Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for faccessat2(2)

[Mimi Zohar <zohar@linux.ibm.com>]

Hi Mickael,

On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün wrote:
> diff --git a/fs/open.c b/fs/open.c
> index 9af548fb841b..879bdfbdc6fa 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -405,9 +405,13 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
>  	if (mode & ~S_IRWXO)	/* where's F_OK, X_OK, W_OK, R_OK? */
>  		return -EINVAL;
>  
> -	if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))
> +	if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH |
> +				AT_INTERPRETED))
>  		return -EINVAL;
>  
> +	/* Only allows X_OK with AT_INTERPRETED for now. */
> +	if ((flags & AT_INTERPRETED) && !(mode & S_IXOTH))
> +		return -EINVAL;
>  	if (flags & AT_SYMLINK_NOFOLLOW)
>  		lookup_flags &= ~LOOKUP_FOLLOW;
>  	if (flags & AT_EMPTY_PATH)
> @@ -426,7 +430,30 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
>  
>  	inode = d_backing_inode(path.dentry);
>  
> -	if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
> +	if ((flags & AT_INTERPRETED)) {
> +		/*
> +		 * For compatibility reasons, without a defined security policy
> +		 * (via sysctl or LSM), using AT_INTERPRETED must map the
> +		 * execute permission to the read permission.  Indeed, from
> +		 * user space point of view, being able to execute data (e.g.
> +		 * scripts) implies to be able to read this data.
> +		 *
> +		 * The MAY_INTERPRETED_EXEC bit is set to enable LSMs to add
> +		 * custom checks, while being compatible with current policies.
> +		 */
> +		if ((mode & MAY_EXEC)) {

Why is the ISREG() test being dropped?   Without dropping it, there
would be no reason for making the existing test an "else" clause.

> +			mode |= MAY_INTERPRETED_EXEC;
> +			/*
> +			 * For compatibility reasons, if the system-wide policy
> +			 * doesn't enforce file permission checks, then
> +			 * replaces the execute permission request with a read
> +			 * permission request.
> +			 */
> +			mode &= ~MAY_EXEC;
> +			/* To be executed *by* user space, files must be readable. */
> +			mode |= MAY_READ;

After this change, I'm wondering if it makes sense to add a call to
security_file_permission().  IMA doesn't currently define it, but
could.

Mimi

> +		}
> +	} else if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
>  		/*
>  		 * MAY_EXEC on regular files is denied if the fs is mounted
>  		 * with the "noexec" flag.