Re: [PATCH] arm64: remove page granularity limitation from KFENCE

On 2021/9/18 19:50, Alexander Potapenko wrote:

On Sat, Sep 18, 2021 at 10:10 AM Liu Shixin [off-list ref] wrote:

quoted

Currently if KFENCE is enabled in arm64, the entire linear map will be
mapped at page granularity which seems overkilled. Actually only the
kfence pool requires to be mapped at page granularity. We can remove the
restriction from KFENCE and force the linear mapping of the kfence pool
at page granularity later in arch_kfence_init_pool().

There was a previous patch by Jisheng Zhang intended to remove this
requirement: https://lore.kernel.org/linux-arm-kernel/20210524180656.395e45f6@xhacker.debian/ (local)
Which of the two is more preferable?

The previous patch by Jisheng Zhang guarantees kfence pool to be mapped at
page granularity by allocating kfence pool before paging_init(), and then map it
at page granularity during map_mem().

The previous patch has a problem: Even if kfence is disabled in cmdline, kfence pool
is still allocated, which is a waste of memory.

I'm sorry for sending it repeatly, and I have no idea how to limit the email format
to TEXT/PLAIN.

thanks.

quoted

Signed-off-by: Liu Shixin <redacted>
---
 arch/arm64/include/asm/kfence.h | 69 ++++++++++++++++++++++++++++++++-
 arch/arm64/mm/mmu.c             |  4 +-
 2 files changed, 70 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/kfence.h b/arch/arm64/include/asm/kfence.h
index aa855c6a0ae6..bee101eced0b 100644
--- a/arch/arm64/include/asm/kfence.h
+++ b/arch/arm64/include/asm/kfence.h

@@ -8,9 +8,76 @@
 #ifndef __ASM_KFENCE_H
 #define __ASM_KFENCE_H

+#include <linux/kfence.h>
 #include <asm/set_memory.h>
+#include <asm/pgalloc.h>

-static inline bool arch_kfence_init_pool(void) { return true; }
+static inline int split_pud_page(pud_t *pud, unsigned long addr)
+{
+       int i;
+       pmd_t *pmd = pmd_alloc_one(&init_mm, addr);
+       unsigned long pfn = PFN_DOWN(__pa(addr));
+
+       if (!pmd)
+               return -ENOMEM;
+
+       for (i = 0; i < PTRS_PER_PMD; i++)
+               set_pmd(pmd + i, pmd_mkhuge(pfn_pmd(pfn + i * PTRS_PER_PTE, PAGE_KERNEL)));
+
+       smp_wmb(); /* See comment in __pte_alloc */
+       pud_populate(&init_mm, pud, pmd);
+       flush_tlb_kernel_range(addr, addr + PUD_SIZE);
+       return 0;
+}
+
+static inline int split_pmd_page(pmd_t *pmd, unsigned long addr)
+{
+       int i;
+       pte_t *pte = pte_alloc_one_kernel(&init_mm);
+       unsigned long pfn = PFN_DOWN(__pa(addr));
+
+       if (!pte)
+               return -ENOMEM;
+
+       for (i = 0; i < PTRS_PER_PTE; i++)
+               set_pte(pte + i, pfn_pte(pfn + i, PAGE_KERNEL));
+
+       smp_wmb(); /* See comment in __pte_alloc */
+       pmd_populate_kernel(&init_mm, pmd, pte);
+
+       flush_tlb_kernel_range(addr, addr + PMD_SIZE);
+       return 0;
+}
+
+static inline bool arch_kfence_init_pool(void)
+{
+       unsigned long addr;
+       pgd_t *pgd;
+       p4d_t *p4d;
+       pud_t *pud;
+       pmd_t *pmd;
+
+       for (addr = (unsigned long)__kfence_pool; is_kfence_address((void *)addr);
+            addr += PAGE_SIZE) {
+               pgd = pgd_offset(&init_mm, addr);
+               if (pgd_leaf(*pgd))
+                       return false;
+               p4d = p4d_offset(pgd, addr);
+               if (p4d_leaf(*p4d))
+                       return false;
+               pud = pud_offset(p4d, addr);
+               if (pud_leaf(*pud)) {
+                       if (split_pud_page(pud, addr & PUD_MASK))
+                               return false;
+               }
+               pmd = pmd_offset(pud, addr);
+               if (pmd_leaf(*pmd)) {
+                       if (split_pmd_page(pmd, addr & PMD_MASK))
+                               return false;
+               }
+       }
+       return true;
+}

 static inline bool kfence_protect_page(unsigned long addr, bool protect)
 {

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index cfd9deb347c3..b2c79ccfb1c5 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c

@@ -516,7 +516,7 @@ static void __init map_mem(pgd_t *pgdp)
         */
        BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));

-       if (can_set_direct_map() || crash_mem_map || IS_ENABLED(CONFIG_KFENCE))
+       if (can_set_direct_map() || crash_mem_map)
                flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;

        /*

@@ -1485,7 +1485,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
         * KFENCE requires linear map to be mapped at page granularity, so that
         * it is possible to protect/unprotect single pages in the KFENCE pool.
         */
-       if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
+       if (can_set_direct_map())
                flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;

        __create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
--

2.18.0.huawei.25

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