Re: KASAN vs. boot-time switching between 4- and 5-level paging

[Andrey Ryabinin]

On 07/11/2017 10:05 PM, Kirill A. Shutemov wrote:
>>> Can use your Signed-off-by for a [cleaned up version of your] patch?
>>
>> Sure.
> 
> Another KASAN-releated issue: dumping page tables for KASAN shadow memory
> region takes unreasonable time due to kasan_zero_p?? mapped there.
> 
> The patch below helps. Any objections?
> 

Well, page tables dump doesn't work at all on 5-level paging.
E.g. I've got this nonsense: 

....
---[ Kernel Space ]---
0xffff800000000000-0xffff808000000000         512G                               pud
---[ Low Kernel Mapping ]---
0xffff808000000000-0xffff810000000000         512G                               pud
---[ vmalloc() Area ]---
0xffff810000000000-0xffff818000000000         512G                               pud
---[ Vmemmap ]---
0xffff818000000000-0xffffff0000000000      128512G                               pud
---[ ESPfix Area ]---
0xffffff0000000000-0x0000000000000000           1T                               pud
0x0000000000000000-0x0000000000000000           0E                               pgd
0x0000000000000000-0x0000000000001000           4K     RW     PCD         GLB NX pte
0x0000000000001000-0x0000000000002000           4K                               pte
0x0000000000002000-0x0000000000003000           4K     ro                 GLB NX pte
0x0000000000003000-0x0000000000004000           4K                               pte
0x0000000000004000-0x0000000000007000          12K     RW                 GLB NX pte
0x0000000000007000-0x0000000000008000           4K                               pte
0x0000000000008000-0x0000000000108000           1M     RW                 GLB NX pte
0x0000000000108000-0x0000000000109000           4K                               pte
0x0000000000109000-0x0000000000189000         512K     RW                 GLB NX pte
0x0000000000189000-0x000000000018a000           4K                               pte
0x000000000018a000-0x000000000018e000          16K     RW                 GLB NX pte
0x000000000018e000-0x000000000018f000           4K                               pte
0x000000000018f000-0x0000000000193000          16K     RW                 GLB NX pte
0x0000000000193000-0x0000000000194000           4K                               pte
... 304 entries skipped ... 
---[ EFI Runtime Services ]---
0xffffffef00000000-0xffffffff80000000          66G                               pud
---[ High Kernel Mapping ]---
0xffffffff80000000-0xffffffffc0000000           1G                               pud
...



As for KASAN, I think it would be better just to make it work faster, the patch below demonstrates the idea.



---
 arch/x86/mm/dump_pagetables.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index 0470826d2bdc..36515fba86b0 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -13,6 +13,7 @@
  */
 
 #include <linux/debugfs.h>
+#include <linux/kasan.h>
 #include <linux/mm.h>
 #include <linux/init.h>
 #include <linux/sched.h>
@@ -307,16 +308,19 @@ static void walk_pte_level(struct seq_file *m, struct pg_state *st, pmd_t addr,
 static void walk_pmd_level(struct seq_file *m, struct pg_state *st, pud_t addr, unsigned long P)
 {
 	int i;
-	pmd_t *start;
+	pmd_t *start, *pmd_addr;
 	pgprotval_t prot;
 
-	start = (pmd_t *)pud_page_vaddr(addr);
+	pmd_addr = start = (pmd_t *)pud_page_vaddr(addr);
 	for (i = 0; i < PTRS_PER_PMD; i++) {
 		st->current_address = normalize_addr(P + i * PMD_LEVEL_MULT);
 		if (!pmd_none(*start)) {
 			if (pmd_large(*start) || !pmd_present(*start)) {
 				prot = pmd_flags(*start);
 				note_page(m, st, __pgprot(prot), 3);
+			} else if (__pa(pmd_addr) == __pa(kasan_zero_pmd)) {
+				prot = pte_flags(kasan_zero_pte[0]);
+				note_page(m, st, __pgprot(prot), 4);
 			} else {
 				walk_pte_level(m, st, *start,
 					       P + i * PMD_LEVEL_MULT);
@@ -349,11 +353,11 @@ static bool pud_already_checked(pud_t *prev_pud, pud_t *pud, bool checkwx)
 static void walk_pud_level(struct seq_file *m, struct pg_state *st, p4d_t addr, unsigned long P)
 {
 	int i;
-	pud_t *start;
+	pud_t *start, *pud_addr;
 	pgprotval_t prot;
 	pud_t *prev_pud = NULL;
 
-	start = (pud_t *)p4d_page_vaddr(addr);
+	pud_addr = start = (pud_t *)p4d_page_vaddr(addr);
 
 	for (i = 0; i < PTRS_PER_PUD; i++) {
 		st->current_address = normalize_addr(P + i * PUD_LEVEL_MULT);
@@ -362,6 +366,9 @@ static void walk_pud_level(struct seq_file *m, struct pg_state *st, p4d_t addr,
 			if (pud_large(*start) || !pud_present(*start)) {
 				prot = pud_flags(*start);
 				note_page(m, st, __pgprot(prot), 2);
+			} else if (__pa(pud_addr) == __pa(kasan_zero_pud)) {
+				prot = pte_flags(kasan_zero_pte[0]);
+				note_page(m, st, __pgprot(prot), 4);
 			} else {
 				walk_pmd_level(m, st, *start,
 					       P + i * PUD_LEVEL_MULT);
@@ -385,10 +392,10 @@ static void walk_pud_level(struct seq_file *m, struct pg_state *st, p4d_t addr,
 static void walk_p4d_level(struct seq_file *m, struct pg_state *st, pgd_t addr, unsigned long P)
 {
 	int i;
-	p4d_t *start;
+	p4d_t *start, *p4d_addr;
 	pgprotval_t prot;
 
-	start = (p4d_t *)pgd_page_vaddr(addr);
+	p4d_addr = start = (p4d_t *)pgd_page_vaddr(addr);
 
 	for (i = 0; i < PTRS_PER_P4D; i++) {
 		st->current_address = normalize_addr(P + i * P4D_LEVEL_MULT);
@@ -396,6 +403,9 @@ static void walk_p4d_level(struct seq_file *m, struct pg_state *st, pgd_t addr,
 			if (p4d_large(*start) || !p4d_present(*start)) {
 				prot = p4d_flags(*start);
 				note_page(m, st, __pgprot(prot), 2);
+			} else if (__pa(p4d_addr) == __pa(kasan_zero_p4d)) {
+				prot = pte_flags(kasan_zero_pte[0]);
+				note_page(m, st, __pgprot(prot), 4);
 			} else {
 				walk_pud_level(m, st, *start,
 					       P + i * P4D_LEVEL_MULT);
-- 
2.13.0