linux-kernel - Re: [PATCH] ext4: Fix refcount leak bug in __ext4_new

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Message-ID: <20211007141522.GB2670@quack2.suse.cz>
Date:   Thu, 7 Oct 2021 16:15:22 +0200
From:   Jan Kara <jack@...e.cz>
To:     Chenyuan Mi <cymi20@...an.edu.cn>
Cc:     yuanxzhang@...an.edu.cn, Xiyu Yang <xiyuyang19@...an.edu.cn>,
        Xin Tan <tanxin.ctf@...il.com>, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: Fix refcount leak bug in __ext4_new_inode()

On Wed 08-09-21 10:46:05, Chenyuan Mi wrote:
> After successfully creating handle by __ext4_journal_start_sb(),
> the function forgets to decrease the refcount of handle in several
> paths, causing refcount leak.
> 
> Fix this issue by recording a flag when successfully getting handle
> by __ext4_journal_start_sb(), and decrease the refcount of handle
> when exiting this function if holding the flag.
> 
> Signed-off-by: Chenyuan Mi <cymi20@...an.edu.cn>
> Signed-off-by: Xiyu Yang <xiyuyang19@...an.edu.cn>
> Signed-off-by: Xin Tan <tanxin.ctf@...il.com>

I'm sorry but this patch is wrong. We are expected to return with handle
running from __ext4_new_inode(). Please have a look into callers of
ext4_new_inode() and ext4_new_inode_start_handle() how they use and then
stop the handle. And similarly as for the second patch I'd note that even a
small amount of testing would show you that this patch does not work.

                                                                Honza

> 
> ---
>  fs/ext4/ialloc.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> index f73e5eb43eae..7563b892c64f 100644
> --- a/fs/ext4/ialloc.c
> +++ b/fs/ext4/ialloc.c
> @@ -944,6 +944,7 @@ struct inode *__ext4_new_inode(struct user_namespace *mnt_userns,
>  	ext4_group_t flex_group;
>  	struct ext4_group_info *grp = NULL;
>  	bool encrypt = false;
> +	bool create_handle = false;
>  
>  	/* Cannot create files in a deleted directory */
>  	if (!dir || !dir->i_nlink)
> @@ -1085,6 +1086,7 @@ struct inode *__ext4_new_inode(struct user_namespace *mnt_userns,
>  				ext4_std_error(sb, err);
>  				goto out;
>  			}
> +			create_handle = true;
>  		}
>  		BUFFER_TRACE(inode_bitmap_bh, "get_write_access");
>  		err = ext4_journal_get_write_access(handle, sb, inode_bitmap_bh,
> @@ -1345,7 +1347,8 @@ struct inode *__ext4_new_inode(struct user_namespace *mnt_userns,
>  		ext4_std_error(sb, err);
>  		goto fail_free_drop;
>  	}
> -
> +	if (create_handle)
> +		ext4_journal_stop(handle);
>  	ext4_debug("allocating inode %lu\n", inode->i_ino);
>  	trace_ext4_allocate_inode(inode, dir, mode);
>  	brelse(inode_bitmap_bh);
> @@ -1357,6 +1360,8 @@ struct inode *__ext4_new_inode(struct user_namespace *mnt_userns,
>  	clear_nlink(inode);
>  	unlock_new_inode(inode);
>  out:
> +	if (create_handle)
> +		ext4_journal_stop(handle);
>  	dquot_drop(inode);
>  	inode->i_flags |= S_NOQUOTA;
>  	iput(inode);
> -- 
> 2.17.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR