Re: [PATCH 01/12] smb: client: fix potential UAF in cifs_debug_files_proc_show()

[Wang Zhaolong <wangzhaolong1@huawei.com>]

Hello,

I have some questions regarding CVE-2024-26928.

I would like to confirm whether the phrase "fix potential UAF in
cifs_debug_files_proc_show()" implies that the UAF issue does not
actually exist, correct?

Based on my analysis, `cifs_tcp_ses_lock` plays a crucial role in
preventing the UAF.

After adding `ses` to the `smb_ses_list` list, the only place where
the `ses` is freed is in the `__cifs_put_smb_ses` function:

```
__cifs_put_smb_ses()
     ...
     spin_lock(&cifs_tcp_ses_lock);
     list_del_init(&ses->smb_ses_list);
     spin_unlock(&cifs_tcp_ses_lock);
     ...
     sesInfoFree(ses);
```

The `ses` is freed only after it is removed from the list, and this
removal is protected by `cifs_tcp_ses_lock`.

In `cifs_debug_files_proc_show()`, the `cifs_tcp_ses_lock` is still
held, ensuring that during access to the `ses` that is about to be
destroyed, the `ses` will not be freed in `__cifs_put_smb_ses()`,
thus preventing a UAF issue.

```
cifs_debug_files_proc_show()
     ...
     spin_lock(&cifs_tcp_ses_lock);
     ...
     list_for_each_entry(ses, &server->smb_ses_list, smb_ses_list)
     ...
     spin_lock(&cifs_tcp_ses_lock);
```
Based on this understanding, I wonder if the issue addressed by
this CVE might not be a genuine problem. I am also curious about
the series of patches considered as fixes for this CVE.

Best regards,
Wang Zhaolong


> Skip sessions that are being teared down (status == SES_EXITING) to
> avoid UAF.
> 
> Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
> ---
>   fs/smb/client/cifs_debug.c |  2 ++
>   fs/smb/client/cifsglob.h   | 10 ++++++++++
>   2 files changed, 12 insertions(+)
> 
> diff --git a/fs/smb/client/cifs_debug.c b/fs/smb/client/cifs_debug.c
> index 226d4835c92d..c9aec9a38ad3 100644
> --- a/fs/smb/client/cifs_debug.c
> +++ b/fs/smb/client/cifs_debug.c
> @@ -250,6 +250,8 @@ static int cifs_debug_files_proc_show(struct seq_file *m, void *v)
>   	spin_lock(&cifs_tcp_ses_lock);
>   	list_for_each_entry(server, &cifs_tcp_ses_list, tcp_ses_list) {
>   		list_for_each_entry(ses, &server->smb_ses_list, smb_ses_list) {
> +			if (cifs_ses_exiting(ses))
> +				continue;
>   			list_for_each_entry(tcon, &ses->tcon_list, tcon_list) {
>   				spin_lock(&tcon->open_file_lock);
>   				list_for_each_entry(cfile, &tcon->openFileList, tlist) {
> diff --git a/fs/smb/client/cifsglob.h b/fs/smb/client/cifsglob.h
> index 286afbe346be..f67607319c43 100644
> --- a/fs/smb/client/cifsglob.h
> +++ b/fs/smb/client/cifsglob.h
> @@ -2322,4 +2322,14 @@ struct smb2_compound_vars {
>   	struct kvec ea_iov;
>   };
>   
> +static inline bool cifs_ses_exiting(struct cifs_ses *ses)
> +{
> +	bool ret;
> +
> +	spin_lock(&ses->ses_lock);
> +	ret = ses->ses_status == SES_EXITING;
> +	spin_unlock(&ses->ses_lock);
> +	return ret;
> +}
> +
>   #endif	/* _CIFS_GLOB_H */