Re: [PATCH 2/2] taskstats: restrict access to user

[Balbir Singh <bsingharora@gmail.com>]

On Fri, Jun 24, 2011 at 5:39 PM, Vasiliy Kulikov <segoon@openwall.com> wrote:
> taskstats information may be used for gathering private information.
> E.g. for openssh and vsftpd daemons read_characters/write_characters may
> be used to learn the precise password length.  Restrict it to processes
> being able to ptrace the target process.
>
> For TASKSTATS_CMD_ATTR_REGISTER_CPUMASK the fix is euid check instead of
> a ptrace check as the handler is processed in the context of the target
> process, not the listener process'.  When ptrace_task_may_access_current()
> is introduced, it should be used instead of euid check.  Currently there
> is a small race when a process temporarily changes its euid (e.g. to
> access user's files), until the process sets euid back user's processes
> may gather privileged process' statistics.
>
> Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
> ---
>  kernel/taskstats.c |   23 ++++++++++++++++++++++-
>  1 files changed, 22 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/taskstats.c b/kernel/taskstats.c
> index 9ffea36..d92c95a 100644
> --- a/kernel/taskstats.c
> +++ b/kernel/taskstats.c
> @@ -27,6 +27,7 @@
>  #include <linux/cgroup.h>
>  #include <linux/fs.h>
>  #include <linux/file.h>
> +#include <linux/ptrace.h>
>  #include <net/genetlink.h>
>  #include <asm/atomic.h>
>
> @@ -132,6 +133,8 @@ static void send_cpu_listeners(struct sk_buff *skb,
>        struct sk_buff *skb_next, *skb_cur = skb;
>        void *reply = genlmsg_data(genlhdr);
>        int rc, delcount = 0;
> +       const struct cred *cred = current_cred();
> +       struct task_struct *task;
>
>        rc = genlmsg_end(skb, reply);
>        if (rc < 0) {
> @@ -142,6 +145,15 @@ static void send_cpu_listeners(struct sk_buff *skb,
>        rc = 0;
>        down_read(&listeners->sem);

Why not grab RCU lock here

>        list_for_each_entry(s, &listeners->list, list) {
> +
> +               rcu_read_lock();

You'll end up grabbing RCU read lock too often here, do you need to?

> +               task = find_task_by_vpid(s->pid);
> +               if (!task || __task_cred(task)->euid != cred->euid) {
> +                       rcu_read_unlock();
> +                       continue;
> +               }
> +               rcu_read_unlock();
> +

Release the lock prior to up_read()

>                skb_next = NULL;
>                if (!list_is_last(&s->list, &listeners->list)) {
>                        skb_next = skb_clone(skb_cur, GFP_KERNEL);
> @@ -199,14 +211,19 @@ static void fill_stats(struct task_struct *tsk, struct taskstats *stats)
>  static int fill_stats_for_pid(pid_t pid, struct taskstats *stats)
>  {
>        struct task_struct *tsk;
> +       int rc = -ESRCH;
>
>        rcu_read_lock();
>        tsk = find_task_by_vpid(pid);
> +       if (tsk && !ptrace_may_access(tsk, PTRACE_MODE_READ)) {
> +               tsk = NULL;
> +               rc = -EACCES;
> +       }
>        if (tsk)
>                get_task_struct(tsk);
>        rcu_read_unlock();
>        if (!tsk)
> -               return -ESRCH;
> +               return rc;
>        fill_stats(tsk, stats);
>        put_task_struct(tsk);
>        return 0;
> @@ -224,6 +241,10 @@ static int fill_stats_for_tgid(pid_t tgid, struct taskstats *stats)
>         */
>        rcu_read_lock();
>        first = find_task_by_vpid(tgid);
> +       if (first && !ptrace_may_access(first, PTRACE_MODE_READ)) {
> +               rc = -EACCES;
> +               goto out;
> +       }
>
>        if (!first || !lock_task_sighand(first, &flags))
>                goto out;

Balbir Singh