[RFC PATCH 1/6] x86/alternative: assert text_mutex is taken

[Nadav Amit]

Use lockdep to ensure that text_mutex is taken when text_poke() is
called.

Actually it is not always taken, specifically when it is called by kgdb,
so take the lock in these cases.

Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
 arch/x86/kernel/alternative.c | 1 +
 arch/x86/kernel/kgdb.c        | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 014f214da581..916c11b410c4 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -699,6 +699,7 @@ void *text_poke(void *addr, const void *opcode, size_t len)
 	 * pages as they are not yet initialized.
 	 */
 	BUG_ON(!after_bootmem);
+	lockdep_assert_held(&text_mutex);
 
 	if (!core_kernel_text((unsigned long)addr)) {
 		pages[0] = vmalloc_to_page(addr);
diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 8e36f249646e..60b99c76086c 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -768,8 +768,12 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
 	 */
 	if (mutex_is_locked(&text_mutex))
 		return -EBUSY;
+
+	/* Take the mutex to avoid lockdep assertion failures. */
+	mutex_lock(&text_mutex);
 	text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
 		  BREAK_INSTR_SIZE);
+	mutex_unlock(&text_mutex);
 	err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
 	if (err)
 		return err;
@@ -793,7 +797,12 @@ int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
 	 */
 	if (mutex_is_locked(&text_mutex))
 		goto knl_write;
+
+	/* Take the mutex to avoid lockdep assertion failures. */
+	mutex_lock(&text_mutex);
 	text_poke((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE);
+	mutex_unlock(&text_mutex);
+
 	err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
 	if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))
 		goto knl_write;
-- 
2.17.1