[patch 111/127] proc: relax /proc/<tid>/timerslack_ns capability requirements

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



From: John Stultz <john.stultz@xxxxxxxxxx>
Subject: 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@xxxxxxxxxx
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
Reviewed-by: Nick Kralevich <nnk@xxxxxxxxxx>
Acked-by: Serge Hallyn <serge@xxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Oren Laadan <orenl@xxxxxxxxxxx>
Cc: Ruchi Kandoi <kandoiruchi@xxxxxxxxxx>
Cc: Rom Lemarchand <romlem@xxxxxxxxxxx>
Cc: Todd Kjos <tkjos@xxxxxxxxxx>
Cc: Colin Cross <ccross@xxxxxxxxxxx>
Cc: Nick Kralevich <nnk@xxxxxxxxxx>
Cc: Dmitry Shmidt <dimitrysh@xxxxxxxxxx>
Cc: Elliott Hughes <enh@xxxxxxxxxx>
Cc: Android Kernel Team <kernel-team@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 fs/proc/base.c |   34 ++++++++++++++++++++--------------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff -puN fs/proc/base.c~proc-relax-proc-tid-timerslack_ns-capability-requirements fs/proc/base.c
--- a/fs/proc/base.c~proc-relax-proc-tid-timerslack_ns-capability-requirements
+++ a/fs/proc/base.c
@@ -2280,16 +2280,19 @@ static ssize_t timerslack_ns_write(struc
 	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
 {
 	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;
_
--
To unsubscribe from this list: send the line "unsubscribe mm-commits" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html



[Index of Archives]     [Kernel Archive]     [IETF Annouce]     [DCCP]     [Netdev]     [Networking]     [Security]     [Bugtraq]     [Yosemite]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Linux SCSI]
  Powered by Linux