Re: [PATCH v2 6/9] procfs: add permission checks on the file's opener of /proc/*/stat

[Andy Lutomirski <luto@amacapital.net>]

On 10/01/2013 01:26 PM, Djalal Harouni wrote:
> Some fields of the /proc/*/stat are sensitive fields that need
> appropriate protection.
> 
> However, /proc file descriptors can be passed to a more privileged
> process (e.g. a suid-exec) which will pass the classic
> ptrace_may_access() permission check during read().
> 
> To prevent it, use proc_same_open_cred() to detect if current's cred
> have changed between ->open() and ->read(), if so, call
> proc_allow_access() to check if the original file's opener had enough
> permissions to read these sensitive fields. This will prevent passing
> file descriptors to a more privileged process to leak data.
> 
> The patch also adds a previously missing signal->cred_guard_mutex lock.
> 
> This patch does not break userspace since it only hides the fields that
> were supposed to be protected.
> 
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
> ---
>  fs/proc/array.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index cbd0f1b..f034e05 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -394,7 +394,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>  	char state;
>  	pid_t ppid = 0, pgid = -1, sid = -1;
>  	int num_threads = 0;
> -	int permitted;
> +	int permitted = 0;
>  	struct mm_struct *mm;
>  	unsigned long long start_time;
>  	unsigned long cmin_flt = 0, cmaj_flt = 0;
> @@ -404,10 +404,22 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>  	unsigned long rsslim = 0;
>  	char tcomm[sizeof(task->comm)];
>  	unsigned long flags;
> +	struct file *file = m->private;
> +	int same_cred = proc_same_open_cred(file->f_cred);
> +	unsigned int ptrace_mode = PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT;
>  
>  	state = *get_task_state(task);
>  	vsize = eip = esp = 0;
> -	permitted = ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT);
> +
> +	if (!mutex_lock_killable(&task->signal->cred_guard_mutex)) {
> +		permitted = ptrace_may_access(task, ptrace_mode);
> +		if (permitted && !same_cred)
> +			permitted = proc_allow_access(file->f_cred,
> +						      task, ptrace_mode);
> +
> +		mutex_unlock(&task->signal->cred_guard_mutex);
> +	}
> +

else permitted = false?

But surely this would be *much* more comprehensible if you had
proc_allow_access do the entire check.

--Andy

--Andy