[PATCH] arm64: mm: ensure that the zero page is visible to the page table walker

On Thu, Dec 10, 2015 at 06:14:12PM +0000, Mark Rutland wrote:

Hi Will,

Hi Mark,

On Thu, Dec 10, 2015 at 05:39:59PM +0000, Will Deacon wrote:

quoted

In paging_init, we allocate the zero page, memset it to zero and then
point TTBR0 to it in order to avoid speculative fetches through the
identity mapping.

In order to guarantee that the freshly zeroed page is indeed visible to
the page table walker, we need to execute a dsb instruction prior to
writing the TTBR.

Cc: <redacted> # v3.14+, for older kernels need to drop the 'ishst'
Signed-off-by: Will Deacon <redacted>
---
 arch/arm64/mm/mmu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index c04def90f3e4..c5bd5bca8e3d 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c

@@ -464,6 +464,9 @@ void __init paging_init(void)
 
 	empty_zero_page = virt_to_page(zero_page);
 
+	/* Ensure the zero page is visible to the page table walker */
+	dsb(ishst);

I think this should live in early_alloc (likewise in late_alloc).

In the other cases we call early_alloc or late_allot we assume the
zeroing is visible to the page table walker.

For example in in alloc_init_pte we do:
	
	if (pmd_none(*pmd) || pmd_sect(*pmd)) {
		pte = alloc(PTRS_PER_PTE * sizeof(pte_t));
		if (pmd_sect(*pmd))
			split_pmd(pmd, pte);
		__pmd_populate(pmd, __pa(pte), PMD_TYPE_TABLE);
		flush_tlb_all();
	}

There's a dsb in __pmd_populate, but it's _after_ the write to the pmd
entry, so the walker might start walking the newly-allocated pte table
before the zeroing is visible.

Urgh. The reason this is a problem is because we're modifying the page
tables live (which I know that you're fixing) without using
break-before-make. Consequently, the usual ordering guarantees that we
get from the tlb flush after installing the invalid entry do not apply
and we end up with the issue you point out.

Either we need a barrier after every alloc, or we fold the barrier into
the two allocation functions.

Could you roll this into your patch that drops the size parameter from
the alloc functions please? Then we can name them {early,late}_alloc_pgtable
and have them do the dsb in there. Maybe we can drop it again when we're
doing proper break-before-make.

Cheers,

Will