[RFC PATCH 4/6] arm64: mm: add helper to fill execmem with trapping instructions

["Adrian Barnaś" <abarnas@google.com>]

Implement the architecture-specific execmem_fill_trapping_insns() helper
to poison executable memory regions.

When CONFIG_ARCH_HAS_EXECMEM_ROX is enabled, the execmem subsystem
requires a way to fill unused or freed executable memory with
architecture-specific trapping instructions. This implementation fills
the specified region with AARCH64_BREAK_FAULT instructions and flushes
the icache to ensure the traps are immediately visible to execution.

Signed-off-by: Adrian Barnaś <abarnas@google.com>
---
 arch/arm64/mm/init.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index c673a9a839dd..71aa745e0bef 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -408,6 +408,20 @@ void dump_mem_limit(void)
 }
 
 #ifdef CONFIG_EXECMEM
+
+#ifdef CONFIG_ARCH_HAS_EXECMEM_ROX
+void execmem_fill_trapping_insns(void *ptr, size_t size)
+{
+	int nr_inst = size / AARCH64_INSN_SIZE;
+	__le32 *updptr = ptr;
+
+	for (int i = 0; i < nr_inst; i++)
+		updptr[i] = cpu_to_le32(AARCH64_BREAK_FAULT);
+
+	flush_icache_range((unsigned long)ptr, (unsigned long)ptr + size);
+}
+#endif
+
 static u64 module_direct_base __ro_after_init = 0;
 static u64 module_plt_base __ro_after_init = 0;
 
-- 
2.54.0.1136.gdb2ca164c4-goog