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Commit 8c06c774 authored by Dave Hansen's avatar Dave Hansen Committed by Ingo Molnar
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x86/pti: Leave kernel text global for !PCID 

Global pages are bad for hardening because they potentially let an
exploit read the kernel image via a Meltdown-style attack which
makes it easier to find gadgets.

But, global pages are good for performance because they reduce TLB
misses when making user/kernel transitions, especially when PCIDs
are not available, such as on older hardware, or where a hypervisor
has disabled them for some reason.

This patch implements a basic, sane policy: If you have PCIDs, you
only map a minimal amount of kernel text global.  If you do not have
PCIDs, you map all kernel text global.

This policy effectively makes PCIDs something that not only adds
performance but a little bit of hardening as well.

I ran a simple "lseek" microbenchmark[1] to test the benefit on
a modern Atom microserver.  Most of the benefit comes from applying
the series before this patch ("entry only"), but there is still a
signifiant benefit from this patch.

  No Global Lines (baseline  ): 6077741 lseeks/sec
  88 Global Lines (entry only): 7528609 lseeks/sec (+23.9%)
  94 Global Lines (this patch): 8433111 lseeks/sec (+38.8%)

[1.] https://github.com/antonblanchard/will-it-scale/blob/master/tests/lseek1.c



Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kees Cook <keescook@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nadav Amit <namit@vmware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20180406205518.E3D989EB@viggo.jf.intel.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 39114b7a

arch/x86/include/asm/pti.h

+2 −0
Original line number Diff line number Diff line
@@ -6,8 +6,10 @@ 
#ifdef CONFIG_PAGE_TABLE_ISOLATION

extern void pti_init(void);

extern void pti_check_boottime_disable(void);

extern void pti_clone_kernel_text(void);

#else

static inline void pti_check_boottime_disable(void) { }

static inline void pti_clone_kernel_text(void) { }

#endif



#endif /* __ASSEMBLY__ */

arch/x86/mm/init_64.c

+6 −0
Original line number Diff line number Diff line
@@ -1290,6 +1290,12 @@ void mark_rodata_ro(void) 
			(unsigned long) __va(__pa_symbol(_sdata)));



	debug_checkwx();



	/*

	 * Do this after all of the manipulation of the

	 * kernel text page tables are complete.

	 */

	pti_clone_kernel_text();

}



int kern_addr_valid(unsigned long addr)

arch/x86/mm/pti.c

+74 −4
Original line number Diff line number Diff line
@@ -66,12 +66,22 @@ static void __init pti_print_if_secure(const char *reason) 
		pr_info("%s\n", reason);

}



enum pti_mode {

	PTI_AUTO = 0,

	PTI_FORCE_OFF,

	PTI_FORCE_ON

} pti_mode;



void __init pti_check_boottime_disable(void)

{

	char arg[5];

	int ret;



	/* Assume mode is auto unless overridden. */

	pti_mode = PTI_AUTO;



	if (hypervisor_is_type(X86_HYPER_XEN_PV)) {

		pti_mode = PTI_FORCE_OFF;

		pti_print_if_insecure("disabled on XEN PV.");

		return;

	}
@@ -79,18 +89,23 @@ void __init pti_check_boottime_disable(void) 
	ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));

	if (ret > 0)  {

		if (ret == 3 && !strncmp(arg, "off", 3)) {

			pti_mode = PTI_FORCE_OFF;

			pti_print_if_insecure("disabled on command line.");

			return;

		}

		if (ret == 2 && !strncmp(arg, "on", 2)) {

			pti_mode = PTI_FORCE_ON;

			pti_print_if_secure("force enabled on command line.");

			goto enable;

		}

		if (ret == 4 && !strncmp(arg, "auto", 4))

		if (ret == 4 && !strncmp(arg, "auto", 4)) {

			pti_mode = PTI_AUTO;

			goto autosel;

		}

	}



	if (cmdline_find_option_bool(boot_command_line, "nopti")) {

		pti_mode = PTI_FORCE_OFF;

		pti_print_if_insecure("disabled on command line.");

		return;

	}
@@ -149,7 +164,7 @@ pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd) 
 *

 * Returns a pointer to a P4D on success, or NULL on failure.

 */

static __init p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)

static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)

{

	pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));

	gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
@@ -177,7 +192,7 @@ static __init p4d_t *pti_user_pagetable_walk_p4d(unsigned long address) 
 *

 * Returns a pointer to a PMD on success, or NULL on failure.

 */

static __init pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)

static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)

{

	gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);

	p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
@@ -267,7 +282,7 @@ static void __init pti_setup_vsyscall(void) 
static void __init pti_setup_vsyscall(void) { }

#endif



static void __init

static void

pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear)

{

	unsigned long addr;
@@ -372,6 +387,58 @@ static void __init pti_clone_entry_text(void) 
		       _PAGE_RW);

}



/*

 * Global pages and PCIDs are both ways to make kernel TLB entries

 * live longer, reduce TLB misses and improve kernel performance.

 * But, leaving all kernel text Global makes it potentially accessible

 * to Meltdown-style attacks which make it trivial to find gadgets or

 * defeat KASLR.

 *

 * Only use global pages when it is really worth it.

 */

static inline bool pti_kernel_image_global_ok(void)

{

	/*

	 * Systems with PCIDs get litlle benefit from global

	 * kernel text and are not worth the downsides.

	 */

	if (cpu_feature_enabled(X86_FEATURE_PCID))

		return false;



	/*

	 * Only do global kernel image for pti=auto.  Do the most

	 * secure thing (not global) if pti=on specified.

	 */

	if (pti_mode != PTI_AUTO)

		return false;



	/*

	 * K8 may not tolerate the cleared _PAGE_RW on the userspace

	 * global kernel image pages.  Do the safe thing (disable

	 * global kernel image).  This is unlikely to ever be

	 * noticed because PTI is disabled by default on AMD CPUs.

	 */

	if (boot_cpu_has(X86_FEATURE_K8))

		return false;



	return true;

}



/*

 * For some configurations, map all of kernel text into the user page

 * tables.  This reduces TLB misses, especially on non-PCID systems.

 */

void pti_clone_kernel_text(void)

{

	unsigned long start = PFN_ALIGN(_text);

	unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);



	if (!pti_kernel_image_global_ok())

		return;



	pti_clone_pmds(start, end, _PAGE_RW);

}



/*

 * This is the only user for it and it is not arch-generic like

 * the other set_memory.h functions.  Just extern it.
@@ -388,6 +455,9 @@ void pti_set_kernel_image_nonglobal(void) 
	unsigned long start = PFN_ALIGN(_text);

	unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);



	if (pti_kernel_image_global_ok())

		return;



	pr_debug("set kernel image non-global\n");



	set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);