linux-kernel - [PATCH] fs: Fix data race in inode_set_ctime_to

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Message-Id: <20241120024306.156920-1-zhenghaoran@buaa.edu.cn>
Date: Wed, 20 Nov 2024 10:43:06 +0800
From: Hao-ran Zheng <zhenghaoran@...a.edu.cn>
To: viro@...iv.linux.org.uk,
	brauner@...nel.org,
	jack@...e.cz,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: baijiaju1990@...il.com,
	zhenghaoran@...a.edu.cn,
	21371365@...a.edu.cn
Subject: [PATCH] fs: Fix data race in inode_set_ctime_to_ts

A data race may occur when the function `inode_set_ctime_to_ts()` and
the function `inode_get_ctime_sec()` are executed concurrently. When
two threads call `aio_read` and `aio_write` respectively, they will
be distributed to the read and write functions of the corresponding
file system respectively. Taking the btrfs file system as an example,
the `btrfs_file_read_iter` and `btrfs_file_write_iter` functions are
finally called. These two functions created a data race when they
finally called `inode_get_ctime_sec()` and `inode_set_ctime_to_ns()`.
The specific call stack that appears during testing is as follows:

```
============DATA_RACE============
btrfs_delayed_update_inode+0x1f61/0x7ce0 [btrfs]
btrfs_update_inode+0x45e/0xbb0 [btrfs]
btrfs_dirty_inode+0x2b8/0x530 [btrfs]
btrfs_update_time+0x1ad/0x230 [btrfs]
touch_atime+0x211/0x440
filemap_read+0x90f/0xa20
btrfs_file_read_iter+0xeb/0x580 [btrfs]
aio_read+0x275/0x3a0
io_submit_one+0xd22/0x1ce0
__se_sys_io_submit+0xb3/0x250
do_syscall_64+0xc1/0x190
entry_SYSCALL_64_after_hwframe+0x77/0x7f
============OTHER_INFO============
btrfs_write_check+0xa15/0x1390 [btrfs]
btrfs_buffered_write+0x52f/0x29d0 [btrfs]
btrfs_do_write_iter+0x53d/0x1590 [btrfs]
btrfs_file_write_iter+0x41/0x60 [btrfs]
aio_write+0x41e/0x5f0
io_submit_one+0xd42/0x1ce0
__se_sys_io_submit+0xb3/0x250
do_syscall_64+0xc1/0x190
entry_SYSCALL_64_after_hwframe+0x77/0x7f
```

The call chain after traceability is as follows:

```
Thread1:
btrfs_delayed_update_inode() ->
fill_stack_inode_item() ->
inode_get_ctime_sec()

Thread2:
btrfs_write_check() ->
update_time_for_write() ->
inode_set_ctime_to_ts()
```

To address this issue, it is recommended to
add WRITE_ONCE when writing the `inode->i_ctime_sec` variable.

Signed-off-by: Hao-ran Zheng <zhenghaoran@...a.edu.cn>
---
 include/linux/fs.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3559446279c1..d11b257a35e1 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1674,8 +1674,8 @@ static inline struct timespec64 inode_get_ctime(const struct inode *inode)
 static inline struct timespec64 inode_set_ctime_to_ts(struct inode *inode,
 						      struct timespec64 ts)
 {
-	inode->i_ctime_sec = ts.tv_sec;
-	inode->i_ctime_nsec = ts.tv_nsec;
+	WRITE_ONCE(inode->i_ctime_sec, ts.tv_sec);
+	WRITE_ONCE(inode->i_ctime_nsec, ts.tv_nsec);
 	return ts;
 }
 
-- 
2.34.1