[RFC PATCH 12/16] arm64: mm: Map p4d/pgd with privileged pkey

[Kevin Brodsky]

If CONFIG_KPKEYS_HARDENED_PGTABLES is enabled, map p4d/pgd pages
using a privileged pkey (KPKEYS_PKEY_PGTABLES), so that they can
only be written under guard(kpkeys_hardened_pgtables).

The case where pgd is not page-sized is not currently handled -
this is pending support for pkeys in kmem_cache.

This patch is a no-op if CONFIG_KPKEYS_HARDENED_PGTABLES is disabled
(default).

Signed-off-by: Kevin Brodsky <kevin.brod...@arm.com>
---
 arch/arm64/include/asm/pgalloc.h | 21 ++++++++++++++++++---
 arch/arm64/mm/pgd.c              | 30 ++++++++++++++++++++++++++++--
 2 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
index e75422864d1b..c006aecd6ba5 100644
--- a/arch/arm64/include/asm/pgalloc.h
+++ b/arch/arm64/include/asm/pgalloc.h
@@ -88,18 +88,33 @@ static inline void pgd_populate(struct mm_struct *mm, pgd_t 
*pgdp, p4d_t *p4dp)
 static inline p4d_t *p4d_alloc_one(struct mm_struct *mm, unsigned long addr)
 {
        gfp_t gfp = GFP_PGTABLE_USER;
+       int ret;
 
        if (mm == &init_mm)
                gfp = GFP_PGTABLE_KERNEL;
-       return (p4d_t *)get_zeroed_page(gfp);
+
+       addr = get_zeroed_page(gfp);
+       if (!addr)
+               return NULL;
+
+       ret = kpkeys_protect_pgtable_memory(addr, 1);
+       if (ret) {
+               free_page(addr);
+               return NULL;
+       }
+
+       return (p4d_t *)addr;
 }
 
 static inline void p4d_free(struct mm_struct *mm, p4d_t *p4d)
 {
+       unsigned long addr = (unsigned long)p4d;
+
        if (!pgtable_l5_enabled())
                return;
-       BUG_ON((unsigned long)p4d & (PAGE_SIZE-1));
-       free_page((unsigned long)p4d);
+       BUG_ON(addr & (PAGE_SIZE-1));
+       kpkeys_unprotect_pgtable_memory(addr, 1);
+       free_page(addr);
 }
 
 #define __p4d_free_tlb(tlb, p4d, addr)  p4d_free((tlb)->mm, p4d)
diff --git a/arch/arm64/mm/pgd.c b/arch/arm64/mm/pgd.c
index 0c501cabc238..3577cc1821af 100644
--- a/arch/arm64/mm/pgd.c
+++ b/arch/arm64/mm/pgd.c
@@ -28,12 +28,38 @@ static bool pgdir_is_page_size(void)
        return false;
 }
 
+static pgd_t *pgd_page_alloc(gfp_t gfp)
+{
+       unsigned long addr;
+       int ret;
+
+       addr = __get_free_page(gfp);
+       if (!addr)
+               return NULL;
+
+       ret = kpkeys_protect_pgtable_memory(addr, 1);
+       if (ret) {
+               free_page(addr);
+               return NULL;
+       }
+
+       return (pgd_t *)addr;
+}
+
+static void pgd_page_free(pgd_t *pgd)
+{
+       unsigned long addr = (unsigned long)pgd;
+
+       kpkeys_unprotect_pgtable_memory(addr, 1);
+       free_page(addr);
+}
+
 pgd_t *pgd_alloc(struct mm_struct *mm)
 {
        gfp_t gfp = GFP_PGTABLE_USER;
 
        if (pgdir_is_page_size())
-               return (pgd_t *)__get_free_page(gfp);
+               return pgd_page_alloc(gfp);
        else
                return kmem_cache_alloc(pgd_cache, gfp);
 }
@@ -41,7 +67,7 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
 void pgd_free(struct mm_struct *mm, pgd_t *pgd)
 {
        if (pgdir_is_page_size())
-               free_page((unsigned long)pgd);
+               pgd_page_free(pgd);
        else
                kmem_cache_free(pgd_cache, pgd);
 }
-- 
2.47.0