On 03/06/2011 14:57, "Li, Xin" <xin.li@xxxxxxxxx> wrote:
> Intel new CPU supports SMEP (Supervisor Mode Execution Protection). SMEP
> prevents software operating with CPL < 3 (supervisor mode) from fetching
> instructions from any linear address with a valid translation for which the
> U/S
> flag (bit 2) is 1 in every paging-structure entry controlling the translation
> for the linear address.
>
> This patch enables SMEP in Xen to protect Xen hypervisor from executing pv
> guest
> instructions, whose translation paging-structure entries' U/S flags are all
> set.
Xin,
I have attached a modified version of your patch which I believe deals with
a number of issues. I briefly outline the major ones here:
1. The initialisation in cpu/common.c is misguided. Features like
set_in_cr4(), setup_clear_cpu_cap(), and cpu_has_xxx, are designed to allow
the feature initialisation to happen close in code to where the feature is
used. Hence I have moved the initialisation into traps.c:trap_init().
2. I have renamed the command-line option to 'smep'. Boolean options can be
inverted by prepending 'no-', or appending '={0,false,off,no}'. We don't
generally encode the negative within the base option name string.
3. I have pushed interpretation of the pf_type enumeration out to the
callers of spurious_page_fault(). This is because a SMEP fault while Xen is
executing should *crash Xen*. So that's what I do. Further, when it is a
guest fault, we should domain_crash() not domain_crash_synchronous() (which
is generally a more dangerous function for Xen's own health).
More generally, I wonder whether SMEP should be enabled only for guests
(even PV guests) which detect it via CPUID and proactively enable it via
their virtualised CR4? I mean, it is off in real hardware by default for a
reason: backward compatibility. Furthermore, we only detect spurious page
faults for buggy old PV guests, the rest will get the SMEP fault punted up
to them, which seems odd if they don't understand SMEP. What do you think?
I have cc'ed Jan since he has commented on previous versions of this patch.
-- Keir
> It crashes 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>
>
> diff -r 0791661a32d8 xen/arch/x86/cpu/common.c
> --- a/xen/arch/x86/cpu/common.c Thu Jun 02 18:46:35 2011 +0100
> +++ b/xen/arch/x86/cpu/common.c Fri Jun 03 19:59:43 2011 +0800
> @@ -222,6 +222,22 @@ static void __init early_cpu_detect(void
> c->x86_capability[4] = cap4;
> }
>
> +/* nosmep: if true, Intel SMEP is disabled. */
> +static bool_t disable_smep __cpuinitdata;
> +boolean_param("nosmep", disable_smep);
> +
> +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);
> + else {
> + set_in_cr4(X86_CR4_SMEP);
> + mmu_cr4_features |= X86_CR4_SMEP;
> + }
> + }
> +}
> +
> void __cpuinit generic_identify(struct cpuinfo_x86 * c)
> {
> u32 tfms, xlvl, capability, excap, ebx;
> @@ -268,6 +284,8 @@ void __cpuinit generic_identify(struct c
> c->x86_capability[X86_FEATURE_FSGSBASE / 32] = ebx;
> }
>
> + setup_smep(c);
> +
> early_intel_workaround(c);
>
> #ifdef CONFIG_X86_HT
> diff -r 0791661a32d8 xen/arch/x86/traps.c
> --- a/xen/arch/x86/traps.c Thu Jun 02 18:46:35 2011 +0100
> +++ b/xen/arch/x86/traps.c Fri Jun 03 19:59:43 2011 +0800
> @@ -1139,7 +1139,13 @@ static int handle_gdt_ldt_mapping_fault(
> (((va) >= HYPERVISOR_VIRT_START))
> #endif
>
> -static int __spurious_page_fault(
> +enum pf_type {
> + real_page_fault,
> + smep_fault,
> + spurious_fault
> +};
> +
> +static enum pf_type __page_fault_type(
> unsigned long addr, unsigned int error_code)
> {
> unsigned long mfn, cr3 = read_cr3();
> @@ -1151,7 +1157,7 @@ static int __spurious_page_fault(
> #endif
> l2_pgentry_t l2e, *l2t;
> l1_pgentry_t l1e, *l1t;
> - unsigned int required_flags, disallowed_flags;
> + unsigned int required_flags, disallowed_flags, u_flag;
>
> /*
> * We do not take spurious page faults in IRQ handlers as we do not
> @@ -1159,11 +1165,11 @@ static int __spurious_page_fault(
> * map_domain_page() is not IRQ-safe.
> */
> if ( in_irq() )
> - return 0;
> + return real_page_fault;
>
> /* Reserved bit violations are never spurious faults. */
> if ( error_code & PFEC_reserved_bit )
> - return 0;
> + return real_page_fault;
>
> required_flags = _PAGE_PRESENT;
> if ( error_code & PFEC_write_access )
> @@ -1175,6 +1181,8 @@ static int __spurious_page_fault(
> if ( error_code & PFEC_insn_fetch )
> disallowed_flags |= _PAGE_NX_BIT;
>
> + u_flag = _PAGE_USER;
> +
> mfn = cr3 >> PAGE_SHIFT;
>
> #if CONFIG_PAGING_LEVELS >= 4
> @@ -1184,7 +1192,9 @@ static int __spurious_page_fault(
> unmap_domain_page(l4t);
> if ( ((l4e_get_flags(l4e) & required_flags) != required_flags) ||
> (l4e_get_flags(l4e) & disallowed_flags) )
> - return 0;
> + return real_page_fault;
> +
> + u_flag &= l4e_get_flags(l4e);
> #endif
>
> #if CONFIG_PAGING_LEVELS >= 3
> @@ -1197,13 +1207,23 @@ static int __spurious_page_fault(
> unmap_domain_page(l3t);
> #if CONFIG_PAGING_LEVELS == 3
> if ( !(l3e_get_flags(l3e) & _PAGE_PRESENT) )
> - return 0;
> + return real_page_fault;
> #else
> if ( ((l3e_get_flags(l3e) & required_flags) != required_flags) ||
> (l3e_get_flags(l3e) & disallowed_flags) )
> - return 0;
> - if ( l3e_get_flags(l3e) & _PAGE_PSE )
> - return 1;
> + return real_page_fault;
> +
> + u_flag &= l3e_get_flags(l3e);
> +
> + 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 && u_flag )
> + return smep_fault;
> +
> + return spurious_fault;
> + }
> #endif
> #endif
>
> @@ -1213,9 +1233,19 @@ static int __spurious_page_fault(
> unmap_domain_page(l2t);
> if ( ((l2e_get_flags(l2e) & required_flags) != required_flags) ||
> (l2e_get_flags(l2e) & disallowed_flags) )
> - return 0;
> - if ( l2e_get_flags(l2e) & _PAGE_PSE )
> - return 1;
> + return real_page_fault;
> +
> + u_flag &= l2e_get_flags(l2e);
> +
> + 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 && u_flag )
> + return smep_fault;
> +
> + return spurious_fault;
> + }
>
> l1t = map_domain_page(mfn);
> l1e = l1e_read_atomic(&l1t[l1_table_offset(addr)]);
> @@ -1223,26 +1253,37 @@ static int __spurious_page_fault(
> unmap_domain_page(l1t);
> if ( ((l1e_get_flags(l1e) & required_flags) != required_flags) ||
> (l1e_get_flags(l1e) & disallowed_flags) )
> - return 0;
> -
> - return 1;
> + return real_page_fault;
> +
> + u_flag &= l1e_get_flags(l1e);
> +
> + /* SMEP fault error code 10001b */
> + if ( (error_code & PFEC_insn_fetch) &&
> + !(error_code & PFEC_user_mode) &&
> + cpu_has_smep && u_flag )
> + return smep_fault;
> +
> + return spurious_fault;
> }
>
> static int spurious_page_fault(
> unsigned long addr, unsigned int error_code)
> {
> unsigned long flags;
> - int is_spurious;
> + enum pf_type pf_type;
>
> /*
> * Disabling interrupts prevents TLB flushing, and hence prevents
> * page tables from becoming invalid under our feet during the walk.
> */
> local_irq_save(flags);
> - is_spurious = __spurious_page_fault(addr, error_code);
> + pf_type = __page_fault_type(addr, error_code);
> local_irq_restore(flags);
>
> - return is_spurious;
> + if ( pf_type == smep_fault )
> + domain_crash_synchronous();
> +
> + return (pf_type == spurious_fault);
> }
>
> static int fixup_page_fault(unsigned long addr, struct cpu_user_regs *regs)
> diff -r 0791661a32d8 xen/include/asm-x86/cpufeature.h
> --- a/xen/include/asm-x86/cpufeature.h Thu Jun 02 18:46:35 2011 +0100
> +++ b/xen/include/asm-x86/cpufeature.h Fri Jun 03 19:59:43 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)
> @@ -202,6 +203,8 @@
> #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 0791661a32d8 xen/include/asm-x86/domain.h
> --- a/xen/include/asm-x86/domain.h Thu Jun 02 18:46:35 2011 +0100
> +++ b/xen/include/asm-x86/domain.h Fri Jun 03 19:59:43 2011 +0800
> @@ -527,12 +527,14 @@ unsigned long pv_guest_cr4_fixup(const s
> /* Convert between guest-visible and real CR4 values. */
> #define pv_guest_cr4_to_real_cr4(v) \
> (((v)->arch.pv_vcpu.ctrlreg[4] \
> - | (mmu_cr4_features & (X86_CR4_PGE | X86_CR4_PSE)) \
> - | ((v)->domain->arch.vtsc ? X86_CR4_TSD : 0) \
> - | ((xsave_enabled(v))? X86_CR4_OSXSAVE : 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))
> + | (mmu_cr4_features \
> + & (X86_CR4_PGE | X86_CR4_PSE | X86_CR4_SMEP)) \
> + | ((v)->domain->arch.vtsc ? X86_CR4_TSD : 0) \
> + | ((xsave_enabled(v))? X86_CR4_OSXSAVE : 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))
>
> void domain_cpuid(struct domain *d,
> unsigned int input,
> diff -r 0791661a32d8 xen/include/asm-x86/processor.h
> --- a/xen/include/asm-x86/processor.h Thu Jun 02 18:46:35 2011 +0100
> +++ b/xen/include/asm-x86/processor.h Fri Jun 03 19:59:43 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.
00-xen-smep
Description: Binary data
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