commit 3edc936354a5a0ad38a4c258e5db8f7c2e2eb505
Author: Jens Axboe <axboe@kernel.dk>
Date:   Mon Nov 25 09:09:43 2019 -0700

    io_uring: async workers should inherit the user creds
    
    If we don't inherit the original task creds, then we can confuse users
    like fuse that pass creds in the request header. See link below on
    identical aio issue.
    
    Link: https://lore.kernel.org/linux-fsdevel/26f0d78e-99ca-2f1b-78b9-433088053a61@scylladb.com/T/#u
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 2c819c3c855d..cbe8dabb6479 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -238,6 +238,8 @@ struct io_ring_ctx {
 
 	struct user_struct	*user;
 
+	struct cred		*creds;
+
 	struct completion	ctx_done;
 
 	struct {
@@ -1752,8 +1754,11 @@ static void io_poll_complete_work(struct work_struct *work)
 	struct io_poll_iocb *poll = &req->poll;
 	struct poll_table_struct pt = { ._key = poll->events };
 	struct io_ring_ctx *ctx = req->ctx;
+	const struct cred *old_cred;
 	__poll_t mask = 0;
 
+	old_cred = override_creds(ctx->creds);
+
 	if (!READ_ONCE(poll->canceled))
 		mask = vfs_poll(poll->file, &pt) & poll->events;
 
@@ -1768,7 +1773,7 @@ static void io_poll_complete_work(struct work_struct *work)
 	if (!mask && !READ_ONCE(poll->canceled)) {
 		add_wait_queue(poll->head, &poll->wait);
 		spin_unlock_irq(&ctx->completion_lock);
-		return;
+		goto out;
 	}
 	list_del_init(&req->list);
 	io_poll_complete(ctx, req, mask);
@@ -1776,6 +1781,8 @@ static void io_poll_complete_work(struct work_struct *work)
 
 	io_cqring_ev_posted(ctx);
 	io_put_req(req);
+out:
+	revert_creds(old_cred);
 }
 
 static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
@@ -2147,10 +2154,12 @@ static void io_sq_wq_submit_work(struct work_struct *work)
 	struct io_ring_ctx *ctx = req->ctx;
 	struct mm_struct *cur_mm = NULL;
 	struct async_list *async_list;
+	const struct cred *old_cred;
 	LIST_HEAD(req_list);
 	mm_segment_t old_fs;
 	int ret;
 
+	old_cred = override_creds(ctx->creds);
 	async_list = io_async_list_from_sqe(ctx, req->submit.sqe);
 restart:
 	do {
@@ -2258,6 +2267,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
 		unuse_mm(cur_mm);
 		mmput(cur_mm);
 	}
+	revert_creds(old_cred);
 }
 
 /*
@@ -2663,6 +2673,7 @@ static int io_sq_thread(void *data)
 {
 	struct io_ring_ctx *ctx = data;
 	struct mm_struct *cur_mm = NULL;
+	const struct cred *old_cred;
 	mm_segment_t old_fs;
 	DEFINE_WAIT(wait);
 	unsigned inflight;
@@ -2672,6 +2683,7 @@ static int io_sq_thread(void *data)
 
 	old_fs = get_fs();
 	set_fs(USER_DS);
+	old_cred = override_creds(ctx->creds);
 
 	timeout = inflight = 0;
 	while (!kthread_should_park()) {
@@ -2782,6 +2794,7 @@ static int io_sq_thread(void *data)
 		unuse_mm(cur_mm);
 		mmput(cur_mm);
 	}
+	revert_creds(old_cred);
 
 	kthread_parkme();
 
@@ -3567,6 +3580,8 @@ static void io_ring_ctx_free(struct io_ring_ctx *ctx)
 		io_unaccount_mem(ctx->user,
 				ring_pages(ctx->sq_entries, ctx->cq_entries));
 	free_uid(ctx->user);
+	if (ctx->creds)
+		put_cred(ctx->creds);
 	kfree(ctx);
 }
 
@@ -3838,6 +3853,12 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
 	ctx->account_mem = account_mem;
 	ctx->user = user;
 
+	ctx->creds = prepare_creds();
+	if (!ctx->creds) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
 	ret = io_allocate_scq_urings(ctx, p);
 	if (ret)
 		goto err;