@@ -2281,16 +2281,19 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
if (!p)
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
+ if (!capable(CAP_SYS_NICE)) {
count = -EPERM;
+ goto out;
+ }
+
+ 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);
+out:
put_task_struct(p);
return count;
@@ -2300,19 +2303,22 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
{
struct inode *inode = m->private;
struct task_struct *p;
- int err = 0;
+ int err = 0;
p = get_proc_task(inode);
if (!p)
return -ESRCH;
- 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
+ if (!capable(CAP_SYS_NICE)) {
err = -EPERM;
+ goto out;
+ }
+ task_lock(p);
+ seq_printf(m, "%llu\n", p->timer_slack_ns);
+ task_unlock(p);
+
+out:
put_task_struct(p);
return err;
When an interface to allow a task to change another tasks timerslack was first proposed, it was suggested that something greater then CAP_SYS_NICE would be needed, as a task could be delayed further then what normally could be done with nice adjustments. So CAP_SYS_PTRACE was adopted instead for what became the /proc/<tid>/timerslack_ns interface. However, for Android (where this feature originates), giving the system_server CAP_SYS_PTRACE would allow it to observe and modify all tasks memory. This is considered too high a privilege level for only needing to change the timerslack. After some discussion, it was realized that a CAP_SYS_NICE process can set a task as SCHED_FIFO, so they could fork some spinning processes and set them all SCHED_FIFO 99, in effect delaying all other tasks for an infinite amount of time. So as a CAP_SYS_NICE task can already cause trouble for other tasks, using it as a required capability for accessing and modifying /proc/<tid>/timerslack_ns seems sufficient. Thus, this patch loosens the capability requirements to CAP_SYS_NICE and removes CAP_SYS_PTRACE, simplifying some of the code flow as well. This is technically an ABI change, but as the feature just landed in 4.6, I suspect no one is yet using it. Cc: Kees Cook <keescook@chromium.org> Cc: "Serge E. Hallyn" <serge@hallyn.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> CC: Arjan van de Ven <arjan@linux.intel.com> Cc: Oren Laadan <orenl@cellrox.com> Cc: Ruchi Kandoi <kandoiruchi@google.com> Cc: Rom Lemarchand <romlem@android.com> Cc: Todd Kjos <tkjos@google.com> Cc: Colin Cross <ccross@android.com> Cc: Nick Kralevich <nnk@google.com> Cc: Dmitry Shmidt <dimitrysh@google.com> Cc: Elliott Hughes <enh@google.com> Cc: Android Kernel Team <kernel-team@android.com> Signed-off-by: John Stultz <john.stultz@linaro.org> --- v2: Removed CAP_SYS_PTRACE check and simplified code flow v3: Tweaked where CAP_SYS_NICE check is made, suggeded by NickK fs/proc/base.c | 34 ++++++++++++++++++++-------------- 1 file changed, 20 insertions(+), 14 deletions(-) -- 1.9.1