linux-kernel - Re: [PATCH] Fix BUG: KCSAN: data-race in xas_find_marked / xas_init

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Message-ID: <87wmhrziwq.fsf@mail.parknet.co.jp>
Date: Tue, 29 Oct 2024 12:29:09 +0900
From: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To: Daniel Yang <danielyangkang@...il.com>
Cc: linux-kernel@...r.kernel.org (open list),
        syzbot+0dd28f0c6293cc87d462@...kaller.appspotmail.com
Subject: Re: [PATCH] Fix BUG: KCSAN: data-race in xas_find_marked /
 xas_init_marks

Daniel Yang <danielyangkang@...il.com> writes:

> The cause of the bug is a call to truncate_setsize() while
> fat_file_fsync() is attempting to read. Although fat_setattr() acquires
> &MSDOS_I(inode)->truncate_lock before calling truncate, fat_file_fsync()
> doesn't acquire the read lock for truncate_lock. The function
> __generic_file_fsync() called in fat_file_fsync() only acquires lock to
> the inode itself, not the &MSDOS_I(inode) it is a member of. This leads
> to the data race where fat_file_fsync is reading during truncation.
>
> To fix: &MSDOS_I(inode)->truncate_lock is acquired for reading before
> calling __generic_file_fsync().

If writepages really races with truncate, why normal writeback path
doesn't need truncate_lock? And this introduces the ABBA locking bug.

Thanks.

> Signed-off-by: Daniel Yang <danielyangkang@...il.com>
> Reported-by: syzbot+0dd28f0c6293cc87d462@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=0dd28f0c6293cc87d462
> ---
>  fs/fat/file.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/fs/fat/file.c b/fs/fat/file.c
> index e887e9ab7..5578b771a 100644
> --- a/fs/fat/file.c
> +++ b/fs/fat/file.c
> @@ -188,7 +188,9 @@ int fat_file_fsync(struct file *filp, loff_t start, loff_t end, int datasync)
>  	struct inode *inode = filp->f_mapping->host;
>  	int err;
>  
> +	down_read(&MSDOS_I(inode)->truncate_lock);
>  	err = __generic_file_fsync(filp, start, end, datasync);
> +	up_read(&MSDOS_I(inode)->truncate_lock);
>  	if (err)
>  		return err;

-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>