The patch titled
Subject: proc: relax /proc/<tid>/timerslack_ns capability requirements
has been added to the -mm tree. Its filename is
proc-relax-proc-tid-timerslack_ns-capability-requirements.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/proc-relax-proc-tid-timerslack_ns-capability-requirements.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/proc-relax-proc-tid-timerslack_ns-capability-requirements.patch
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------------------------------------------------------
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
@@ -2285,16 +2285,19 @@ 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;
@@ -2304,19 +2307,22 @@ 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;
_
Patches currently in -mm which might be from john.stultz@xxxxxxxxxx are
proc-relax-proc-tid-timerslack_ns-capability-requirements.patch
proc-add-lsm-hook-checks-to-proc-tid-timerslack_ns.patch
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