On Mon, Aug 22, 2016 at 7:01 PM, John Stultz <[email protected]> wrote:
> In changing from checking ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)
> to capable(CAP_SYS_NICE), I missed that ptrace_my_access succeeds
> when p == current, but the CAP_SYS_NICE doesn't.
>
> Thus while the previous commit was intended to loosen the needed
> privledges to modify a processes timerslack, it needlessly restricted
> a task modifying its own timerslack via the proc/<tid>/timerslack_ns
> (which is permitted also via the PR_SET_TIMERSLACK method).
>
> This patch corrects this by checking if p == current before checking
> the CAP_SYS_NICE value.
>
> This patch applies on top of my two previous patches currently in -mm
>
> 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]>
> Acked-by: Kees Cook <[email protected]>
> Signed-off-by: John Stultz <[email protected]>
Andrew, can you take this for v4.8?
-Kees
> ---
> fs/proc/base.c | 34 +++++++++++++++++++---------------
> 1 file changed, 19 insertions(+), 15 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 02f8389..01c3c2d 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2281,15 +2281,17 @@ static ssize_t timerslack_ns_write(struct file *file,
> const char __user *buf,
> if (!p)
> return -ESRCH;
>
> - if (!capable(CAP_SYS_NICE)) {
> - count = -EPERM;
> - goto out;
> - }
> + if (p != current) {
> + if (!capable(CAP_SYS_NICE)) {
> + count = -EPERM;
> + goto out;
> + }
>
> - err = security_task_setscheduler(p);
> - if (err) {
> - count = err;
> - goto out;
> + err = security_task_setscheduler(p);
> + if (err) {
> + count = err;
> + goto out;
> + }
> }
>
> task_lock(p);
> @@ -2315,14 +2317,16 @@ static int timerslack_ns_show(struct seq_file *m,
> void *v)
> if (!p)
> return -ESRCH;
>
> - if (!capable(CAP_SYS_NICE)) {
> - err = -EPERM;
> - goto out;
> - }
> + if (p != current) {
>
> - err = security_task_getscheduler(p);
> - if (err)
> - goto out;
> + if (!capable(CAP_SYS_NICE)) {
> + err = -EPERM;
> + goto out;
> + }
> + err = security_task_getscheduler(p);
> + if (err)
> + goto out;
> + }
>
> task_lock(p);
> seq_printf(m, "%llu\n", p->timer_slack_ns);
> --
> 1.9.1
>
--
Kees Cook
Nexus Security
