[PATCH] solve SMAP/SMEP issues with 32-bit pv guests

[PATCH] solve SMAP/SMEP issues with 32-bit pv guests

[He Chen]

This patch is going to solve SMAP/SMEP issues with 32-bit pv guests by
adding new xen command line options "xen_smap" and "xen_smep".

For the details, please see: https://lists.xen.org/archives/html/xen-devel/2016-06/msg03441.html

I am sorry that I don't have 32-bit PV environment to test this patch,
I would really appreciate it if someone could help verify this patch :)

He Chen (1):
  xen: enable/disable SMAP/SMEP for Xen itself

 docs/misc/xen-command-line.markdown | 14 ++++++++++++++
 xen/arch/x86/setup.c                | 12 ++++++++++--
 2 files changed, 24 insertions(+), 2 deletions(-)

-- 
1.9.1


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[PATCH] xen: enable/disable SMAP/SMEP for Xen itself

[He Chen]

SMAP/SMEP may affect the 32-bit pv guests.
Users can determine whether turn SMAP/SMEP on for Xen hyperviosr when
running 32-bit pv guests.

Signed-off-by: He Chen <he.chen@linux.intel.com>
---
 docs/misc/xen-command-line.markdown | 14 ++++++++++++++
 xen/arch/x86/setup.c                | 12 ++++++++++--
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 3a250cb..a48c4aa 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1434,6 +1434,13 @@ Set the serial transmit buffer size.
 
 Flag to enable Supervisor Mode Execution Protection
 
+### xen_smep
+> `= <boolean>`
+
+> Default: `true`
+
+Flag to enable SMEP for Xen itself
+
 ### smap
 > `= <boolean>`
 
@@ -1441,6 +1448,13 @@ Flag to enable Supervisor Mode Execution Protection
 
 Flag to enable Supervisor Mode Access Prevention
 
+### xen_smap
+> `= <boolean>`
+
+> Default: `true`
+
+Flag to enable SMAP for Xen itself
+
 ### snb\_igd\_quirk
 > `= <boolean> | cap | <integer>`
 
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 217c775..84debc3 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -65,10 +65,18 @@ integer_param("maxcpus", max_cpus);
 static bool_t __initdata opt_smep = 1;
 boolean_param("smep", opt_smep);
 
+/* xen_smep: Enable/disable SMEP for Xen itself (default on). */
+static bool_t __initdata opt_xen_smep = 1;
+boolean_param("xen_smep", opt_xen_smep);
+
 /* smap: Enable/disable Supervisor Mode Access Prevention (default on). */
 static bool_t __initdata opt_smap = 1;
 boolean_param("smap", opt_smap);
 
+/* xen_smap: Enable/disable SMAP for Xen itself (default on). */
+static bool_t __initdata opt_xen_smap = 1;
+boolean_param("xen_smap", opt_xen_smap);
+
 unsigned long __read_mostly cr4_pv32_mask;
 
 /* Boot dom0 in pvh mode */
@@ -1403,12 +1411,12 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 
     if ( !opt_smep )
         setup_clear_cpu_cap(X86_FEATURE_SMEP);
-    if ( cpu_has_smep )
+    if ( cpu_has_smep && opt_xen_smep )
         set_in_cr4(X86_CR4_SMEP);
 
     if ( !opt_smap )
         setup_clear_cpu_cap(X86_FEATURE_SMAP);
-    if ( cpu_has_smap )
+    if ( cpu_has_smap && opt_xen_smap )
         set_in_cr4(X86_CR4_SMAP);
 
     cr4_pv32_mask = mmu_cr4_features & XEN_CR4_PV32_BITS;
-- 
1.9.1


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Re: [PATCH] xen: enable/disable SMAP/SMEP for Xen itself

[Jan Beulich]

>>> On 09.08.16 at 11:13, <he.chen@linux.intel.com> wrote:
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -1434,6 +1434,13 @@ Set the serial transmit buffer size.
>  
>  Flag to enable Supervisor Mode Execution Protection
>  
> +### xen_smep
> +> `= <boolean>`
> +
> +> Default: `true`
> +
> +Flag to enable SMEP for Xen itself
> +
>  ### smap
>  > `= <boolean>`
>  
> @@ -1441,6 +1448,13 @@ Flag to enable Supervisor Mode Execution Protection
>  
>  Flag to enable Supervisor Mode Access Prevention
>  
> +### xen_smap
> +> `= <boolean>`
> +
> +> Default: `true`
> +
> +Flag to enable SMAP for Xen itself
> +
>  ### snb\_igd\_quirk
>  > `= <boolean> | cap | <integer>`
>  

If these were to stay, please obey to the (mostly) alphabetical
sorting. But I'd much rather see the "smap=" and "smep="
options enhanced, e.g. with allowing "hvm" as a value alongside
the currently permitted boolean indicators.

> @@ -1403,12 +1411,12 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>  
>      if ( !opt_smep )
>          setup_clear_cpu_cap(X86_FEATURE_SMEP);
> -    if ( cpu_has_smep )
> +    if ( cpu_has_smep && opt_xen_smep )
>          set_in_cr4(X86_CR4_SMEP);
>  
>      if ( !opt_smap )
>          setup_clear_cpu_cap(X86_FEATURE_SMAP);
> -    if ( cpu_has_smap )
> +    if ( cpu_has_smap && opt_xen_smap )
>          set_in_cr4(X86_CR4_SMAP);
>  
>      cr4_pv32_mask = mmu_cr4_features & XEN_CR4_PV32_BITS;

It's extremely hard to believe that this would be all it takes:
Already the two cpu_has_smap uses further down in this file
would need similar adjustment afaict. Furthermore
mmu_cr4_features (which now won't have the two flags set
if their use for Xen gets disabled) gets used, among other
places, in vmx.c. If indeed no other changes were necessary,
the minimum imo would be for you to justify this in the patch
description.

Jan


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Re: [PATCH] solve SMAP/SMEP issues with 32-bit pv guests

[George Dunlap]

On Tue, Aug 9, 2016 at 10:13 AM, He Chen <he.chen@linux.intel.com> wrote:
> This patch is going to solve SMAP/SMEP issues with 32-bit pv guests by
> adding new xen command line options "xen_smap" and "xen_smep".
>
> For the details, please see: https://lists.xen.org/archives/html/xen-devel/2016-06/msg03441.html
>
> I am sorry that I don't have 32-bit PV environment to test this patch,
> I would really appreciate it if someone could help verify this patch :)
>
> He Chen (1):
>   xen: enable/disable SMAP/SMEP for Xen itself
>
>  docs/misc/xen-command-line.markdown | 14 ++++++++++++++
>  xen/arch/x86/setup.c                | 12 ++++++++++--
>  2 files changed, 24 insertions(+), 2 deletions(-)
>
> --
> 1.9.1

There doesn't seem to be any actual content here.

 -George

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