On Wed, Feb 17, 2016 at 9:59 PM, John Stultz <john.stu...@linaro.org> wrote:
> This patch adjusts the timerslack_ns file permissions to be
> 0666 but requires PTRACE_MODE_ATTACH_FSCREDS to read or write
> the value.
>
> This allows tasks with sufficient privledges (CAP_SYS_PTRACE)
> to be able to modify a the timerslack for proccesses owned by
> a different user.
>
> This patch also fixes a return value from EINVAL to EPERM,
> and does task locking consistently, given we're handling u64s
> on 32bit systems. It also makes use of kstrtoull_from_user
> which simplifies some code.
>
> Cc: Arjan van de Ven <ar...@linux.intel.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Oren Laadan <or...@cellrox.com>
> Cc: Ruchi Kandoi <kandoiru...@google.com>
> Cc: Rom Lemarchand <rom...@android.com>
> Cc: Kees Cook <keesc...@chromium.org>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Android Kernel Team <kernel-t...@android.com>
> Signed-off-by: John Stultz <john.stu...@linaro.org>
Acked-by: Kees Cook <keesc...@chromium.org>
> ---
> This patch applies on top of the previous two patches
> which Andrew already added to -mm. It can be folded
> down or kept separate as desired.
Probably best to fold them together.
-Kees
>
> I've also wired up the Android userspace side to use
> this interface, and tested it there, and things seem
> to be working properly ( - with some selinux noise, I
> still need to figure out the selinux policy changes,
> but its working with permissive mode).
>
> fs/proc/base.c | 28 +++++++++++++---------------
> 1 file changed, 13 insertions(+), 15 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index d7c51ca..35f583a 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2262,18 +2262,10 @@ static ssize_t timerslack_ns_write(struct file *file,
> const char __user *buf,
> {
> struct inode *inode = file_inode(file);
> struct task_struct *p;
> - char buffer[PROC_NUMBUF];
> u64 slack_ns;
> int err;
>
> - memset(buffer, 0, sizeof(buffer));
> - if (count > sizeof(buffer) - 1)
> - count = sizeof(buffer) - 1;
> -
> - if (copy_from_user(buffer, buf, count))
> - return -EFAULT;
> -
> - err = kstrtoull(strstrip(buffer), 10, &slack_ns);
> + err = kstrtoull_from_user(buf, count, 10, &slack_ns);
> if (err < 0)
> return err;
>
> @@ -2282,12 +2274,14 @@ static ssize_t timerslack_ns_write(struct file *file,
> const char __user *buf,
> return -ESRCH;
>
> if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> + task_lock(p);
> if (slack_ns == 0)
> p->timer_slack_ns = p->default_timer_slack_ns;
> else
> p->timer_slack_ns = slack_ns;
> + task_unlock(p);
> } else
> - count = -EINVAL;
> + count = -EPERM;
>
> put_task_struct(p);
>
> @@ -2298,18 +2292,22 @@ static int timerslack_ns_show(struct seq_file *m,
> void *v)
> {
> struct inode *inode = m->private;
> struct task_struct *p;
> + int err = 0;
>
> p = get_proc_task(inode);
> if (!p)
> return -ESRCH;
>
> - task_lock(p);
> - seq_printf(m, "%llu\n", p->timer_slack_ns);
> - task_unlock(p);
> + if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> + task_lock(p);
> + seq_printf(m, "%llu\n", p->timer_slack_ns);
> + task_unlock(p);
> + } else
> + err = -EPERM;
>
> put_task_struct(p);
>
> - return 0;
> + return err;
> }
>
> static int timerslack_ns_open(struct inode *inode, struct file *filp)
> @@ -2899,7 +2897,7 @@ static const struct pid_entry tgid_base_stuff[] = {
> #ifdef CONFIG_CHECKPOINT_RESTORE
> REG("timers", S_IRUGO, proc_timers_operations),
> #endif
> - REG("timerslack_ns", S_IRUGO|S_IWUSR,
> proc_pid_set_timerslack_ns_operations),
> + REG("timerslack_ns", S_IRUGO|S_IWUGO,
> proc_pid_set_timerslack_ns_operations),
> };
>
> static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx)
> --
> 1.9.1
>
--
Kees Cook
Chrome OS & Brillo Security
