Commit 7abbaf94 authored Oct 07, 2016 by John Stultz Committed by Linus Torvalds Oct 07, 2016

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>

parent e16e2d8e

fs/proc/base.c

+20 −14

Original line number	Diff line number	Diff line
		@@ -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)) {
		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);
		} else
		count = -EPERM;

		out:
		put_task_struct(p);

		return count;
		@@ -2305,13 +2308,16 @@ static int timerslack_ns_show(struct seq_file m, void v)
		if (!p)
		return -ESRCH;

		if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
		if (!capable(CAP_SYS_NICE)) {
		err = -EPERM;
		goto out;
		}

		task_lock(p);
		seq_printf(m, "%llu\n", p->timer_slack_ns);
		task_unlock(p);
		} else
		err = -EPERM;

		out:
		put_task_struct(p);

		return err;

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