[PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.

[PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.

[Tetsuo Handa]

>From 482be6384379072eb4c0d45d0ab8a25df4f59ed7 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Tue, 24 Dec 2013 18:04:14 +0900
Subject: [PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.

Since get_pid_task() grabs a reference on the task_struct, we have to drop the
refcount after reading that task's comm name. Also, directly reading like
get_pid_task()->comm can trigger an oops when get_pid_task() returned NULL.

This patch fixes both problems.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 drivers/gpu/drm/i915/i915_debugfs.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 6ed45a9..d0a8e0a 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -406,11 +406,20 @@ static int i915_gem_object_info(struct seq_file *m, void* data)
 	seq_putc(m, '\n');
 	list_for_each_entry_reverse(file, &dev->filelist, lhead) {
 		struct file_stats stats;
+		struct task_struct *task;
+		char name[TASK_COMM_LEN];
 
 		memset(&stats, 0, sizeof(stats));
 		idr_for_each(&file->object_idr, per_file_stats, &stats);
+		task = get_pid_task(file->pid, PIDTYPE_PID);
+		if (task) {
+			get_task_comm(name, task);
+			put_task_struct(task);
+		} else {
+			strlcpy(name, "<unknown>", sizeof(name));
+		}
 		seq_printf(m, "%s: %u objects, %zu bytes (%zu active, %zu inactive, %zu unbound)\n",
-			   get_pid_task(file->pid, PIDTYPE_PID)->comm,
+			   name,
 			   stats.count,
 			   stats.total,
 			   stats.active,
-- 
1.7.1

Re: [PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.

[Chris Wilson]

On Tue, Dec 24, 2013 at 08:50:23PM +0900, Tetsuo Handa wrote:
> >From 482be6384379072eb4c0d45d0ab8a25df4f59ed7 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Date: Tue, 24 Dec 2013 18:04:14 +0900
> Subject: [PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.
> 
> Since get_pid_task() grabs a reference on the task_struct, we have to drop the
> refcount after reading that task's comm name. Also, directly reading like
> get_pid_task()->comm can trigger an oops when get_pid_task() returned NULL.

The second issue is moot as file itself cannot exist if the task_struct
is NULL, and the task_struct cannot be destroyed until we finish the
function. The simpler fix would appear to be s/get_pid_task/pid_task/
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

Re: [PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.

[Tetsuo Handa]

Hello.

Chris Wilson wrote:
> > Since get_pid_task() grabs a reference on the task_struct, we have to drop the
> > refcount after reading that task\'s comm name. Also, directly reading like
> > get_pid_task()->comm can trigger an oops when get_pid_task() returned NULL.
> 
> The second issue is moot as file itself cannot exist if the task_struct
> is NULL, and the task_struct cannot be destroyed until we finish the
> function. The simpler fix would appear to be s/get_pid_task/pid_task/

I didn\'t catch. Please see below sample module.

pid_task() will return NULL if the target task has already died.
The target task could be killed (e.g. by the OOM killer) at any time, and
whether the target task\'s task_struct is not yet destroyed is irrelevant
for pid_task().

get_pid_task(file->pid, PIDTYPE_PID)->comm by chance didn\'t trigger
oops because offsetof(struct task_struct, comm) < PAGE_SIZE is true.

---------- sample start ----------
#include <linux/module.h>
#include <linux/delay.h>
#include <linux/kthread.h>

static int test_thread(void *unused)
{
        return 0;
}

static int __init test_init(void)
{
        struct task_struct *task = kthread_create(test_thread, NULL, \"test\");
        if (!IS_ERR(task)) {
                struct pid *pid;
                struct task_struct *t;
                get_task_struct(task);
                pid = get_pid(task->pids[PIDTYPE_PID].pid);
                rcu_read_lock();
                t = pid_task(pid, PIDTYPE_PID);
                rcu_read_unlock();
                printk(\"pid_task(%p)=%p\\n\", pid, t);
                wake_up_process(task);
                ssleep(1);
                rcu_read_lock();
                t = pid_task(pid, PIDTYPE_PID);
                rcu_read_unlock();
                printk(\"pid_task(%p)=%p\\n\", pid, t);
                put_task_struct(task);
                put_pid(pid);
        }
        return -EINVAL;
}

module_init(test_init);
MODULE_LICENSE(\"GPL\");
---------- sample end ----------

---------- output start ----------
pid_task(ffff8800474bff80)=ffff8800435b0e10
pid_task(ffff8800474bff80)=          (null)
---------- output end ----------

Re: [PATCH] drm/i915: Fix refcount leak and possible NULL pointerdereference.

[Tetsuo Handa]

Chris Wilson wrote:
> On Tue, Dec 24, 2013 at 08:50:23PM +0900, Tetsuo Handa wrote:
> > >From 482be6384379072eb4c0d45d0ab8a25df4f59ed7 Mon Sep 17 00:00:00 2001
> > From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > Date: Tue, 24 Dec 2013 18:04:14 +0900
> > Subject: [PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.
> > 
> > Since get_pid_task() grabs a reference on the task_struct, we have to drop the
> > refcount after reading that task's comm name. Also, directly reading like
> > get_pid_task()->comm can trigger an oops when get_pid_task() returned NULL.
> 
> The second issue is moot as file itself cannot exist if the task_struct
> is NULL, and the task_struct cannot be destroyed until we finish the
> function. The simpler fix would appear to be s/get_pid_task/pid_task/

If I understand correctly, priv->pid = get_pid(task_pid(current)); in
drm_open_helper() grabs a reference on "struct pid" before adding to
&dev->filelist, and put_pid(file_priv->pid); in drm_release() releases that
reference after removing from &dev->filelist.

So, you meant that mutex_lock_interruptible(&dev->struct_mutex); in
i915_gem_object_info() prevents drm_release() from calling put_pid() ?

Then, this file->pid in &dev->filelist keeps at least one reference.
OK. Updated patch follows.
----------
>From 5ea824bc84f65d2265addc81e1adacc8baf82d48 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Fri, 3 Jan 2014 20:30:41 +0900
Subject: [PATCH v2] drm/i915: Fix refcount leak and possible NULL pointer dereference.

Since get_pid_task() grabs a reference on the task_struct, we have to drop the
refcount after reading that task's comm name. Use pid_task() with RCU instead.

Also, avoid directly reading like pid_task()->comm because
pid_task() will return NULL if the task have already exit()ed.

This patch fixes both problems.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 drivers/gpu/drm/i915/i915_debugfs.c |   12 +++++++++++-
 1 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 6ed45a9..91c26b6 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -406,16 +406,26 @@ static int i915_gem_object_info(struct seq_file *m, void* data)
 	seq_putc(m, '\n');
 	list_for_each_entry_reverse(file, &dev->filelist, lhead) {
 		struct file_stats stats;
+		struct task_struct *task;
 
 		memset(&stats, 0, sizeof(stats));
 		idr_for_each(&file->object_idr, per_file_stats, &stats);
+		/*
+		 * Although we have a valid reference on file->pid, that does
+		 * not guarantee that the task_struct who called get_pid() is
+		 * still alive (e.g. get_pid(current) => fork() => exit()).
+		 * Therefore, we need to protect this ->comm access using RCU.
+		 */
+		rcu_read_lock();
+		task = pid_task(file->pid, PIDTYPE_PID);
 		seq_printf(m, "%s: %u objects, %zu bytes (%zu active, %zu inactive, %zu unbound)\n",
-			   get_pid_task(file->pid, PIDTYPE_PID)->comm,
+			   task ? task->comm : "<unknown>",
 			   stats.count,
 			   stats.total,
 			   stats.active,
 			   stats.inactive,
 			   stats.unbound);
+		rcu_read_unlock();
 	}
 
 	mutex_unlock(&dev->struct_mutex);
-- 
1.7.1

Re: [PATCH] drm/i915: Fix refcount leak and possible NULL pointerdereference.

[Chris Wilson]

On Fri, Jan 03, 2014 at 08:42:18PM +0900, Tetsuo Handa wrote:
> Chris Wilson wrote:
> > On Tue, Dec 24, 2013 at 08:50:23PM +0900, Tetsuo Handa wrote:
> > > >From 482be6384379072eb4c0d45d0ab8a25df4f59ed7 Mon Sep 17 00:00:00 2001
> > > From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > > Date: Tue, 24 Dec 2013 18:04:14 +0900
> > > Subject: [PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.
> > > 
> > > Since get_pid_task() grabs a reference on the task_struct, we have to drop the
> > > refcount after reading that task's comm name. Also, directly reading like
> > > get_pid_task()->comm can trigger an oops when get_pid_task() returned NULL.
> > 
> > The second issue is moot as file itself cannot exist if the task_struct
> > is NULL, and the task_struct cannot be destroyed until we finish the
> > function. The simpler fix would appear to be s/get_pid_task/pid_task/
> 
> If I understand correctly, priv->pid = get_pid(task_pid(current)); in
> drm_open_helper() grabs a reference on "struct pid" before adding to
> &dev->filelist, and put_pid(file_priv->pid); in drm_release() releases that
> reference after removing from &dev->filelist.
> 
> So, you meant that mutex_lock_interruptible(&dev->struct_mutex); in
> i915_gem_object_info() prevents drm_release() from calling put_pid() ?

Right, my understanding is that since we take the struct_mutex during
release of the filp (and across this list walker) that is sufficient
serialisation to prevent the task struct from disappearing from
underneath us.
 
> Then, this file->pid in &dev->filelist keeps at least one reference.
> OK. Updated patch follows.

Looks good to me, and the comment is great.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

Re: [PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.

[Tetsuo Handa]

Chris Wilson wrote:
> > Then, this file->pid in &dev->filelist keeps at least one reference.
> > OK. Updated patch follows.
> 
> Looks good to me, and the comment is great.
> -Chris

Please pick up this patch via your git tree.

Also, please change from "<unknown>" to whatever you like. For example,
dump_holder() in fs/gfs2/glock.c uses "(ended)" if comm is not available.

Re: [PATCH] drm/i915: Fix refcount leak and possible NULL pointer dereference.

[Daniel Vetter]

On Sun, Jan 05, 2014 at 10:08:42AM +0900, Tetsuo Handa wrote:
> Chris Wilson wrote:
> > > Then, this file->pid in &dev->filelist keeps at least one reference.
> > > OK. Updated patch follows.
> > 
> > Looks good to me, and the comment is great.
> > -Chris
> 
> Please pick up this patch via your git tree.
> 
> Also, please change from "<unknown>" to whatever you like. For example,
> dump_holder() in fs/gfs2/glock.c uses "(ended)" if comm is not available.

Queued for -next, thanks for the patch.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch