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
