linux-kernel - [tip:x86/mm] x86/mm/ptdump: Optimize check for W+X mappings for CONFIG

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <tip-243b72aae28ca1032284028323bb81c9235b15c9@git.kernel.org>
Date:   Thu, 16 Feb 2017 10:58:32 -0800
From:   tip-bot for Andrey Ryabinin <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     dvyukov@...gle.com, aryabinin@...tuozzo.com, hpa@...or.com,
        glider@...gle.com, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, tobias.regnery@...il.com,
        mingo@...nel.org, mark.rutland@....com
Subject: [tip:x86/mm] x86/mm/ptdump: Optimize check for W+X mappings for
 CONFIG_KASAN=y

Commit-ID:  243b72aae28ca1032284028323bb81c9235b15c9
Gitweb:     http://git.kernel.org/tip/243b72aae28ca1032284028323bb81c9235b15c9
Author:     Andrey Ryabinin <aryabinin@...tuozzo.com>
AuthorDate: Tue, 14 Feb 2017 13:08:38 +0300
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Thu, 16 Feb 2017 19:53:25 +0100

x86/mm/ptdump: Optimize check for W+X mappings for CONFIG_KASAN=y

Enabling both DEBUG_WX=y and KASAN=y options significantly increases
boot time (dozens of seconds at least).
KASAN fills kernel page tables with repeated values to map several
TBs of the virtual memory to the single kasan_zero_page:

    kasan_zero_pud ->
        kasan_zero_pmd->
            kasan_zero_pte->
                kasan_zero_page

So, the page table walker used to find W+X mapping check the same
kasan_zero_p?d page table entries a lot more than once.
With patch pud walker will skip the pud if it has the same value as
the previous one . Skipping done iff we search for W+X mappings,
so this optimization won't affect the page table dump via debugfs.

This dropped time spend in W+X check from ~30 sec to reasonable 0.1 sec:

Before:
[    4.579991] Freeing unused kernel memory: 1000K
[   35.257523] x86/mm: Checked W+X mappings: passed, no W+X pages found.

After:
[    5.138756] Freeing unused kernel memory: 1000K
[    5.266496] x86/mm: Checked W+X mappings: passed, no W+X pages found.

Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
Reviewed-by: Alexander Potapenko <glider@...gle.com>
Cc: Mark Rutland <mark.rutland@....com>
Cc: kasan-dev@...glegroups.com
Cc: Tobias Regnery <tobias.regnery@...il.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>
Link: http://lkml.kernel.org/r/20170214100839.17186-1-aryabinin@virtuozzo.com
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

---
 arch/x86/mm/dump_pagetables.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index 8aa6bea..0813534 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -327,18 +327,31 @@ static void walk_pmd_level(struct seq_file *m, struct pg_state *st, pud_t addr,
 
 #if PTRS_PER_PUD > 1
 
+/*
+ * This is an optimization for CONFIG_DEBUG_WX=y + CONFIG_KASAN=y
+ * KASAN fills page tables with the same values. Since there is no
+ * point in checking page table more than once we just skip repeated
+ * entries. This saves us dozens of seconds during boot.
+ */
+static bool pud_already_checked(pud_t *prev_pud, pud_t *pud, bool checkwx)
+{
+	return checkwx && prev_pud && (pud_val(*prev_pud) == pud_val(*pud));
+}
+
 static void walk_pud_level(struct seq_file *m, struct pg_state *st, pgd_t addr,
 							unsigned long P)
 {
 	int i;
 	pud_t *start;
 	pgprotval_t prot;
+	pud_t *prev_pud = NULL;
 
 	start = (pud_t *) pgd_page_vaddr(addr);
 
 	for (i = 0; i < PTRS_PER_PUD; i++) {
 		st->current_address = normalize_addr(P + i * PUD_LEVEL_MULT);
-		if (!pud_none(*start)) {
+		if (!pud_none(*start) &&
+		    !pud_already_checked(prev_pud, start, st->check_wx)) {
 			if (pud_large(*start) || !pud_present(*start)) {
 				prot = pud_flags(*start);
 				note_page(m, st, __pgprot(prot), 2);
@@ -349,6 +362,7 @@ static void walk_pud_level(struct seq_file *m, struct pg_state *st, pgd_t addr,
 		} else
 			note_page(m, st, __pgprot(0), 2);
 
+		prev_pud = start;
 		start++;
 	}
 }