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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH v3] xen: support enabling SMEP/SMAP for HVM only
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.
A 32-bit PV guest will suffer unknown SMEP/SMAP page fault when guest
kernel attempt to access user address although SMEP/SMAP is close for
PV guests already.
This patch is going to support enabling SMEP/SMAP for HVM but disabling
them for Xen hypervisor. Users can choose whether opening them for Xen,
especially when they are going to run 32-bit PV guests.
Signed-off-by: He Chen <he.chen@xxxxxxxxxxxxxxx>
---
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 | 61 ++++++++++++++++++++++++++++++++-----
xen/arch/x86/traps.c | 7 +++++
xen/include/asm-x86/setup.h | 6 ++++
4 files changed, 68 insertions(+), 8 deletions(-)
diff --git a/docs/misc/xen-command-line.markdown
b/docs/misc/xen-command-line.markdown
index 3a250cb..b15f3e7 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 enable SMEP for 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 enable SMAP for 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..a428558 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -62,12 +62,12 @@ 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);
+static void parse_smep_param(char *s);
+custom_param("smep", parse_smep_param);
/* smap: Enable/disable Supervisor Mode Access Prevention (default on). */
-static bool_t __initdata opt_smap = 1;
-boolean_param("smap", opt_smap);
+static void parse_smap_param(char *s);
+custom_param("smap", parse_smap_param);
unsigned long __read_mostly cr4_pv32_mask;
@@ -111,6 +111,40 @@ struct cpuinfo_x86 __read_mostly boot_cpu_data = { 0, 0,
0, 0, -1 };
unsigned long __read_mostly mmu_cr4_features = XEN_MINIMAL_CR4;
+int opt_smep = 1;
+static void __init parse_smep_param(char *s)
+{
+ if ( !strcmp(s, "hvm") )
+ {
+ opt_smep = SMEP_HVM_ONLY;
+ }
+ else if ( !parse_bool(s) )
+ {
+ opt_smep = 0;
+ }
+ else if ( parse_bool(s) && opt_smep != SMEP_HVM_ONLY )
+ {
+ opt_smep = 1;
+ }
+}
+
+int opt_smap = 1;
+static void __init parse_smap_param(char *s)
+{
+ if ( !strcmp(s, "hvm") )
+ {
+ opt_smap = SMAP_HVM_ONLY;
+ }
+ else if ( !parse_bool(s) )
+ {
+ opt_smap = 0;
+ }
+ else if ( parse_bool(s) && opt_smap != SMAP_HVM_ONLY )
+ {
+ opt_smap = 1;
+ }
+}
+
bool_t __read_mostly acpi_disabled;
bool_t __initdata acpi_force;
static char __initdata acpi_param[10] = "";
@@ -1403,12 +1437,12 @@ 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_smep != SMEP_HVM_ONLY )
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_smap != SMAP_HVM_ONLY )
set_in_cr4(X86_CR4_SMAP);
cr4_pv32_mask = mmu_cr4_features & XEN_CR4_PV32_BITS;
@@ -1430,8 +1464,19 @@ void __init noreturn __start_xen(unsigned long mbi_p)
arch_init_memory();
+ /*
+ * Temporarily clear SMAP in internal feature bitmap to avoid
+ * patching unnecessary SMAP instructions when SMAP is disabled in
+ * Xen hypervisor.
+ */
+ if ( opt_smap == SMAP_HVM_ONLY )
+ __clear_bit(X86_FEATURE_SMAP, boot_cpu_data.x86_capability);
+
alternative_instructions();
+ if ( opt_smap == SMAP_HVM_ONLY )
+ __set_bit(X86_FEATURE_SMAP, boot_cpu_data.x86_capability);
+
local_irq_enable();
pt_pci_init();
@@ -1550,7 +1595,7 @@ 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 ( cpu_has_smap && opt_smap != SMAP_HVM_ONLY )
{
cr4_pv32_mask &= ~X86_CR4_SMAP;
write_cr4(read_cr4() & ~X86_CR4_SMAP);
@@ -1570,7 +1615,7 @@ 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 ( cpu_has_smap && opt_smap != SMAP_HVM_ONLY )
{
write_cr4(read_cr4() | X86_CR4_SMAP);
cr4_pv32_mask |= X86_CR4_SMAP;
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index e822719..58989c4 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -77,6 +77,7 @@
#include <public/arch-x86/cpuid.h>
#include <asm/cpuid.h>
#include <xsm/xsm.h>
+#include <asm/setup.h>
/*
* opt_nmi: one of 'ignore', 'dom0', or 'fatal'.
@@ -1098,6 +1099,12 @@ void pv_cpuid(struct cpu_user_regs *regs)
b |= (host_featureset[FEATURESET_7b0] &
special_features[FEATURESET_7b0]);
+ if ( opt_smep == SMEP_HVM_ONLY )
+ b &= ~cpufeat_mask(X86_FEATURE_SMEP);
+
+ if ( opt_smap == SMAP_HVM_ONLY )
+ b &= ~cpufeat_mask(X86_FEATURE_SMAP);
+
c &= pv_featureset[FEATURESET_7c0];
if ( !is_pvh_domain(currd) )
diff --git a/xen/include/asm-x86/setup.h b/xen/include/asm-x86/setup.h
index c65b79c..7102b5c 100644
--- a/xen/include/asm-x86/setup.h
+++ b/xen/include/asm-x86/setup.h
@@ -51,6 +51,12 @@ void microcode_grab_module(
extern uint8_t kbd_shift_flags;
+#define SMEP_HVM_ONLY -1
+extern int opt_smep;
+
+#define SMAP_HVM_ONLY -1
+extern int opt_smap;
+
#ifdef NDEBUG
# define highmem_start 0
#else
--
1.9.1
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