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SubjectRe: [PATCH] proc: Fix timerslack_ns CAP_SYS_NICE check when adjusting self
On Tue, Aug 9, 2016 at 4:54 PM, John Stultz <john.stultz@linaro.org> 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 <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 | 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;
> + }
> }

This entirely bypasses LSM when p == current. Is that intended?

-Kees

>
> 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

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