      \
-            X86_CR4_OSXSAVE | X86_CR4_FSGSBASE))            \
+            X86_CR4_SMAP | X86_CR4_OSXSAVE |                \
+            X86_CR4_FSGSBASE))                              \
       | ((v)->domain->arch.vtsc ? X86_CR4_TSD : 0))         \
      & ~X86_CR4_DE)
 #define real_cr4_to_pv_guest_cr4(c)                         \
     ((c) & ~(X86_CR4_PGE | X86_CR4_PSE | X86_CR4_TSD |      \
-             X86_CR4_OSXSAVE | X86_CR4_SMEP | X86_CR4_FSGSBASE))
+             X86_CR4_OSXSAVE | X86_CR4_SMEP |               \
+             X86_CR4_FSGSBASE | X86_CR4_SMAP))
 
 void domain_cpuid(struct domain *d,
                   unsigned int  input,
-- 
1.8.3.1


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