evie/android_kernel_oneplus_msm8998
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

FROMLIST: proc: Relax /proc/<tid>/timerslack_ns capability requirements

Browse source 
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>
Reviewed-by: Nick Kralevich <nnk@google.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>

FROMLIST URL: https://lkml.org/lkml/2016/7/21/522
Change-Id: Ia75481402e3948165a1b7c1551c539530cb25509
(Cherry picked against common/android-4.4)
Signed-off-by: John Stultz <john.stultz@linaro.org>
This commit is contained in:
John Stultz 2016-07-14 11:20:55 -07:00
parent 85a09f2732
commit 419d43edc5
1 changed files with 20 additions and 14 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

34
fs/proc/base.c
View file

@ -2260,16 +2260,19 @@ 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;
@ -2279,19 +2282,22 @@ 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;