linux-kernel - [PATCH] eventfd: strictly check the count parameter of eventfd

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Date: Wed,  7 Feb 2024 00:35:18 +0800
From: wenyang.linux@...mail.com
To: Christian Brauner <brauner@...nel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Alexander Viro <viro@...iv.linux.org.uk>
Cc: Wen Yang <wenyang.linux@...mail.com>,
	Jan Kara <jack@...e.cz>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Matthew Wilcox <willy@...radead.org>,
	Eric Biggers <ebiggers@...gle.com>,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] eventfd: strictly check the count parameter of eventfd_write to avoid inputting illegal strings

From: Wen Yang <wenyang.linux@...mail.com>

Since eventfd's document has clearly stated: A write(2) call adds
the 8-byte integer value supplied in its buffer to the counter.

However, in the current implementation, the following code snippet
did not cause an error:

	char str[16] = "hello world";
	uint64_t value;
	ssize_t size;
	int fd;

	fd = eventfd(0, 0);
	size = write(fd, &str, strlen(str));
	printf("eventfd: test writing a string, size=%ld\n", size);
	size = read(fd, &value, sizeof(value));
	printf("eventfd: test reading as uint64, size=%ld, valus=0x%lX\n",
	       size, value);

	close(fd);

And its output is:
eventfd: test writing a string, size=8
eventfd: test reading as uint64, size=8, valus=0x6F77206F6C6C6568

By checking whether count is equal to sizeof(ucnt), such errors
could be detected. It also follows the requirements of the manual.

Signed-off-by: Wen Yang <wenyang.linux@...mail.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Jan Kara <jack@...e.cz>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: Eric Biggers <ebiggers@...gle.com>
Cc: <linux-fsdevel@...r.kernel.org>
Cc: <linux-kernel@...r.kernel.org>
---
 fs/eventfd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/eventfd.c b/fs/eventfd.c
index fc4d81090763..9afdb722fa92 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -251,7 +251,7 @@ static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t c
 	ssize_t res;
 	__u64 ucnt;
 
-	if (count < sizeof(ucnt))
+	if (count != sizeof(ucnt))
 		return -EINVAL;
 	if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
 		return -EFAULT;
-- 
2.25.1