[PATCH v5] xen/sm{e, a}p: allow disabling sm{e, a}p for Xen itself

[PATCH v5] xen/sm{e, a}p: allow disabling sm{e, a}p for Xen itself

[He Chen]

SMEP/SMAP is a security feature to prevent kernel executing/accessing
user address involuntarily, any such behavior will lead to a page fault.

SMEP/SMAP is open (in CR4) for both Xen and HVM guest in earlier code.
SMEP/SMAP bit set in Xen CR4 would enforce security checking for 32-bit
PV guest which will suffer unknown SMEP/SMAP page fault when guest
kernel attempt to access user address although SMEP/SMAP is close for
PV guests.

This patch introduces a new boot option value "hvm" for "sm{e,a}p", it
is going to diable SMEP/SMAP for Xen hypervisor while enable them for
HVM. In this way, 32-bit PV guest will not suffer SMEP/SMAP security
issue. Users can choose whether open SMEP/SMAP for Xen itself,
especially when they are going to run 32-bit PV guests.

Signed-off-by: He Chen <he.chen@linux.intel.com>

---
Changes in v5:
* refine sm{e,a}p parameters parser flow.
* replace cpu_has_sm{e,a}p with boot_cpu_has(X86_FEATURE_XEN_SM{E,A}P).
* refine docs.

Changes in v4:
* introduce 2 new synthetic features X86_FEATURE_XEN_SMEP and
  X86_FEATURE_XEN_SMAP for Xen itself.
* adjust SM{E,A}P related instruction patching code.
* commit message refinement.

Changes in v3:
* fix boot options.
* fix CR4 & mmu_cr4_features operations.
* disable SMEP/SMAP for Dom0.
* commit message refinement.

Changes in v2:
* allow "hvm" as a value to "smep" and "smap" command line options.
* clear SMEP/SMAP CPUID bits for pv guests if they are set to hvm only.
* refine docs.
* rewrite commit message.
---
 docs/misc/xen-command-line.markdown |  2 ++
 xen/arch/x86/setup.c                | 54 ++++++++++++++++++++++++++++---------
 xen/include/asm-x86/asm_defns.h     | 10 +++----
 xen/include/asm-x86/cpufeature.h    |  4 +--
 4 files changed, 51 insertions(+), 19 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 3a250cb..0225974 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1433,6 +1433,7 @@ Set the serial transmit buffer size.
 > Default: `true`
 
 Flag to enable Supervisor Mode Execution Protection
+Use `smep=hvm` to allow SMEP use by HVM guests only.
 
 ### smap
 > `= <boolean>`
@@ -1440,6 +1441,7 @@ Flag to enable Supervisor Mode Execution Protection
 > Default: `true`
 
 Flag to enable Supervisor Mode Access Prevention
+Use `smap=hvm` to allow SMAP use by HVM guests only.
 
 ### snb\_igd\_quirk
 > `= <boolean> | cap | <integer>`
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 217c775..5256e99 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -61,14 +61,6 @@ boolean_param("nosmp", opt_nosmp);
 static unsigned int __initdata max_cpus;
 integer_param("maxcpus", max_cpus);
 
-/* smep: Enable/disable Supervisor Mode Execution Protection (default on). */
-static bool_t __initdata opt_smep = 1;
-boolean_param("smep", opt_smep);
-
-/* smap: Enable/disable Supervisor Mode Access Prevention (default on). */
-static bool_t __initdata opt_smap = 1;
-boolean_param("smap", opt_smap);
-
 unsigned long __read_mostly cr4_pv32_mask;
 
 /* Boot dom0 in pvh mode */
@@ -111,6 +103,44 @@ struct cpuinfo_x86 __read_mostly boot_cpu_data = { 0, 0, 0, 0, -1 };
 
 unsigned long __read_mostly mmu_cr4_features = XEN_MINIMAL_CR4;
 
+/* smep: Enable/disable Supervisor Mode Execution Protection (default on). */
+#define SMEP_HVM_ONLY (-1)
+static s8 __initdata opt_smep = 1;
+static void __init parse_smep_param(char *s)
+{
+    if ( !parse_bool(s) )
+    {
+        opt_smep = 0;
+    }
+    else if ( !strcmp(s, "hvm") )
+    {
+        opt_smep = SMEP_HVM_ONLY;
+    }
+
+    if ( opt_smep == 1 )
+        __set_bit(X86_FEATURE_XEN_SMEP, boot_cpu_data.x86_capability);
+}
+custom_param("smep", parse_smep_param);
+
+/* smap: Enable/disable Supervisor Mode Access Prevention (default on). */
+#define SMAP_HVM_ONLY (-1)
+static s8 __initdata opt_smap = 1;
+static void __init parse_smap_param(char *s)
+{
+    if ( !parse_bool(s) )
+    {
+        opt_smap = 0;
+    }
+    else if ( !strcmp(s, "hvm") )
+    {
+        opt_smap = SMAP_HVM_ONLY;
+    }
+
+    if ( opt_smap == 1 )
+        __set_bit(X86_FEATURE_XEN_SMAP, boot_cpu_data.x86_capability);
+}
+custom_param("smap", parse_smap_param);
+
 bool_t __read_mostly acpi_disabled;
 bool_t __initdata acpi_force;
 static char __initdata acpi_param[10] = "";
@@ -1403,12 +1433,12 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 
     if ( !opt_smep )
         setup_clear_cpu_cap(X86_FEATURE_SMEP);
-    if ( cpu_has_smep )
+    if ( boot_cpu_has(X86_FEATURE_XEN_SMEP) )
         set_in_cr4(X86_CR4_SMEP);
 
     if ( !opt_smap )
         setup_clear_cpu_cap(X86_FEATURE_SMAP);
-    if ( cpu_has_smap )
+    if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
         set_in_cr4(X86_CR4_SMAP);
 
     cr4_pv32_mask = mmu_cr4_features & XEN_CR4_PV32_BITS;
@@ -1550,7 +1580,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
      * This saves a large number of corner cases interactions with
      * copy_from_user().
      */
-    if ( cpu_has_smap )
+    if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
     {
         cr4_pv32_mask &= ~X86_CR4_SMAP;
         write_cr4(read_cr4() & ~X86_CR4_SMAP);
@@ -1570,7 +1600,7 @@ 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 )
+    if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
     {
         write_cr4(read_cr4() | X86_CR4_SMAP);
         cr4_pv32_mask |= X86_CR4_SMAP;
diff --git a/xen/include/asm-x86/asm_defns.h b/xen/include/asm-x86/asm_defns.h
index e36e78f..f1c6fa1 100644
--- a/xen/include/asm-x86/asm_defns.h
+++ b/xen/include/asm-x86/asm_defns.h
@@ -205,7 +205,7 @@ void ret_from_intr(void);
         .popsection;                                                   \
         .pushsection .altinstructions, "a";                            \
         altinstruction_entry 661b, 661b, X86_FEATURE_ALWAYS, 3, 0;     \
-        altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;       \
+        altinstruction_entry 661b, 662b, X86_FEATURE_XEN_SMAP, 3, 3;       \
         .popsection
 
 #define ASM_STAC ASM_AC(STAC)
@@ -217,21 +217,21 @@ void ret_from_intr(void);
         668: call cr4_pv32_restore;                                \
         .section .altinstructions, "a";                            \
         altinstruction_entry 667b, 667b, X86_FEATURE_ALWAYS, 5, 0; \
-        altinstruction_entry 667b, 668b, X86_FEATURE_SMEP, 5, 5;   \
-        altinstruction_entry 667b, 668b, X86_FEATURE_SMAP, 5, 5;   \
+        altinstruction_entry 667b, 668b, X86_FEATURE_XEN_SMEP, 5, 5;   \
+        altinstruction_entry 667b, 668b, X86_FEATURE_XEN_SMAP, 5, 5;   \
         .popsection
 
 #else
 static always_inline void clac(void)
 {
     /* Note: a barrier is implicit in alternative() */
-    alternative(ASM_NOP3, __stringify(__ASM_CLAC), X86_FEATURE_SMAP);
+    alternative(ASM_NOP3, __stringify(__ASM_CLAC), X86_FEATURE_XEN_SMAP);
 }
 
 static always_inline void stac(void)
 {
     /* Note: a barrier is implicit in alternative() */
-    alternative(ASM_NOP3, __stringify(__ASM_STAC), X86_FEATURE_SMAP);
+    alternative(ASM_NOP3, __stringify(__ASM_STAC), X86_FEATURE_XEN_SMAP);
 }
 #endif
 
diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index bcdf5d6..0f6810a 100644
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -17,6 +17,8 @@ XEN_CPUFEATURE(CPUID_FAULTING,  (FSCAPINTS+0)*32+ 6) /* cpuid faulting */
 XEN_CPUFEATURE(CLFLUSH_MONITOR, (FSCAPINTS+0)*32+ 7) /* clflush reqd with monitor */
 XEN_CPUFEATURE(APERFMPERF,      (FSCAPINTS+0)*32+ 8) /* APERFMPERF */
 XEN_CPUFEATURE(MFENCE_RDTSC,    (FSCAPINTS+0)*32+ 9) /* MFENCE synchronizes RDTSC */
+XEN_CPUFEATURE(XEN_SMEP,        (FSCAPINTS+0)*32+ 10) /* SMEP gets used by Xen itself */
+XEN_CPUFEATURE(XEN_SMAP,        (FSCAPINTS+0)*32+ 11) /* SMAP gets used by Xen itself */
 
 #define NCAPINTS (FSCAPINTS + 1) /* N 32-bit words worth of info */
 
@@ -66,8 +68,6 @@ XEN_CPUFEATURE(MFENCE_RDTSC,    (FSCAPINTS+0)*32+ 9) /* MFENCE synchronizes RDTS
 #define cpu_has_page1gb		boot_cpu_has(X86_FEATURE_PAGE1GB)
 #define cpu_has_fsgsbase	boot_cpu_has(X86_FEATURE_FSGSBASE)
 #define cpu_has_aperfmperf	boot_cpu_has(X86_FEATURE_APERFMPERF)
-#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) \
                                  && boot_cpu_has(X86_FEATURE_FFXSR))
-- 
1.9.1


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Re: [PATCH v5] xen/sm{e, a}p: allow disabling sm{e, a}p for Xen itself

[Jan Beulich]

>>> On 02.09.16 at 10:20, <he.chen@linux.intel.com> wrote:
> +/* smep: Enable/disable Supervisor Mode Execution Protection (default on). */
> +#define SMEP_HVM_ONLY (-1)
> +static s8 __initdata opt_smep = 1;
> +static void __init parse_smep_param(char *s)
> +{
> +    if ( !parse_bool(s) )
> +    {
> +        opt_smep = 0;
> +    }
> +    else if ( !strcmp(s, "hvm") )
> +    {
> +        opt_smep = SMEP_HVM_ONLY;
> +    }
> +
> +    if ( opt_smep == 1 )
> +        __set_bit(X86_FEATURE_XEN_SMEP, boot_cpu_data.x86_capability);
> +}
> +custom_param("smep", parse_smep_param);

The pointless braces are still there, and it still doesn't look like e.g.
"smep=0 smep=1" would work. Did you take the time to look at
other callers of parse_bool()?

And then - I'm sorry for not having noticed before - setting the
feature flag here means it won't get set if no "smep=" was given.
I.e. you rather want to move that ...

> @@ -1403,12 +1433,12 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>  
>      if ( !opt_smep )
>          setup_clear_cpu_cap(X86_FEATURE_SMEP);

... around here.

Jan


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