RE: [Xen-devel] [Patch] Enable SMEP CPU feature support for XEN itself

["Li, Xin" <xin.li@xxxxxxxxx>]

> > Intel new CPU supports SMEP (Supervisor Mode Execution Protection). SMEP
> > prevents
> > kernel from executing code in user. Updated Intel SDM describes this CPU
> > feature.
> > The document will be published soon.
> 
> Then I can hardly be expected to review this patch. I have no idea what it
> does!

Let me explain it a bit:  when CPU running in kernel mode tries to fetch and 
execute
code whose user bit in all level page table entries are set, it will trigger an 
instruction
fetch page fault.  one of its usage is to prevent application to use kernel 
null pointer
to get root privilege.
Thanks!
-Xin

> 
>  -- Keir
> 
> > This patch enables SMEP in Xen to protect Xen hypervisor from executing pv
> > guest
> > code, and kills a pv guest triggering SMEP fault.
> >
> >  Signed-off-by: Yang, Wei <wei.y.yang@xxxxxxxxx>
> >  Signed-off-by: Shan, Haitao <haitao.shan@xxxxxxxxx>
> >  Signed-off-by: Li, Xin <xin.li@xxxxxxxxx>
> >
> > ---
> >  arch/x86/cpu/common.c        |   16 ++++++++++++++++
> >  arch/x86/traps.c             |   43
> > +++++++++++++++++++++++++++++++++++++++++--
> >  include/asm-x86/cpufeature.h |    5 ++++-
> >  include/asm-x86/processor.h  |    1 +
> >  4 files changed, 62 insertions(+), 3 deletions(-)
> >
> > diff -r d4f6310f1ef5 xen/arch/x86/cpu/common.c
> > --- a/xen/arch/x86/cpu/common.c Wed Jun 01 11:11:43 2011 +0100
> > +++ b/xen/arch/x86/cpu/common.c Wed Jun 01 19:53:52 2011 +0800
> > @@ -28,6 +28,9 @@
> >  integer_param("cpuid_mask_ext_ecx", opt_cpuid_mask_ext_ecx);
> >  unsigned int __devinitdata opt_cpuid_mask_ext_edx = ~0u;
> >  integer_param("cpuid_mask_ext_edx", opt_cpuid_mask_ext_edx);
> > +/* nosmep: if true, Intel SMEP is disabled. */
> > +static bool_t __initdata disable_smep;
> > +boolean_param("nosmep", disable_smep);
> >
> >  struct cpu_dev * cpu_devs[X86_VENDOR_NUM] = {};
> >
> > @@ -222,6 +225,17 @@
> > c->x86_capability[4] = cap4;
> >  }
> >
> > +static void __cpuinit setup_smep(struct cpuinfo_x86 *c)
> > +{
> > + if ( cpu_has(c, X86_FEATURE_SMEP) ) {
> > +  if( unlikely(disable_smep) ) {
> > +   setup_clear_cpu_cap(X86_FEATURE_SMEP);
> > +   clear_in_cr4(X86_CR4_SMEP);
> > +  } else
> > +   set_in_cr4(X86_CR4_SMEP);
> > + }
> > +}
> > +
> >  void __cpuinit generic_identify(struct cpuinfo_x86 * c)
> >  {
> > u32 tfms, xlvl, capability, excap, ebx;
> > @@ -268,6 +282,8 @@
> > c->x86_capability[X86_FEATURE_FSGSBASE / 32] = ebx;
> > }
> >
> > + setup_smep(c);
> > +
> > early_intel_workaround(c);
> >
> >  #ifdef CONFIG_X86_HT
> > diff -r d4f6310f1ef5 xen/arch/x86/traps.c
> > --- a/xen/arch/x86/traps.c Wed Jun 01 11:11:43 2011 +0100
> > +++ b/xen/arch/x86/traps.c Wed Jun 01 19:53:52 2011 +0800
> > @@ -1195,8 +1195,16 @@
> >      if ( ((l3e_get_flags(l3e) & required_flags) != required_flags) ||
> >           (l3e_get_flags(l3e) & disallowed_flags) )
> >          return 0;
> > -    if ( l3e_get_flags(l3e) & _PAGE_PSE )
> > +    if ( l3e_get_flags(l3e) & _PAGE_PSE ) {
> > +        /* SMEP fault error code 10001b */
> > +        if ( (error_code & PFEC_insn_fetch) &&
> > +             !(error_code & PFEC_user_mode) &&
> > +             cpu_has_smep &&
> > +             (_PAGE_USER & l4e_get_flags(l4e) & l3e_get_flags(l3e)) )
> > +            return 2;
> > +
> >          return 1;
> > +    }
> >  #endif
> >  #endif
> >
> > @@ -1207,8 +1215,21 @@
> >      if ( ((l2e_get_flags(l2e) & required_flags) != required_flags) ||
> >           (l2e_get_flags(l2e) & disallowed_flags) )
> >          return 0;
> > -    if ( l2e_get_flags(l2e) & _PAGE_PSE )
> > +    if ( l2e_get_flags(l2e) & _PAGE_PSE ) {
> > +        /* SMEP fault error code 10001b */
> > +        if ( (error_code & PFEC_insn_fetch) &&
> > +             !(error_code & PFEC_user_mode) &&
> > +             cpu_has_smep &&
> > +             (_PAGE_USER &
> > +#if CONFIG_PAGING_LEVELS >= 4
> > +              l4e_get_flags(l4e) &
> > +              l3e_get_flags(l3e) &
> > +#endif
> > +              l2e_get_flags(l2e)) )
> > +            return 2;
> > +
> >          return 1;
> > +    }
> >
> >      l1t = map_domain_page(mfn);
> >      l1e = l1e_read_atomic(&l1t[l1_table_offset(addr)]);
> > @@ -1218,6 +1239,18 @@
> >           (l1e_get_flags(l1e) & disallowed_flags) )
> >          return 0;
> >
> > +    /* SMEP fault error code 10001b */
> > +    if ( (error_code & PFEC_insn_fetch) &&
> > +         !(error_code & PFEC_user_mode) &&
> > +         cpu_has_smep &&
> > +         (_PAGE_USER &
> > +#if CONFIG_PAGING_LEVELS >= 4
> > +          l4e_get_flags(l4e) &
> > +          l3e_get_flags(l3e) &
> > +#endif
> > +          l2e_get_flags(l2e) & l1e_get_flags(l1e)) )
> > +        return 2;
> > +
> >      return 1;
> >  }
> >
> > @@ -1235,6 +1268,12 @@
> >      is_spurious = __spurious_page_fault(addr, error_code);
> >      local_irq_restore(flags);
> >
> > +    if ( is_spurious == 2 ) {
> > +        printk("SMEP fault at address %lx, crashing current domain %d\n",
> > +               addr, current->domain->domain_id);
> > +        domain_crash_synchronous();
> > +    }
> > +
> >      return is_spurious;
> >  }
> >
> > diff -r d4f6310f1ef5 xen/include/asm-x86/cpufeature.h
> > --- a/xen/include/asm-x86/cpufeature.h Wed Jun 01 11:11:43 2011 +0100
> > +++ b/xen/include/asm-x86/cpufeature.h Wed Jun 01 19:53:52 2011 +0800
> > @@ -141,8 +141,9 @@
> >  #define X86_FEATURE_TBM         (6*32+21) /* trailing bit manipulations */
> >  #define X86_FEATURE_TOPOEXT     (6*32+22) /* topology extensions CPUID
> leafs
> > */
> >
> > -/* Intel-defined CPU features, CPUID level 0x00000007:0 (ebx), word 9 */
> > +/* Intel-defined CPU features, CPUID level 0x00000007:0 (ebx), word 7 */
> >  #define X86_FEATURE_FSGSBASE (7*32+ 0) /* {RD,WR}{FS,GS}BASE instructions
> */
> > +#define X86_FEATURE_SMEP (7*32+ 7) /* Supervisor Mode Execution Protection
> */
> >
> >  #define cpu_has(c, bit)  test_bit(bit, (c)->x86_capability)
> >  #define boot_cpu_has(bit) test_bit(bit, boot_cpu_data.x86_capability)
> > @@ -201,6 +202,8 @@
> >  #define cpu_has_fsgsbase boot_cpu_has(X86_FEATURE_FSGSBASE)
> >  #endif
> >
> > +#define cpu_has_smep            boot_cpu_has(X86_FEATURE_SMEP)
> > +
> >  #define cpu_has_ffxsr           ((boot_cpu_data.x86_vendor ==
> X86_VENDOR_AMD)
> > \
> >                                   &&
> boot_cpu_has(X86_FEATURE_FFXSR))
> >
> > diff -r d4f6310f1ef5 xen/include/asm-x86/processor.h
> > --- a/xen/include/asm-x86/processor.h Wed Jun 01 11:11:43 2011 +0100
> > +++ b/xen/include/asm-x86/processor.h Wed Jun 01 19:53:52 2011 +0800
> > @@ -85,6 +85,7 @@
> >  #define X86_CR4_SMXE  0x4000  /* enable SMX */
> >  #define X86_CR4_FSGSBASE 0x10000 /* enable {rd,wr}{fs,gs}base */
> >  #define X86_CR4_OSXSAVE 0x40000 /* enable XSAVE/XRSTOR */
> > +#define X86_CR4_SMEP  0x100000/* enable SMEP */
> >
> >  /*
> >   * Trap/fault mnemonics.
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> 


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel