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Subject: Re: [PATCH] fs/io_uring: fix O_PATH fds in openat, openat2, statx
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Max Kellermann <mk@cm4all.com>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
References: <20200507185725.15840-1-mk@cm4all.com>
 <20200507190131.GF23230@ZenIV.linux.org.uk>
 <4cac0e53-656c-50f0-3766-ae3cc6c0310a@kernel.dk>
 <20200507192903.GG23230@ZenIV.linux.org.uk>
 <8e3c88cc-027b-4f90-b4f8-a20d11d35c4b@kernel.dk>
 <20200507220637.GH23230@ZenIV.linux.org.uk>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <283c8edb-fea2-5192-f1d6-3cc57815b1e2@kernel.dk>
Date:   Thu, 7 May 2020 16:25:24 -0600
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On 5/7/20 4:06 PM, Al Viro wrote:
> On Thu, May 07, 2020 at 02:53:30PM -0600, Jens Axboe wrote:
> 
>> I think the patch is correct as-is, I took a good look at how we're
>> currently handling it. None of those three ops should fiddle with
>> the fd at all, and all of them do forbid the use of fixed files (the
>> descriptor table look-alikes), so that part is fine, too.
>>
>> There's some low hanging fruit around optimizing and improving it,
>> I'm including an updated version below. Max, can you double check
>> with your testing?
> 
> <looks>
> 
> Could you explain WTF is io_issue_sqe() doing in case of IORING_OP_CLOSE?
> Specifically, what is the value of
>         req->close.fd = READ_ONCE(sqe->fd);
>         if (req->file->f_op == &io_uring_fops ||
>             req->close.fd == req->ctx->ring_fd)
>                 return -EBADF;
> in io_close_prep()?  And what does happen if some joker does dup2()
> of something with io_uring_fops into our slot right after that check?
> Before the subsequent
>         ret = __close_fd_get_file(req->close.fd, &req->close.put_file);
> that is.

I agree, there's a gap there. We should do the check in the op handler,
and under the files_struct lock. How about something like the below?


diff --git a/fs/file.c b/fs/file.c
index c8a4e4c86e55..50ee73b76d17 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -646,18 +646,13 @@ int __close_fd(struct files_struct *files, unsigned fd)
 }
 EXPORT_SYMBOL(__close_fd); /* for ksys_close() */
 
-/*
- * variant of __close_fd that gets a ref on the file for later fput.
- * The caller must ensure that filp_close() called on the file, and then
- * an fput().
- */
-int __close_fd_get_file(unsigned int fd, struct file **res)
+int __close_fd_get_file_locked(struct files_struct *files, unsigned int fd,
+			       struct file **res)
+	__releases(&files->file_lock)
 {
-	struct files_struct *files = current->files;
 	struct file *file;
 	struct fdtable *fdt;
 
-	spin_lock(&files->file_lock);
 	fdt = files_fdtable(files);
 	if (fd >= fdt->max_fds)
 		goto out_unlock;
@@ -677,6 +672,19 @@ int __close_fd_get_file(unsigned int fd, struct file **res)
 	return -ENOENT;
 }
 
+/*
+ * variant of __close_fd that gets a ref on the file for later fput.
+ * The caller must ensure that filp_close() called on the file, and then
+ * an fput().
+ */
+int __close_fd_get_file(unsigned int fd, struct file **res)
+{
+	struct files_struct *files = current->files;
+
+	spin_lock(&files->file_lock);
+	return __close_fd_get_file_locked(files, fd, res);
+}
+
 void do_close_on_exec(struct files_struct *files)
 {
 	unsigned i;
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 979d9f977409..740547106717 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3399,10 +3399,6 @@ 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->f_op == &io_uring_fops ||
-	    req->close.fd == req->ctx->ring_fd)
-		return -EBADF;
-
 	return 0;
 }
 
@@ -3430,10 +3426,19 @@ static void io_close_finish(struct io_wq_work **workptr)
 
 static int io_close(struct io_kiocb *req, bool force_nonblock)
 {
+	struct files_struct *files = current->files;
 	int ret;
 
 	req->close.put_file = NULL;
-	ret = __close_fd_get_file(req->close.fd, &req->close.put_file);
+	spin_lock(&files->file_lock);
+	if (req->file->f_op == &io_uring_fops ||
+	    req->close.fd == req->ctx->ring_fd) {
+		spin_unlock(&files->file_lock);
+		return -EBADF;
+	}
+
+	ret = __close_fd_get_file_locked(files, req->close.fd,
+						&req->close.put_file);
 	if (ret < 0)
 		return ret;
 
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index f07c55ea0c22..11d19303af46 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -122,6 +122,8 @@ extern void __fd_install(struct files_struct *files,
 extern int __close_fd(struct files_struct *files,
 		      unsigned int fd);
 extern int __close_fd_get_file(unsigned int fd, struct file **res);
+extern int __close_fd_get_file_locked(struct files_struct *files,
+				      unsigned int fd, struct file **res);
 
 extern struct kmem_cache *files_cachep;
 

-- 
Jens Axboe