[PATCH] arm/patch: call flush_icache ASAP after writing new instruction.

[Mikhail Iakhiaev]

The patch moves flush_icache before the patch_unmap call.
The change avoids the possibility of the CPU seeing
partially-patched instructions if a function from patch_unmap
call tree has just been patched. That HAS been observed
in practice, leading to kernel panic or freezing in early boot:
https://bugzilla.kernel.org/show_bug.cgi?id=219089
Specifically, the patch_unmap invokes _raw_spin_unlock_irqrestore
(could be non-inlined) and that function is being patched
during the ftrace_init, so the original code would run the
patched code BEFORE flushing the icache.

Note, some arches are more careful about flushing icache early. E.g.
arch/riscv/kernel/patch.c:
  __patch_insn_set and __patch_insn_write call the
  local_flush_icache_range before the patch_unmap and have an
  explicit comment about this.
arch/x86/kernel/alternative.c:
  text_poke_early calls sync_core (flushes icache) before
  local_irq_restore.

Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219089
Signed-off-by: Mikhail Iakhiaev <mikhailai at gmail.com>
---
 arch/arm/kernel/patch.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/arm/kernel/patch.c b/arch/arm/kernel/patch.c
index e9e828b6bb30..f003867f085a 100644
--- a/arch/arm/kernel/patch.c
+++ b/arch/arm/kernel/patch.c
@@ -99,13 +99,19 @@ void __kprobes __patch_text_real(void *addr, unsigned int insn, bool remap)
 		size = sizeof(u32);
 	}
 
-	if (waddr != addr) {
+	if (waddr != addr)
 		flush_kernel_vmap_range(waddr, twopage ? size / 2 : size);
-		patch_unmap(FIX_TEXT_POKE0, &flags);
-	}
 
 	flush_icache_range((uintptr_t)(addr),
 			   (uintptr_t)(addr) + size);
+
+	/* Can only call 'patch_unmap' after flushing dcache and icache,
+	 * because it calls 'raw_spin_unlock_irqrestore', but that may
+	 * happen to be the very function we're currently patching
+	 * (as it happens during the ftrace init).
+	 */
+	if (waddr != addr)
+		patch_unmap(FIX_TEXT_POKE0, &flags);
 }
 
 static int __kprobes patch_text_stop_machine(void *data)
-- 
2.34.1