[RFC PATCH 6/6] arm64/mm: use independent physical allocation for pgdir segment

[Ard Biesheuvel]

In order to avoid leaking the physical placement of the kernel via
the value off TTBR1_EL1 on platforms that are affected by variant 3a,
replace the statically allocated page table region with a dynamically
allocated buffer whose placement in the physical address space does
not correlate with the placement of the kernel itself.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
 arch/arm64/mm/mmu.c | 41 ++++++++------------
 1 file changed, 16 insertions(+), 25 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 55c84d63244d..6c16e71c26e2 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -572,7 +572,7 @@ core_initcall(map_entry_trampoline);
 /*
  * Create fine-grained mappings for the kernel.
  */
-static void __init map_kernel(pgd_t *pgdp)
+static void __init map_kernel(pgd_t *pgdp, phys_addr_t pgdir_phys)
 {
 	static struct vm_struct vmlinux_text, vmlinux_rodata, vmlinux_inittext,
 				vmlinux_initdata, vmlinux_data, vmlinux_pgdir;
@@ -603,8 +603,7 @@ static void __init map_kernel(pgd_t *pgdp)
 			   __pa_symbol(_data), PAGE_KERNEL,
 			   &vmlinux_data, 0, VM_NO_GUARD);
 	map_kernel_segment(pgdp, __pgdir_segment_start, __pgdir_segment_end,
-			   __pa_symbol(__pgdir_segment_start), PAGE_KERNEL,
-			   &vmlinux_pgdir, 0, 0);
+			   pgdir_phys, PAGE_KERNEL, &vmlinux_pgdir, 0, 0);
 
 	if (!READ_ONCE(pgd_val(*pgd_offset_raw(pgdp, FIXADDR_START)))) {
 		/*
@@ -639,36 +638,28 @@ 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);
+	int pgdir_segment_size = __pgdir_segment_end - __pgdir_segment_start;
+	phys_addr_t pgdir_phys = memblock_alloc(pgdir_segment_size, PAGE_SIZE);
+	phys_addr_t p;
+	pgd_t *pgdp;
+
+	for (p = 0; p < pgdir_segment_size; p += PAGE_SIZE)
+		clear_page_phys(p);
 
-	__pa_swapper_pg_dir = __pa_symbol(swapper_pg_dir);
+	__pa_swapper_pg_dir = pgdir_phys + (u64)swapper_pg_dir -
+			      (u64)__pgdir_segment_start;
 
-	map_kernel(pgdp);
+	pgdp = pgd_set_fixmap(__pa_swapper_pg_dir);
+
+	map_kernel(pgdp, pgdir_phys);
 	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(pgd_phys);
-	memcpy(swapper_pg_dir, pgdp, PGD_SIZE);
 	cpu_replace_ttbr1(__pa_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);
+	/* the statically allocated pgdir is no longer used after this point */
+	memblock_free(__pa_symbol(__pgdir_segment_start), pgdir_segment_size);
 }
 
 /*
-- 
2.11.0