Re: [PATCH -tip 1/2] x86/alternative: Sync bp_patching update for avoiding NULL pointer exception

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 27, 2019 at 02:56:52PM +0900, Masami Hiramatsu wrote:

> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 4552795a8df4..9505096e2cd1 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1134,8 +1134,14 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
>  	 * sync_core() implies an smp_mb() and orders this store against
>  	 * the writing of the new instruction.
>  	 */
> -	bp_patching.vec = NULL;
>  	bp_patching.nr_entries = 0;
> +	/*
> +	 * This sync_core () ensures that all int3 handlers in progress
> +	 * have finished. This allows poke_int3_handler () after this to
> +	 * avoid touching bp_paching.vec by checking nr_entries == 0.
> +	 */
> +	text_poke_sync();
> +	bp_patching.vec = NULL;
>  }

How's something like this instead? Under the assumption that it is rare
to actually hit the INT3 and even more rare to actually hit this race,
the below should be a lot cheaper.

---
 arch/x86/kernel/alternative.c | 69 +++++++++++++++++++++++++++++++++----------
 1 file changed, 53 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 30e86730655c..12f2d193109d 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -953,6 +953,8 @@ static struct bp_patching_desc {
 	int nr_entries;
 } bp_patching;
 
+static atomic_t bp_handlers;
+
 static inline void *text_poke_addr(struct text_poke_loc *tp)
 {
 	return _stext + tp->rel_addr;
@@ -973,8 +975,8 @@ NOKPROBE_SYMBOL(patch_cmp);
 int notrace poke_int3_handler(struct pt_regs *regs)
 {
 	struct text_poke_loc *tp;
+	int nr, len, ret = 0;
 	void *ip;
-	int len;
 
 	/*
 	 * Having observed our INT3 instruction, we now must observe
@@ -987,12 +989,21 @@ int notrace poke_int3_handler(struct pt_regs *regs)
 	 * Idem for other elements in bp_patching.
 	 */
 	smp_rmb();
-
-	if (likely(!bp_patching.nr_entries))
+	if (!READ_ONCE(bp_patching.nr_entries))
 		return 0;
 
+	atomic_inc(&bp_handlers);
+	/*
+	 * 'ACQUIRE', everything happens after the increment.
+	 */
+	smp_mb__after_atomic();
+
+	nr = smp_load_acquire(&bp_patching.nr_entries);
+	if (likely(!nr))
+		goto out;
+
 	if (user_mode(regs))
-		return 0;
+		goto out;
 
 	/*
 	 * Discount the INT3. See text_poke_bp_batch().
@@ -1002,16 +1013,16 @@ int notrace poke_int3_handler(struct pt_regs *regs)
 	/*
 	 * Skip the binary search if there is a single member in the vector.
 	 */
-	if (unlikely(bp_patching.nr_entries > 1)) {
-		tp = bsearch(ip, bp_patching.vec, bp_patching.nr_entries,
+	if (unlikely(nr > 1)) {
+		tp = bsearch(ip, bp_patching.vec, nr,
 			     sizeof(struct text_poke_loc),
 			     patch_cmp);
 		if (!tp)
-			return 0;
+			goto out;
 	} else {
 		tp = bp_patching.vec;
 		if (text_poke_addr(tp) != ip)
-			return 0;
+			goto out;
 	}
 
 	len = text_opcode_size(tp->opcode);
@@ -1023,7 +1034,7 @@ int notrace poke_int3_handler(struct pt_regs *regs)
 		 * Someone poked an explicit INT3, they'll want to handle it,
 		 * do not consume.
 		 */
-		return 0;
+		goto out;
 
 	case CALL_INSN_OPCODE:
 		int3_emulate_call(regs, (long)ip + tp->rel32);
@@ -1038,7 +1049,14 @@ int notrace poke_int3_handler(struct pt_regs *regs)
 		BUG();
 	}
 
-	return 1;
+	ret = 1;
+out:
+	/*
+	 * 'RELEASE", everything happens before the decrement.
+	 */
+	smp_mb__before_atomic();
+	atomic_dec(&bp_handlers);
+	return ret;
 }
 NOKPROBE_SYMBOL(poke_int3_handler);
 
@@ -1076,7 +1094,12 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
 	lockdep_assert_held(&text_mutex);
 
 	bp_patching.vec = tp;
-	bp_patching.nr_entries = nr_entries;
+	/*
+	 * bp_patching.vec = tp			nr = bp_patching.nr_entries
+	 * REL					ACQ
+	 * bp_patching.nr_entries = nr_entries	tp = bp_patching.vec[]
+	 */
+	smp_store_release(&bp_patching.nr_entries, nr_entries);
 
 	/*
 	 * Corresponding read barrier in int3 notifier for making sure the
@@ -1134,13 +1157,27 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
 	 * sync_core() implies an smp_mb() and orders this store against
 	 * the writing of the new instruction.
 	 */
-	bp_patching.nr_entries = 0;
+	WRITE_ONCE(bp_patching.nr_entries, 0);
 	/*
-	 * This sync_core () call ensures that all INT3 handlers in progress
-	 * have finished. This allows poke_int3_handler() after this to
-	 * avoid touching bp_paching.vec by checking nr_entries == 0.
+	 * nr_entries = 0	bp_handlers++
+	 * MB			MB
+	 * VAL = bp_handlers	nr = nr_entries
+	 */
+	smp_mb();
+	/*
+	 * Guarantee all poke_int3_handler()s that have observed
+	 * @bp_patching.nr_enties have completed before we clear
+	 * bp_patching.vec.
+	 *
+	 * We can't do this before text_poke_sync() because then there
+	 * might still be observable INT3 instructions.
+	 */
+	atomic_cond_read_acquire(&bp_handlers, !VAL);
+	/*
+	 * bp_handlers == 0		tp = bp_patching.vec[]
+	 * ACQ				MB
+	 * bp_patching.vec = NULL	bp_handlers--;
 	 */
-	text_poke_sync();
 	bp_patching.vec = NULL;
 }