linux-kernel - Re: [PATCH v2] exec: Set file unwritable before LSM check

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Date:   Tue, 13 Mar 2018 16:16:04 +1100 (AEDT)
From:   James Morris <jmorris@...ei.org>
To:     Kees Cook <keescook@...omium.org>, Al Viro <viro@....linux.org.uk>
cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>
Subject: Re: [PATCH v2] exec: Set file unwritable before LSM check

On Fri, 9 Mar 2018, Kees Cook wrote:

> The LSM check should happen after the file has been confirmed to be
> unchanging. Without this, we could have a race between the Time of Check
> (the call to security_kernel_read_file() which could read the file and
> make access policy decisions) and the Time of Use (starting with
> kernel_read_file()'s reading of the file contents). In theory, file
> contents could change between the two.
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> v2: Clarify the ToC/ToU race (Linus)


I'll merge this unless Al objects (cc'd).

> 
> Only loadpin and SELinux currently implement this hook. From what
> I can see, this won't change anything for either of them. IMA calls
> kernel_read_file(), but looking there it seems those callers won't be
> negatively impacted either. Can folks double-check this and send an
> Ack please?
> ---
>  fs/exec.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 7eb8d21bcab9..a919a827d181 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -895,13 +895,13 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
>  	if (!S_ISREG(file_inode(file)->i_mode) || max_size < 0)
>  		return -EINVAL;
>  
> -	ret = security_kernel_read_file(file, id);
> +	ret = deny_write_access(file);
>  	if (ret)
>  		return ret;
>  
> -	ret = deny_write_access(file);
> +	ret = security_kernel_read_file(file, id);
>  	if (ret)
> -		return ret;
> +		goto out;
>  
>  	i_size = i_size_read(file_inode(file));
>  	if (max_size > 0 && i_size > max_size) {
> 

-- 
James Morris
<jmorris@...ei.org>