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Re: [PATCH v4 3/3] powerpc/32: Add KASAN support

Hi Andrey,

Le 08/02/2019 à 18:40, Andrey Konovalov a écrit :
On Fri, Feb 8, 2019 at 6:17 PM Christophe Leroy <christophe.leroy@xxxxxx> wrote:

Hi Daniel,

Le 08/02/2019 à 17:18, Daniel Axtens a écrit :
Hi Christophe,

I've been attempting to port this to 64-bit Book3e nohash (e6500),
although I think I've ended up with an approach more similar to Aneesh's
much earlier (2015) series for book3s.

Part of this is just due to the changes between 32 and 64 bits - we need
to hack around the discontiguous mappings - but one thing that I'm
particularly puzzled by is what the kasan_early_init is supposed to do.

It should be a problem as my patch uses a 'for_each_memblock(memory,
reg)' loop.

+void __init kasan_early_init(void)
+    unsigned long addr = KASAN_SHADOW_START;
+    unsigned long end = KASAN_SHADOW_END;
+    unsigned long next;
+    pmd_t *pmd = pmd_offset(pud_offset(pgd_offset_k(addr), addr), addr);
+    int i;
+    phys_addr_t pa = __pa(kasan_early_shadow_page);
+    if (early_mmu_has_feature(MMU_FTR_HPTE_TABLE))
+            panic("KASAN not supported with Hash MMU\n");
+    for (i = 0; i < PTRS_PER_PTE; i++)
+            __set_pte_at(&init_mm, (unsigned long)kasan_early_shadow_page,
+                         kasan_early_shadow_pte + i,
+                         pfn_pte(PHYS_PFN(pa), PAGE_KERNEL_RO), 0);
+    do {
+            next = pgd_addr_end(addr, end);
+            pmd_populate_kernel(&init_mm, pmd, kasan_early_shadow_pte);
+    } while (pmd++, addr = next, addr != end);

As far as I can tell it's mapping the early shadow page, read-only, over
the KASAN_SHADOW_START->KASAN_SHADOW_END range, and it's using the early
shadow PTE array from the generic code.

I haven't been able to find an answer to why this is in the docs, so I
was wondering if you or anyone else could explain the early part of
kasan init a bit better.

See https://www.kernel.org/doc/html/latest/dev-tools/kasan.html for an
explanation of the shadow.

When shadow is 0, it means the memory area is entirely accessible.

It is necessary to setup a shadow area as soon as possible because all
data accesses check the shadow area, from the begining (except for a few
files where sanitizing has been disabled in Makefiles).

Until the real shadow area is set, all access are granted thanks to the
zero shadow area beeing for of zeros.

Not entirely correct. kasan_early_init() indeed maps the whole shadow
memory range to the same kasan_early_shadow_page. However as kernel
loads and memory gets allocated this shadow page gets rewritten with
non-zero values by different KASAN allocator hooks. Since these values
come from completely different parts of the kernel, but all land on
the same page, kasan_early_shadow_page's content can be considered
garbage. When KASAN checks memory accesses for validity it detects
these garbage shadow values, but doesn't print any reports, as the
reporting routine bails out on the current->kasan_depth check (which
has the value of 1 initially). Only after kasan_init() completes, when
the proper shadow memory is mapped, current->kasan_depth gets set to 0
and we start reporting bad accesses.

That's surprising, because in the early phase I map the shadow area read-only, so I do not expect it to get modified unless RO protection is failing for some reason.

Next week I'll add a test in early_init() to check the content of the early shadow area.


I mainly used ARM arch as an exemple when I implemented KASAN for ppc32.

At the moment, I don't do any early init, and like Aneesh's series for
book3s, I end up needing a special flag to disable kasan until after
kasan_init. Also, as with Balbir's seris for Radix, some tests didn't
fire, although my missing tests are a superset of his. I suspect the
early init has something to do with these...?

I think you should really focus on establishing a zero shadow area as
early as possible instead of trying to ack the core parts of KASAN.

(I'm happy to collate answers into a patch to the docs, btw!)

We can also have the discussion going via

In the long term I hope to revive Aneesh's and Balbir's series for hash
and radix as well.




+static void __init kasan_init_region(struct memblock_region *reg)
+    void *start = __va(reg->base);
+    void *end = __va(reg->base + reg->size);
+    unsigned long k_start, k_end, k_cur, k_next;
+    pmd_t *pmd;
+    if (start >= end)
+            return;
+    k_start = (unsigned long)kasan_mem_to_shadow(start);
+    k_end = (unsigned long)kasan_mem_to_shadow(end);
+    pmd = pmd_offset(pud_offset(pgd_offset_k(k_start), k_start), k_start);
+    for (k_cur = k_start; k_cur != k_end; k_cur = k_next, pmd++) {
+            k_next = pgd_addr_end(k_cur, k_end);
+            if ((void *)pmd_page_vaddr(*pmd) == kasan_early_shadow_pte) {
+                    pte_t *new = pte_alloc_one_kernel(&init_mm);
+                    if (!new)
+                            panic("kasan: pte_alloc_one_kernel() failed");
+                    memcpy(new, kasan_early_shadow_pte, PTE_TABLE_SIZE);
+                    pmd_populate_kernel(&init_mm, pmd, new);
+            }
+    };
+    for (k_cur = k_start; k_cur < k_end; k_cur += PAGE_SIZE) {
+            void *va = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
+            pte_t pte = pfn_pte(PHYS_PFN(__pa(va)), PAGE_KERNEL);
+            if (!va)
+                    panic("kasan: memblock_alloc() failed");
+            pmd = pmd_offset(pud_offset(pgd_offset_k(k_cur), k_cur), k_cur);
+            pte_update(pte_offset_kernel(pmd, k_cur), ~0, pte_val(pte));
+    }
+    flush_tlb_kernel_range(k_start, k_end);
+void __init kasan_init(void)
+    struct memblock_region *reg;
+    for_each_memblock(memory, reg)
+            kasan_init_region(reg);
+    kasan_init_tags();
+    /* At this point kasan is fully initialized. Enable error messages */
+    init_task.kasan_depth = 0;
+    pr_info("KASAN init done\n");
diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
index 33cc6f676fa6..ae7db88b72d6 100644
--- a/arch/powerpc/mm/mem.c
+++ b/arch/powerpc/mm/mem.c
@@ -369,6 +369,10 @@ void __init mem_init(void)
      pr_info("  * 0x%08lx..0x%08lx  : highmem PTEs\n",
   #endif /* CONFIG_HIGHMEM */
+    pr_info("  * 0x%08lx..0x%08lx  : kasan shadow mem\n",
      pr_info("  * 0x%08lx..0x%08lx  : consistent mem\n",

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