Re: [PATCH v7 3/7] exec: Move path_noexec() check earlier

[Eric W. Biederman]

Mickaël Salaün <mic@xxxxxxxxxxx> writes:

> From: Kees Cook <keescook@xxxxxxxxxxxx>
>
> The path_noexec() check, like the regular file check, was happening too
> late, letting LSMs see impossible execve()s. Check it earlier as well
> in may_open() and collect the redundant fs/exec.c path_noexec() test
> under the same robustness comment as the S_ISREG() check.
>
> My notes on the call path, and related arguments, checks, etc:

A big question arises, that I think someone already asked.

Why perform this test in may_open directly instead of moving
it into inode_permission.  That way the code can be shared with
faccessat, and any other code path that wants it?

That would look to provide a more maintainable kernel.

Eric


> do_open_execat()
>     struct open_flags open_exec_flags = {
>         .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>         .acc_mode = MAY_EXEC,
>         ...
>     do_filp_open(dfd, filename, open_flags)
>         path_openat(nameidata, open_flags, flags)
>             file = alloc_empty_file(open_flags, current_cred());
>             do_open(nameidata, file, open_flags)
>                 may_open(path, acc_mode, open_flag)
>                     /* new location of MAY_EXEC vs path_noexec() test */
>                     inode_permission(inode, MAY_OPEN | acc_mode)
>                         security_inode_permission(inode, acc_mode)
>                 vfs_open(path, file)
>                     do_dentry_open(file, path->dentry->d_inode, open)
>                         security_file_open(f)
>                         open()
>     /* old location of path_noexec() test */
>
> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20200605160013.3954297-4-keescook@xxxxxxxxxxxx
> ---
>  fs/exec.c  | 12 ++++--------
>  fs/namei.c |  4 ++++
>  2 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index bdc6a6eb5dce..4eea20c27b01 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -147,10 +147,8 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
>  	 * and check again at the very end too.
>  	 */
>  	error = -EACCES;
> -	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> -		goto exit;
> -
> -	if (path_noexec(&file->f_path))
> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> +			 path_noexec(&file->f_path)))
>  		goto exit;
>  
>  	fsnotify_open(file);
> @@ -897,10 +895,8 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
>  	 * and check again at the very end too.
>  	 */
>  	err = -EACCES;
> -	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> -		goto exit;
> -
> -	if (path_noexec(&file->f_path))
> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> +			 path_noexec(&file->f_path)))
>  		goto exit;
>  
>  	err = deny_write_access(file);
> diff --git a/fs/namei.c b/fs/namei.c
> index a559ad943970..ddc9b25540fe 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2863,6 +2863,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
>  			return -EACCES;
>  		flag &= ~O_TRUNC;
>  		break;
> +	case S_IFREG:
> +		if ((acc_mode & MAY_EXEC) && path_noexec(path))
> +			return -EACCES;
> +		break;
>  	}
>  
>  	error = inode_permission(inode, MAY_OPEN | acc_mode);