io_uring: don't rely on weak ->files references

Grab actual references to the files_struct. To avoid circular references
issues due to this, we add a per-task note that keeps track of what
io_uring contexts a task has used. When the tasks execs or exits its
assigned files, we cancel requests based on this tracking.

With that, we can grab proper references to the files table, and no
longer need to rely on stashing away ring_fd and ring_file to check
if the ring_fd may have been closed.

Cc: stable@vger.kernel.org # v5.5+
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/exec.c

@@ -62,6 +62,7 @@
 #include <linux/oom.h>
 #include <linux/compat.h>
 #include <linux/vmalloc.h>
+#include <linux/io_uring.h>
 
 #include <linux/uaccess.h>
 #include <asm/mmu_context.h>
@@ -1895,6 +1896,11 @@ static int bprm_execve(struct linux_binprm *bprm,
 	struct files_struct *displaced;
 	int retval;
 
+	/*
+	 * Cancel any io_uring activity across execve
+	 */
+	io_uring_task_cancel();
+
 	retval = unshare_files(&displaced);
 	if (retval)
 		return retval;

fs/file.c

@@ -21,6 +21,7 @@
 #include <linux/rcupdate.h>
 #include <linux/close_range.h>
 #include <net/sock.h>
+#include <linux/io_uring.h>
 
 unsigned int sysctl_nr_open __read_mostly = 1024*1024;
 unsigned int sysctl_nr_open_min = BITS_PER_LONG;
@@ -452,6 +453,7 @@ void exit_files(struct task_struct *tsk)
 	struct files_struct * files = tsk->files;
 
 	if (files) {
+		io_uring_files_cancel(files);
 		task_lock(tsk);
 		tsk->files = NULL;
 		task_unlock(tsk);

fs/io_uring.c

@@ -79,6 +79,7 @@
 #include <linux/splice.h>
 #include <linux/task_work.h>
 #include <linux/pagemap.h>
+#include <linux/io_uring.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/io_uring.h>
@@ -284,8 +285,6 @@ struct io_ring_ctx {
 	 */
 	struct fixed_file_data	*file_data;
 	unsigned		nr_user_files;
-	int 			ring_fd;
-	struct file 		*ring_file;
 
 	/* if used, fixed mapped user buffers */
 	unsigned		nr_user_bufs;
@@ -1433,7 +1432,12 @@ static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags)
 		WRITE_ONCE(cqe->user_data, req->user_data);
 		WRITE_ONCE(cqe->res, res);
 		WRITE_ONCE(cqe->flags, cflags);
-	} else if (ctx->cq_overflow_flushed) {
+	} else if (ctx->cq_overflow_flushed || req->task->io_uring->in_idle) {
+		/*
+		 * If we're in ring overflow flush mode, or in task cancel mode,
+		 * then we cannot store the request for later flushing, we need
+		 * to drop it on the floor.
+		 */
 		WRITE_ONCE(ctx->rings->cq_overflow,
 				atomic_inc_return(&ctx->cached_cq_overflow));
 	} else {
@@ -1591,8 +1595,12 @@ static bool io_dismantle_req(struct io_kiocb *req)
 
 static void __io_free_req_finish(struct io_kiocb *req)
 {
+	struct io_uring_task *tctx = req->task->io_uring;
 	struct io_ring_ctx *ctx = req->ctx;
 
+	atomic_long_inc(&tctx->req_complete);
+	if (tctx->in_idle)
+		wake_up(&tctx->wait);
 	put_task_struct(req->task);
 
 	if (likely(!io_is_fallback_req(req)))
@@ -1907,6 +1915,7 @@ static void io_req_free_batch_finish(struct io_ring_ctx *ctx,
 	if (rb->to_free)
 		__io_req_free_batch_flush(ctx, rb);
 	if (rb->task) {
+		atomic_long_add(rb->task_refs, &rb->task->io_uring->req_complete);
 		put_task_struct_many(rb->task, rb->task_refs);
 		rb->task = NULL;
 	}
@@ -1922,8 +1931,10 @@ static void io_req_free_batch(struct req_batch *rb, struct io_kiocb *req)
 		io_queue_next(req);
 
 	if (req->task != rb->task) {
-		if (rb->task)
+		if (rb->task) {
+			atomic_long_add(rb->task_refs, &rb->task->io_uring->req_complete);
 			put_task_struct_many(rb->task, rb->task_refs);
+		}
 		rb->task = req->task;
 		rb->task_refs = 0;
 	}
@@ -3978,8 +3989,7 @@ static int io_close_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		return -EBADF;
 
 	req->close.fd = READ_ONCE(sqe->fd);
-	if ((req->file && req->file->f_op == &io_uring_fops) ||
-	    req->close.fd == req->ctx->ring_fd)
+	if ((req->file && req->file->f_op == &io_uring_fops))
 		return -EBADF;
 
 	req->close.put_file = NULL;
@@ -5667,6 +5677,7 @@ static void io_req_drop_files(struct io_kiocb *req)
 		wake_up(&ctx->inflight_wait);
 	spin_unlock_irqrestore(&ctx->inflight_lock, flags);
 	req->flags &= ~REQ_F_INFLIGHT;
+	put_files_struct(req->work.files);
 	req->work.files = NULL;
 }
 
@@ -6067,34 +6078,20 @@ static int io_req_set_file(struct io_submit_state *state, struct io_kiocb *req,
 
 static int io_grab_files(struct io_kiocb *req)
 {
-	int ret = -EBADF;
 	struct io_ring_ctx *ctx = req->ctx;
 
 	io_req_init_async(req);
 
 	if (req->work.files || (req->flags & REQ_F_NO_FILE_TABLE))
 		return 0;
-	if (!ctx->ring_file)
-		return -EBADF;
 
-	rcu_read_lock();
+	req->work.files = get_files_struct(current);
+	req->flags |= REQ_F_INFLIGHT;
+
 	spin_lock_irq(&ctx->inflight_lock);
-	/*
-	 * We use the f_ops->flush() handler to ensure that we can flush
-	 * out work accessing these files if the fd is closed. Check if
-	 * the fd has changed since we started down this path, and disallow
-	 * this operation if it has.
-	 */
-	if (fcheck(ctx->ring_fd) == ctx->ring_file) {
-		list_add(&req->inflight_entry, &ctx->inflight_list);
-		req->flags |= REQ_F_INFLIGHT;
-		req->work.files = current->files;
-		ret = 0;
-	}
+	list_add(&req->inflight_entry, &ctx->inflight_list);
 	spin_unlock_irq(&ctx->inflight_lock);
-	rcu_read_unlock();
-
-	return ret;
+	return 0;
 }
 
 static inline int io_prep_work_files(struct io_kiocb *req)
@@ -6459,6 +6456,7 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	refcount_set(&req->refs, 2);
 	req->task = current;
 	get_task_struct(req->task);
+	atomic_long_inc(&req->task->io_uring->req_issue);
 	req->result = 0;
 
 	if (unlikely(req->opcode >= IORING_OP_LAST))
@@ -6494,8 +6492,7 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	return io_req_set_file(state, req, READ_ONCE(sqe->fd));
 }
 
-static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
-			  struct file *ring_file, int ring_fd)
+static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 {
 	struct io_submit_state state;
 	struct io_kiocb *link = NULL;
@@ -6516,9 +6513,6 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 
 	io_submit_state_start(&state, ctx, nr);
 
-	ctx->ring_fd = ring_fd;
-	ctx->ring_file = ring_file;
-
 	for (i = 0; i < nr; i++) {
 		const struct io_uring_sqe *sqe;
 		struct io_kiocb *req;
@@ -6687,7 +6681,7 @@ static int io_sq_thread(void *data)
 
 		mutex_lock(&ctx->uring_lock);
 		if (likely(!percpu_ref_is_dying(&ctx->refs)))
-			ret = io_submit_sqes(ctx, to_submit, NULL, -1);
+			ret = io_submit_sqes(ctx, to_submit);
 		mutex_unlock(&ctx->uring_lock);
 		timeout = jiffies + ctx->sq_thread_idle;
 	}
@@ -7516,6 +7510,34 @@ static int io_init_wq_offload(struct io_ring_ctx *ctx,
 	return ret;
 }
 
+static int io_uring_alloc_task_context(struct task_struct *task)
+{
+	struct io_uring_task *tctx;
+
+	tctx = kmalloc(sizeof(*tctx), GFP_KERNEL);
+	if (unlikely(!tctx))
+		return -ENOMEM;
+
+	xa_init(&tctx->xa);
+	init_waitqueue_head(&tctx->wait);
+	tctx->last = NULL;
+	tctx->in_idle = 0;
+	atomic_long_set(&tctx->req_issue, 0);
+	atomic_long_set(&tctx->req_complete, 0);
+	task->io_uring = tctx;
+	return 0;
+}
+
+void __io_uring_free(struct task_struct *tsk)
+{
+	struct io_uring_task *tctx = tsk->io_uring;
+
+	WARN_ON_ONCE(!xa_empty(&tctx->xa));
+	xa_destroy(&tctx->xa);
+	kfree(tctx);
+	tsk->io_uring = NULL;
+}
+
 static int io_sq_offload_start(struct io_ring_ctx *ctx,
 			       struct io_uring_params *p)
 {
@@ -7551,6 +7573,9 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
 			ctx->sqo_thread = NULL;
 			goto err;
 		}
+		ret = io_uring_alloc_task_context(ctx->sqo_thread);
+		if (ret)
+			goto err;
 		wake_up_process(ctx->sqo_thread);
 	} else if (p->flags & IORING_SETUP_SQ_AFF) {
 		/* Can't have SQ_AFF without SQPOLL */
@@ -8063,7 +8088,7 @@ static bool io_wq_files_match(struct io_wq_work *work, void *data)
 {
 	struct files_struct *files = data;
 
-	return work->files == files;
+	return !files || work->files == files;
 }
 
 /*
@@ -8218,7 +8243,7 @@ static bool io_uring_cancel_files(struct io_ring_ctx *ctx,
 
 		spin_lock_irq(&ctx->inflight_lock);
 		list_for_each_entry(req, &ctx->inflight_list, inflight_entry) {
-			if (req->work.files != files)
+			if (files && req->work.files != files)
 				continue;
 			/* req is being completed, ignore */
 			if (!refcount_inc_not_zero(&req->refs))
@@ -8254,18 +8279,217 @@ static bool io_cancel_task_cb(struct io_wq_work *work, void *data)
 	return io_task_match(req, task);
 }
 
+static bool __io_uring_cancel_task_requests(struct io_ring_ctx *ctx,
+					    struct task_struct *task,
+					    struct files_struct *files)
+{
+	bool ret;
+
+	ret = io_uring_cancel_files(ctx, files);
+	if (!files) {
+		enum io_wq_cancel cret;
+
+		cret = io_wq_cancel_cb(ctx->io_wq, io_cancel_task_cb, task, true);
+		if (cret != IO_WQ_CANCEL_NOTFOUND)
+			ret = true;
+
+		/* SQPOLL thread does its own polling */
+		if (!(ctx->flags & IORING_SETUP_SQPOLL)) {
+			while (!list_empty_careful(&ctx->iopoll_list)) {
+				io_iopoll_try_reap_events(ctx);
+				ret = true;
+			}
+		}
+
+		ret |= io_poll_remove_all(ctx, task);
+		ret |= io_kill_timeouts(ctx, task);
+	}
+
+	return ret;
+}
+
+/*
+ * We need to iteratively cancel requests, in case a request has dependent
+ * hard links. These persist even for failure of cancelations, hence keep
+ * looping until none are found.
+ */
+static void io_uring_cancel_task_requests(struct io_ring_ctx *ctx,
+					  struct files_struct *files)
+{
+	struct task_struct *task = current;
+
+	if (ctx->flags & IORING_SETUP_SQPOLL)
+		task = ctx->sqo_thread;
+
+	io_cqring_overflow_flush(ctx, true, task, files);
+
+	while (__io_uring_cancel_task_requests(ctx, task, files)) {
+		io_run_task_work();
+		cond_resched();
+	}
+}
+
+/*
+ * Note that this task has used io_uring. We use it for cancelation purposes.
+ */
+static int io_uring_add_task_file(struct file *file)
+{
+	if (unlikely(!current->io_uring)) {
+		int ret;
+
+		ret = io_uring_alloc_task_context(current);
+		if (unlikely(ret))
+			return ret;
+	}
+	if (current->io_uring->last != file) {
+		XA_STATE(xas, &current->io_uring->xa, (unsigned long) file);
+		void *old;
+
+		rcu_read_lock();
+		old = xas_load(&xas);
+		if (old != file) {
+			get_file(file);
+			xas_lock(&xas);
+			xas_store(&xas, file);
+			xas_unlock(&xas);
+		}
+		rcu_read_unlock();
+		current->io_uring->last = file;
+	}
+
+	return 0;
+}
+
+/*
+ * Remove this io_uring_file -> task mapping.
+ */
+static void io_uring_del_task_file(struct file *file)
+{
+	struct io_uring_task *tctx = current->io_uring;
+	XA_STATE(xas, &tctx->xa, (unsigned long) file);
+
+	if (tctx->last == file)
+		tctx->last = NULL;
+
+	xas_lock(&xas);
+	file = xas_store(&xas, NULL);
+	xas_unlock(&xas);
+
+	if (file)
+		fput(file);
+}
+
+static void __io_uring_attempt_task_drop(struct file *file)
+{
+	XA_STATE(xas, &current->io_uring->xa, (unsigned long) file);
+	struct file *old;
+
+	rcu_read_lock();
+	old = xas_load(&xas);
+	rcu_read_unlock();
+
+	if (old == file)
+		io_uring_del_task_file(file);
+}
+
+/*
+ * Drop task note for this file if we're the only ones that hold it after
+ * pending fput()
+ */
+static void io_uring_attempt_task_drop(struct file *file, bool exiting)
+{
+	if (!current->io_uring)
+		return;
+	/*
+	 * fput() is pending, will be 2 if the only other ref is our potential
+	 * task file note. If the task is exiting, drop regardless of count.
+	 */
+	if (!exiting && atomic_long_read(&file->f_count) != 2)
+		return;
+
+	__io_uring_attempt_task_drop(file);
+}
+
+void __io_uring_files_cancel(struct files_struct *files)
+{
+	struct io_uring_task *tctx = current->io_uring;
+	XA_STATE(xas, &tctx->xa, 0);
+
+	/* make sure overflow events are dropped */
+	tctx->in_idle = true;
+
+	do {
+		struct io_ring_ctx *ctx;
+		struct file *file;
+
+		xas_lock(&xas);
+		file = xas_next_entry(&xas, ULONG_MAX);
+		xas_unlock(&xas);
+
+		if (!file)
+			break;
+
+		ctx = file->private_data;
+
+		io_uring_cancel_task_requests(ctx, files);
+		if (files)
+			io_uring_del_task_file(file);
+	} while (1);
+}
+
+static inline bool io_uring_task_idle(struct io_uring_task *tctx)
+{
+	return atomic_long_read(&tctx->req_issue) ==
+		atomic_long_read(&tctx->req_complete);
+}
+
+/*
+ * Find any io_uring fd that this task has registered or done IO on, and cancel
+ * requests.
+ */
+void __io_uring_task_cancel(void)
+{
+	struct io_uring_task *tctx = current->io_uring;
+	DEFINE_WAIT(wait);
+	long completions;
+
+	/* make sure overflow events are dropped */
+	tctx->in_idle = true;
+
+	while (!io_uring_task_idle(tctx)) {
+		/* read completions before cancelations */
+		completions = atomic_long_read(&tctx->req_complete);
+		__io_uring_files_cancel(NULL);
+
+		prepare_to_wait(&tctx->wait, &wait, TASK_UNINTERRUPTIBLE);
+
+		/*
+		 * If we've seen completions, retry. This avoids a race where
+		 * a completion comes in before we did prepare_to_wait().
+		 */
+		if (completions != atomic_long_read(&tctx->req_complete))
+			continue;
+		if (io_uring_task_idle(tctx))
+			break;
+		schedule();
+	}
+
+	finish_wait(&tctx->wait, &wait);
+	tctx->in_idle = false;
+}
+
 static int io_uring_flush(struct file *file, void *data)
 {
 	struct io_ring_ctx *ctx = file->private_data;
 
-	io_uring_cancel_files(ctx, data);
-
 	/*
 	 * If the task is going away, cancel work it may have pending
 	 */
 	if (fatal_signal_pending(current) || (current->flags & PF_EXITING))
-		io_wq_cancel_cb(ctx->io_wq, io_cancel_task_cb, current, true);
+		data = NULL;
 
+	io_uring_cancel_task_requests(ctx, data);
+	io_uring_attempt_task_drop(file, !data);
 	return 0;
 }
 
@@ -8379,8 +8603,11 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 			wake_up(&ctx->sqo_wait);
 		submitted = to_submit;
 	} else if (to_submit) {
+		ret = io_uring_add_task_file(f.file);
+		if (unlikely(ret))
+			goto out;
 		mutex_lock(&ctx->uring_lock);
-		submitted = io_submit_sqes(ctx, to_submit, f.file, fd);
+		submitted = io_submit_sqes(ctx, to_submit);
 		mutex_unlock(&ctx->uring_lock);
 
 		if (submitted != to_submit)
@@ -8590,6 +8817,7 @@ static int io_uring_get_fd(struct io_ring_ctx *ctx)
 	file = anon_inode_getfile("[io_uring]", &io_uring_fops, ctx,
 					O_RDWR | O_CLOEXEC);
 	if (IS_ERR(file)) {
+err_fd:
 		put_unused_fd(ret);
 		ret = PTR_ERR(file);
 		goto err;
@@ -8598,6 +8826,10 @@ static int io_uring_get_fd(struct io_ring_ctx *ctx)
 #if defined(CONFIG_UNIX)
 	ctx->ring_sock->file = file;
 #endif
+	if (unlikely(io_uring_add_task_file(file))) {
+		file = ERR_PTR(-ENOMEM);
+		goto err_fd;
+	}
 	fd_install(ret, file);
 	return ret;
 err:

include/linux/io_uring.h

@@ -0,0 +1,53 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _LINUX_IO_URING_H
+#define _LINUX_IO_URING_H
+
+#include <linux/sched.h>
+#include <linux/xarray.h>
+#include <linux/percpu-refcount.h>
+
+struct io_uring_task {
+	/* submission side */
+	struct xarray		xa;
+	struct wait_queue_head	wait;
+	struct file		*last;
+	atomic_long_t		req_issue;
+
+	/* completion side */
+	bool			in_idle ____cacheline_aligned_in_smp;
+	atomic_long_t		req_complete;
+};
+
+#if defined(CONFIG_IO_URING)
+void __io_uring_task_cancel(void);
+void __io_uring_files_cancel(struct files_struct *files);
+void __io_uring_free(struct task_struct *tsk);
+
+static inline void io_uring_task_cancel(void)
+{
+	if (current->io_uring && !xa_empty(&current->io_uring->xa))
+		__io_uring_task_cancel();
+}
+static inline void io_uring_files_cancel(struct files_struct *files)
+{
+	if (current->io_uring && !xa_empty(&current->io_uring->xa))
+		__io_uring_files_cancel(files);
+}
+static inline void io_uring_free(struct task_struct *tsk)
+{
+	if (tsk->io_uring)
+		__io_uring_free(tsk);
+}
+#else
+static inline void io_uring_task_cancel(void)
+{
+}
+static inline void io_uring_files_cancel(struct files_struct *files)
+{
+}
+static inline void io_uring_free(struct task_struct *tsk)
+{
+}
+#endif
+
+#endif

include/linux/sched.h

@@ -63,6 +63,7 @@ struct sighand_struct;
 struct signal_struct;
 struct task_delay_info;
 struct task_group;
+struct io_uring_task;
 
 /*
  * Task state bitmask. NOTE! These bits are also
@@ -935,6 +936,10 @@ struct task_struct {
 	/* Open file information: */
 	struct files_struct		*files;
 
+#ifdef CONFIG_IO_URING
+	struct io_uring_task		*io_uring;
+#endif
+
 	/* Namespaces: */
 	struct nsproxy			*nsproxy;
 

init/init_task.c

@@ -114,6 +114,9 @@ struct task_struct init_task
 	.thread		= INIT_THREAD,
 	.fs		= &init_fs,
 	.files		= &init_files,
+#ifdef CONFIG_IO_URING
+	.io_uring	= NULL,
+#endif
 	.signal		= &init_signals,
 	.sighand	= &init_sighand,
 	.nsproxy	= &init_nsproxy,

kernel/fork.c

@@ -95,6 +95,7 @@
 #include <linux/stackleak.h>
 #include <linux/kasan.h>
 #include <linux/scs.h>
+#include <linux/io_uring.h>
 
 #include <asm/pgalloc.h>
 #include <linux/uaccess.h>
@@ -728,6 +729,7 @@ void __put_task_struct(struct task_struct *tsk)
 	WARN_ON(refcount_read(&tsk->usage));
 	WARN_ON(tsk == current);
 
+	io_uring_free(tsk);
 	cgroup_free(tsk);
 	task_numa_free(tsk, true);
 	security_task_free(tsk);
@@ -1983,6 +1985,10 @@ static __latent_entropy struct task_struct *copy_process(
 	p->vtime.state = VTIME_INACTIVE;
 #endif
 
+#ifdef CONFIG_IO_URING
+	p->io_uring = NULL;
+#endif
+
 #if defined(SPLIT_RSS_COUNTING)
 	memset(&p->rss_stat, 0, sizeof(p->rss_stat));
 #endif