[PATCH v2] io_uring/fdinfo: park SQ thread while retrieving cpu/pid

[Jens Axboe <axboe@kernel.dk>]

We could race with SQ thread exit, and if we do, we'll hit a NULL pointer
dereference when the thread is cleared. Grab the SQPOLL data lock before
attempting to get the task cpu and pid for fdinfo, this ensures we have a
stable view of it.

Cc: stable@vger.kernel.org
Link: https://bugzilla.kernel.org/show_bug.cgi?id=218032
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

v2: use the sq_data lock rather than doing a full park/unpark

diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
index c53678875416..af1bdcc0703e 100644
--- a/io_uring/fdinfo.c
+++ b/io_uring/fdinfo.c
@@ -53,7 +53,6 @@ static __cold int io_uring_show_cred(struct seq_file *m, unsigned int id,
 __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
 {
 	struct io_ring_ctx *ctx = f->private_data;
-	struct io_sq_data *sq = NULL;
 	struct io_overflow_cqe *ocqe;
 	struct io_rings *r = ctx->rings;
 	unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
@@ -64,6 +63,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
 	unsigned int cq_shift = 0;
 	unsigned int sq_shift = 0;
 	unsigned int sq_entries, cq_entries;
+	int sq_pid = -1, sq_cpu = -1;
 	bool has_lock;
 	unsigned int i;
 
@@ -143,13 +143,18 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
 	has_lock = mutex_trylock(&ctx->uring_lock);
 
 	if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
-		sq = ctx->sq_data;
-		if (!sq->thread)
-			sq = NULL;
+		struct io_sq_data *sq = ctx->sq_data;
+
+		mutex_lock(&sq->lock);
+		if (sq->thread) {
+			sq_pid = task_pid_nr(sq->thread);
+			sq_cpu = task_cpu(sq->thread);
+		}
+		mutex_unlock(&sq->lock);
 	}
 
-	seq_printf(m, "SqThread:\t%d\n", sq ? task_pid_nr(sq->thread) : -1);
-	seq_printf(m, "SqThreadCpu:\t%d\n", sq ? task_cpu(sq->thread) : -1);
+	seq_printf(m, "SqThread:\t%d\n", sq_pid);
+	seq_printf(m, "SqThreadCpu:\t%d\n", sq_cpu);
 	seq_printf(m, "UserFiles:\t%u\n", ctx->nr_user_files);
 	for (i = 0; has_lock && i < ctx->nr_user_files; i++) {
 		struct file *f = io_file_from_index(&ctx->file_table, i);

-- 
Jens Axboe