[PATCH v3 1/4] kasan: introduce set_pmd_early_shadow()

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

It is incorrect to call pmd_populate_kernel() multiple times for the
same page table from inside Xen PV domains. Xen notices it during
kasan_populate_early_shadow():

    (XEN) mm.c:3222:d155v0 mfn 3704b already pinned

This happens for kasan_early_shadow_pte when USE_SPLIT_PTE_PTLOCKS is
enabled. Fix this by introducing set_pmd_early_shadow() which calls
pmd_populate_kernel() only once and uses set_pmd() afterwards.

Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
---
v2 --> v3: no changes

v1 --> v2:
- Fix compilation without CONFIG_XEN_PV
- Slightly updated description

RFC --> v1:
- New patch
---
 mm/kasan/init.c | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/mm/kasan/init.c b/mm/kasan/init.c
index ce45c491ebcd..7791fe0a7704 100644
--- a/mm/kasan/init.c
+++ b/mm/kasan/init.c
@@ -81,6 +81,26 @@ static inline bool kasan_early_shadow_page_entry(pte_t pte)
 	return pte_page(pte) == virt_to_page(lm_alias(kasan_early_shadow_page));
 }
 
+#ifdef CONFIG_XEN_PV
+static inline void set_pmd_early_shadow(pmd_t *pmd)
+{
+	static bool pmd_populated = false;
+	pte_t *early_shadow = lm_alias(kasan_early_shadow_pte);
+
+	if (likely(pmd_populated)) {
+		set_pmd(pmd, __pmd(__pa(early_shadow) | _PAGE_TABLE));
+	} else {
+		pmd_populate_kernel(&init_mm, pmd, early_shadow);
+		pmd_populated = true;
+	}
+}
+#else
+static inline void set_pmd_early_shadow(pmd_t *pmd)
+{
+	pmd_populate_kernel(&init_mm, pmd, lm_alias(kasan_early_shadow_pte));
+}
+#endif /* ifdef CONFIG_XEN_PV */
+
 static __init void *early_alloc(size_t size, int node)
 {
 	void *ptr = memblock_alloc_try_nid(size, size, __pa(MAX_DMA_ADDRESS),
@@ -120,8 +140,7 @@ static int __ref zero_pmd_populate(pud_t *pud, unsigned long addr,
 		next = pmd_addr_end(addr, end);
 
 		if (IS_ALIGNED(addr, PMD_SIZE) && end - addr >= PMD_SIZE) {
-			pmd_populate_kernel(&init_mm, pmd,
-					lm_alias(kasan_early_shadow_pte));
+			set_pmd_early_shadow(pmd);
 			continue;
 		}
 
@@ -157,8 +176,7 @@ static int __ref zero_pud_populate(p4d_t *p4d, unsigned long addr,
 			pud_populate(&init_mm, pud,
 					lm_alias(kasan_early_shadow_pmd));
 			pmd = pmd_offset(pud, addr);
-			pmd_populate_kernel(&init_mm, pmd,
-					lm_alias(kasan_early_shadow_pte));
+			set_pmd_early_shadow(pmd);
 			continue;
 		}
 
@@ -198,8 +216,7 @@ static int __ref zero_p4d_populate(pgd_t *pgd, unsigned long addr,
 			pud_populate(&init_mm, pud,
 					lm_alias(kasan_early_shadow_pmd));
 			pmd = pmd_offset(pud, addr);
-			pmd_populate_kernel(&init_mm, pmd,
-					lm_alias(kasan_early_shadow_pte));
+			set_pmd_early_shadow(pmd);
 			continue;
 		}
 
@@ -271,8 +288,7 @@ int __ref kasan_populate_early_shadow(const void *shadow_start,
 			pud_populate(&init_mm, pud,
 					lm_alias(kasan_early_shadow_pmd));
 			pmd = pmd_offset(pud, addr);
-			pmd_populate_kernel(&init_mm, pmd,
-					lm_alias(kasan_early_shadow_pte));
+			set_pmd_early_shadow(pmd);
 			continue;
 		}
 
-- 
2.17.1