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arm64/mm: Separate boot-time page tables from swapper_pg_dir

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Since the address of swapper_pg_dir is fixed for a given kernel image,
it is an attractive target for manipulation via an arbitrary write. To
mitigate this we'd like to make it read-only by moving it into the
rodata section.

We require that swapper_pg_dir is at a fixed offset from tramp_pg_dir
and reserved_ttbr0, so these will also need to move into rodata.
However, swapper_pg_dir is allocated along with some transient page
tables used for boot which we do not want to move into rodata.

As a step towards this, this patch separates the boot-time page tables
into a new init_pg_dir, and reduces swapper_pg_dir to the single page it
needs to be. This allows us to retain the relationship between
swapper_pg_dir, tramp_pg_dir, and swapper_pg_dir, while cleanly
separating these from the boot-time page tables.

The init_pg_dir holds all of the pgd/pud/pmd/pte levels needed during
boot, and all of these levels will be freed when we switch to the
swapper_pg_dir, which is initialized by the existing code in
paging_init(). Since we start off on the init_pg_dir, we no longer need
to allocate a transient page table in paging_init() in order to ensure
that swapper_pg_dir isn't live while we initialize it.

There should be no functional change as a result of this patch.

Signed-off-by: Jun Yao <yaojun8558363@gmail.com>
Reviewed-by: James Morse <james.morse@arm.com>
[Mark: place init_pg_dir after BSS, fold mm changes, commit message]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Jun Yao
2018-09-24 15:47:49 +01:00
committed by Catalin Marinas
parent 693d5639b4
commit 2b5548b681
6 changed files with 21 additions and 33 deletions
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30
arch/arm64/mm/mmu.c
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@@ -629,34 +629,14 @@ static void __init map_kernel(pgd_t *pgdp)
*/
void __init paging_init(void)
{
phys_addr_t pgd_phys = early_pgtable_alloc();
pgd_t *pgdp = pgd_set_fixmap(pgd_phys);
map_kernel(swapper_pg_dir);
map_mem(swapper_pg_dir);
map_kernel(pgdp);
map_mem(pgdp);
/*
* We want to reuse the original swapper_pg_dir so we don't have to
* communicate the new address to non-coherent secondaries in
* secondary_entry, and so cpu_switch_mm can generate the address with
* adrp+add rather than a load from some global variable.
*
* To do this we need to go via a temporary pgd.
*/
cpu_replace_ttbr1(__va(pgd_phys));
memcpy(swapper_pg_dir, pgdp, PGD_SIZE);
cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
init_mm.pgd = swapper_pg_dir;
pgd_clear_fixmap();
memblock_free(pgd_phys, PAGE_SIZE);
/*
* We only reuse the PGD from the swapper_pg_dir, not the pud + pmd
* allocated with it.
*/
memblock_free(__pa_symbol(swapper_pg_dir) + PAGE_SIZE,
__pa_symbol(swapper_pg_end) - __pa_symbol(swapper_pg_dir)
- PAGE_SIZE);
memblock_free(__pa_symbol(init_pg_dir),
__pa_symbol(init_pg_end) - __pa_symbol(init_pg_dir));
}
/*