linux-kernel - Re: [PATCH] configfs: fix a race in configfs

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Message-ID: <AE8DCEC7-6056-4D70-9EA6-F312350BA767@purdue.edu>
Date:   Wed, 29 Dec 2021 02:23:31 +0000
From:   "Gong, Sishuai" <sishuai@...due.edu>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Christoph Hellwig <hch@....de>,
        "jlbec@...lplan.org" <jlbec@...lplan.org>
Subject: Re: [PATCH] configfs: fix a race in configfs_lookup()

Sorry, this email was delayed by several months due to some network 
issues on my machine. 

Please simply ignore it, since the bug mentioned is fixed already by 
the commit c42dd069be8dfc9b2239a5c89e73bbd08ab35de0.

> On Dec 28, 2021, at 8:35 PM, Sishuai Gong <sishuai@...due.edu> wrote:
> 
> From: sishuaigong <sishuai@...due.edu>
> 
> When configfs_lookup() is executing list_for_each_entry(),
> it is possible that configfs_dir_lseek() is calling list_del().
> Some unfortunate interleavings of them can cause a kernel NULL
> pointer dereference error
> 
> Thread 1                  Thread 2
> //configfs_dir_lseek()    //configfs_lookup()
> list_del(&cursor->s_sibling);
>                          list_for_each_entry(sd, ...)
> 
> Fix this bug by using list_for_each_entry_safe() instead.
> 
> Reported-by: Sishuai Gong <sishuai@...due.edu>
> Signed-off-by: sishuaigong <sishuai@...due.edu>
> ---
> fs/configfs/dir.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
> index ac5e0c0e9181..8f5d0309fb4a 100644
> --- a/fs/configfs/dir.c
> +++ b/fs/configfs/dir.c
> @@ -452,7 +452,7 @@ static struct dentry * configfs_lookup(struct inode *dir,
>                       unsigned int flags)
> {
>    struct configfs_dirent * parent_sd = dentry->d_parent->d_fsdata;
> -    struct configfs_dirent * sd;
> +    struct configfs_dirent *sd, *tmp;
>    int found = 0;
>    int err;
> 
> @@ -468,7 +468,7 @@ static struct dentry * configfs_lookup(struct inode *dir,
>    if (!configfs_dirent_is_ready(parent_sd))
>        goto out;
> 
> -    list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
> +    list_for_each_entry_safe(sd, tmp, &parent_sd->s_children, s_sibling) {
>        if (sd->s_type & CONFIGFS_NOT_PINNED) {
>            const unsigned char * name = configfs_get_name(sd);
> 
> -- 
> 2.17.1
>