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Subject[PATCH] proc: Fix timerslack_ns CAP_SYS_NICE check when adjusting self
Date
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;
+ }
}

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
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