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

commit 32d6397805d00573ce1fa55f408ce2bca15b0ad3 upstream.

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.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index c43f1dd194898d36e9ccb8ffe64d48cd4dadeca6..703e5692dc3a0ff27d98548d16b235f064d9b16e 100644 (file)
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -351,6 +351,9 @@ 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);
+
        /*
         * TTBR0 is only used for the identity mapping at this stage. Make it
         * point to zero page to avoid speculatively fetching new entries.