[lkml]   [2016]   [Jul]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[RFC][PATCH] 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

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

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 <>
Cc: "Serge E. Hallyn" <>
Cc: Andrew Morton <>
Cc: Thomas Gleixner <>
CC: Arjan van de Ven <>
Cc: Oren Laadan <>
Cc: Ruchi Kandoi <>
Cc: Rom Lemarchand <>
Cc: Todd Kjos <>
Cc: Colin Cross <>
Cc: Nick Kralevich <>
Cc: Dmitry Shmidt <>
Cc: Elliott Hughes <>
Cc: Android Kernel Team <>
Signed-off-by: John Stultz <>
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)) {
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)) {
seq_printf(m, "%llu\n", p->timer_slack_ns);
 \ /
  Last update: 2016-07-14 21:21    [W:0.031 / U:2.880 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site