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proc: relax /proc/<tid>/timerslack_ns capability requirements 

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.

Link: http://lkml.kernel.org/r/1469132667-17377-1-git-send-email-john.stultz@linaro.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Nick Kralevich <nnk@google.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
John Stultz 2016-10-07 17:02:26 -07:00 committed by Linus Torvalds
parent e16e2d8e14
commit 7abbaf9404
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
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@ -2280,16 +2280,19 @@ 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;
@ -2299,19 +2302,22 @@ 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;