Messages in this thread Patch in this message | | | From | John Stultz <> | Subject | [RFC][PATCH 1/2 v2] proc: Relax /proc/<tid>/timerslack_ns capability requirements | Date | Fri, 15 Jul 2016 10:24:19 -0700 |
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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> Signed-off-by: John Stultz <john.stultz@linaro.org> --- v2: Removed CAP_SYS_PTRACE check and simplified code flow
fs/proc/base.c | 33 ++++++++++++++++----------------- 1 file changed, 16 insertions(+), 17 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c index a11eb71..8f4f8d7 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -2277,19 +2277,19 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf, if (err < 0) return err; + if (!capable(CAP_SYS_NICE)) + return -EPERM; + p = get_proc_task(inode); 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 - count = -EPERM; + 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); put_task_struct(p); @@ -2300,22 +2300,21 @@ static int timerslack_ns_show(struct seq_file *m, void *v) { struct inode *inode = m->private; struct task_struct *p; - int err = 0; + + if (!capable(CAP_SYS_NICE)) + return -EPERM; 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 - err = -EPERM; + task_lock(p); + seq_printf(m, "%llu\n", p->timer_slack_ns); + task_unlock(p); put_task_struct(p); - return err; + return 0; } static int timerslack_ns_open(struct inode *inode, struct file *filp) -- 1.9.1
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