| Message ID | 1468522244-28297-1-git-send-email-john.stultz@linaro.org |
|---|---|
| State | New |
| Headers | show |
On Thu, Jul 14, 2016 at 11:52 AM, Kees Cook <keescook@chromium.org> wrote: > On Thu, Jul 14, 2016 at 11:50 AM, John Stultz <john.stultz@linaro.org> wrote: >> >> For ABI preservation, it still allows CAP_SYS_PTRACE tasks to >> access/modify timerslack values, but I'm fine with removing >> this if others agree. > > Is anything actually using this ABI yet? (Regardless, I'm fine > allowing both caps.) Not that I'm aware of... but there may be someone somewhere. Given it landed in 4.6, I suspect its fairly safe to remove. thanks -john
diff --git a/fs/proc/base.c b/fs/proc/base.c index a11eb71..d32033e 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -2281,7 +2281,8 @@ 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 (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS) || + capable(CAP_SYS_NICE)) { task_lock(p); if (slack_ns == 0) p->timer_slack_ns = p->default_timer_slack_ns; @@ -2306,7 +2307,8 @@ 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 (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS) || + capable(CAP_SYS_NICE)) { task_lock(p); seq_printf(m, "%llu\n", p->timer_slack_ns); task_unlock(p);
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. For ABI preservation, it still allows CAP_SYS_PTRACE tasks to access/modify timerslack values, but I'm fine with removing this if others agree. 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> --- fs/proc/base.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) -- 1.9.1