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 <[email protected]> Cc: "Serge E. Hallyn" <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Thomas Gleixner <[email protected]> CC: Arjan van de Ven <[email protected]> Cc: Oren Laadan <[email protected]> Cc: Ruchi Kandoi <[email protected]> Cc: Rom Lemarchand <[email protected]> Cc: Todd Kjos <[email protected]> Cc: Colin Cross <[email protected]> Cc: Nick Kralevich <[email protected]> Cc: Dmitry Shmidt <[email protected]> Cc: Elliott Hughes <[email protected]> Cc: Android Kernel Team <[email protected]> Signed-off-by: John Stultz <[email protected]> --- fs/proc/base.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) 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); -- 1.9.1
