ARM: DMA: ensure that old section mappings are flushed from the TLB

When setting up the CMA region, we must ensure that the old section
mappings are flushed from the TLB before replacing them with page
tables, otherwise we can suffer from mismatched aliases if the CPU
speculatively prefetches from these mappings at an inopportune time.

A mismatched alias can occur when the TLB contains a section mapping,
but a subsequent prefetch causes it to load a page table mapping,
resulting in the possibility of the TLB containing two matching
mappings for the same virtual address region.
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/dma-mapping.c

@@ -461,12 +461,21 @@ void __init dma_contiguous_remap(void)
 		map.type = MT_MEMORY_DMA_READY;
 
 		/*
-		 * Clear previous low-memory mapping
+		 * Clear previous low-memory mapping to ensure that the
+		 * TLB does not see any conflicting entries, then flush
+		 * the TLB of the old entries before creating new mappings.
+		 *
+		 * This ensures that any speculatively loaded TLB entries
+		 * (even though they may be rare) can not cause any problems,
+		 * and ensures that this code is architecturally compliant.
 		 */
 		for (addr = __phys_to_virt(start); addr < __phys_to_virt(end);
 		     addr += PMD_SIZE)
 			pmd_clear(pmd_off_k(addr));
 
+		flush_tlb_kernel_range(__phys_to_virt(start),
+				       __phys_to_virt(end));
+
 		iotable_init(&map, 1);
 	}
 }