blk_check_plugged() will return a pointer
to an object linked on current->plug->cb_list.
That list may "at any time" be implicitly cleared by
blk_flush_plug_list()
flush_plug_callbacks()
either as a result of blk_finish_plug(),
or implicitly by schedule() [and maybe other implicit mechanisms?]
If there is no protection against an implicit unplug
between the call to blk_check_plug() and using its return value,
that implicit unplug may have already happened,
even before the plug is actually initialized or populated,
and we may be using a pointer to already free()d data.
I suggest that both raid1 and raid10 can easily be fixed
by moving the call to blk_check_plugged() inside the spinlock.
For md/raid5 and btrfs/raid56,
I'm unsure how (if) this needs to be fixed.
The other current in-tree users of blk_check_plugged()
are mm_check_plugged(), and mddev_check_plugged().
mm_check_plugged() is already used safely inside a spinlock.
with mddev_check_plugged() I'm unsure, at least on a preempt kernel.
Did I overlook any magic that protects against such implicit unplug?
Also, why pretend that a custom plug struct (such as raid1_plug_cb)
may have its member "struct blk_plug_cb cb" at an arbitrary offset?
As it is, raid1_check_plugged() below is actually just a cast.
Signed-off-by: Lars Ellenberg <[email protected]>
---
drivers/md/raid1.c | 19 +++++++++++++------
drivers/md/raid10.c | 21 +++++++++++++--------
drivers/md/raid5.c | 5 +++++
fs/btrfs/raid56.c | 5 +++++
4 files changed, 36 insertions(+), 14 deletions(-)
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 39fb21e..55dc960 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1044,6 +1044,18 @@ static void raid1_unplug(struct blk_plug_cb *cb, bool from_schedule)
kfree(plug);
}
+static struct raid1_plug_cb *raid1_check_plugged(struct mddev *mddev)
+{
+ /* return (struct raid1_plug_cb*)blk_check_plugged(...); */
+ struct blk_plug_cb *cb;
+ struct raid1_plug_cb *plug = NULL;
+
+ cb = blk_check_plugged(raid1_unplug, mddev, sizeof(*plug));
+ if (cb)
+ plug = container_of(cb, struct raid1_plug_cb, cb);
+ return plug;
+}
+
static void raid1_make_request(struct mddev *mddev, struct bio * bio)
{
struct r1conf *conf = mddev->private;
@@ -1060,7 +1072,6 @@ static void raid1_make_request(struct mddev *mddev, struct bio * bio)
& (REQ_DISCARD | REQ_SECURE));
const unsigned long do_same = (bio->bi_rw & REQ_WRITE_SAME);
struct md_rdev *blocked_rdev;
- struct blk_plug_cb *cb;
struct raid1_plug_cb *plug = NULL;
int first_clone;
int sectors_handled;
@@ -1382,12 +1393,8 @@ read_again:
atomic_inc(&r1_bio->remaining);
- cb = blk_check_plugged(raid1_unplug, mddev, sizeof(*plug));
- if (cb)
- plug = container_of(cb, struct raid1_plug_cb, cb);
- else
- plug = NULL;
spin_lock_irqsave(&conf->device_lock, flags);
+ plug = raid1_check_plugged(mddev);
if (plug) {
bio_list_add(&plug->pending, mbio);
plug->pending_cnt++;
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index e3fd725..d7d4397 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1052,6 +1052,18 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
kfree(plug);
}
+static struct raid10_plug_cb *raid10_check_plugged(struct mddev *mddev)
+{
+ /* return (struct raid1_plug_cb*)blk_check_plugged(...); */
+ struct blk_plug_cb *cb;
+ struct raid10_plug_cb *plug = NULL;
+
+ cb = blk_check_plugged(raid10_unplug, mddev, sizeof(*plug));
+ if (cb)
+ plug = container_of(cb, struct raid10_plug_cb, cb);
+ return plug;
+}
+
static void __make_request(struct mddev *mddev, struct bio *bio)
{
struct r10conf *conf = mddev->private;
@@ -1066,7 +1078,6 @@ static void __make_request(struct mddev *mddev, struct bio *bio)
const unsigned long do_same = (bio->bi_rw & REQ_WRITE_SAME);
unsigned long flags;
struct md_rdev *blocked_rdev;
- struct blk_plug_cb *cb;
struct raid10_plug_cb *plug = NULL;
int sectors_handled;
int max_sectors;
@@ -1369,14 +1380,8 @@ retry_write:
atomic_inc(&r10_bio->remaining);
- cb = blk_check_plugged(raid10_unplug, mddev,
- sizeof(*plug));
- if (cb)
- plug = container_of(cb, struct raid10_plug_cb,
- cb);
- else
- plug = NULL;
spin_lock_irqsave(&conf->device_lock, flags);
+ plug = raid10_check_plugged(mddev);
if (plug) {
bio_list_add(&plug->pending, mbio);
plug->pending_cnt++;
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 8ab8b65..4e3b02b 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5034,6 +5034,11 @@ static void release_stripe_plug(struct mddev *mddev,
}
cb = container_of(blk_cb, struct raid5_plug_cb, cb);
+/* FIXME
+ * Nothing protects current from being scheduled, which means cb, aka plug,
+ * may implicitly be "unplugged" any time now, before it even is initialized,
+ * and will then be a pointer to free()d space.
+ */
if (cb->list.next == NULL) {
int i;
diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
index 0b7792e..17757d4 100644
--- a/fs/btrfs/raid56.c
+++ b/fs/btrfs/raid56.c
@@ -1774,6 +1774,11 @@ int raid56_parity_write(struct btrfs_root *root, struct bio *bio,
cb = blk_check_plugged(btrfs_raid_unplug, root->fs_info,
sizeof(*plug));
if (cb) {
+/* FIXME
+ * Nothing protects current from being scheduled, which means cb, aka plug,
+ * may implicitly be "unplugged" any time now, before it even is initialized,
+ * and will then be a pointer to free()d space.
+ */
plug = container_of(cb, struct btrfs_plug_cb, cb);
if (!plug->info) {
plug->info = root->fs_info;
--
1.9.1
On Tue, Apr 05 2016, Lars Ellenberg wrote:
> blk_check_plugged() will return a pointer
> to an object linked on current->plug->cb_list.
>
> That list may "at any time" be implicitly cleared by
> blk_flush_plug_list()
> flush_plug_callbacks()
> either as a result of blk_finish_plug(),
> or implicitly by schedule() [and maybe other implicit mechanisms?]
I think the only risk here is preemption, so
preempt_disable() / preempt_enable()
or as you say a spinlock, is sufficient protection.
I would suggest preempt_{dis,en}able for the raid5 code.
Maybe for raid1/raid10 too just for consistency.
>
> If there is no protection against an implicit unplug
> between the call to blk_check_plug() and using its return value,
> that implicit unplug may have already happened,
> even before the plug is actually initialized or populated,
> and we may be using a pointer to already free()d data.
>
> I suggest that both raid1 and raid10 can easily be fixed
> by moving the call to blk_check_plugged() inside the spinlock.
>
> For md/raid5 and btrfs/raid56,
> I'm unsure how (if) this needs to be fixed.
>
> The other current in-tree users of blk_check_plugged()
> are mm_check_plugged(), and mddev_check_plugged().
>
> mm_check_plugged() is already used safely inside a spinlock.
>
> with mddev_check_plugged() I'm unsure, at least on a preempt kernel.
I think this is only an issue on a preempt kernel, and in that case: yes
- mddev_check_plugged() needs protection. Maybe preempt enable/disable
could be done in blk_check_plugged() so those calls which don't
dereference the pointer don't need further protection.
Or maybe blk_check_plugged should have WARN_ON_ONCE(!in_atomic());
>
> Did I overlook any magic that protects against such implicit unplug?
Just the fortunate lack of preemption probably.
>
> Also, why pretend that a custom plug struct (such as raid1_plug_cb)
> may have its member "struct blk_plug_cb cb" at an arbitrary offset?
> As it is, raid1_check_plugged() below is actually just a cast.
Fair point. I generally prefer container_of to casts because it is more
obviously correct, and type checked.
However as blk_check_plugged performs the allocation, the blk_plug_cb
must be at the start of the containing structure, so the complex tests
for handling NULL are just noise.
I'd be happy for that to be changed.
Thanks,
NeilBrown
>
> Signed-off-by: Lars Ellenberg <[email protected]>
> ---
> drivers/md/raid1.c | 19 +++++++++++++------
> drivers/md/raid10.c | 21 +++++++++++++--------
> drivers/md/raid5.c | 5 +++++
> fs/btrfs/raid56.c | 5 +++++
> 4 files changed, 36 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index 39fb21e..55dc960 100644
> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -1044,6 +1044,18 @@ static void raid1_unplug(struct blk_plug_cb *cb, bool from_schedule)
> kfree(plug);
> }
>
> +static struct raid1_plug_cb *raid1_check_plugged(struct mddev *mddev)
> +{
> + /* return (struct raid1_plug_cb*)blk_check_plugged(...); */
> + struct blk_plug_cb *cb;
> + struct raid1_plug_cb *plug = NULL;
> +
> + cb = blk_check_plugged(raid1_unplug, mddev, sizeof(*plug));
> + if (cb)
> + plug = container_of(cb, struct raid1_plug_cb, cb);
> + return plug;
> +}
> +
> static void raid1_make_request(struct mddev *mddev, struct bio * bio)
> {
> struct r1conf *conf = mddev->private;
> @@ -1060,7 +1072,6 @@ static void raid1_make_request(struct mddev *mddev, struct bio * bio)
> & (REQ_DISCARD | REQ_SECURE));
> const unsigned long do_same = (bio->bi_rw & REQ_WRITE_SAME);
> struct md_rdev *blocked_rdev;
> - struct blk_plug_cb *cb;
> struct raid1_plug_cb *plug = NULL;
> int first_clone;
> int sectors_handled;
> @@ -1382,12 +1393,8 @@ read_again:
>
> atomic_inc(&r1_bio->remaining);
>
> - cb = blk_check_plugged(raid1_unplug, mddev, sizeof(*plug));
> - if (cb)
> - plug = container_of(cb, struct raid1_plug_cb, cb);
> - else
> - plug = NULL;
> spin_lock_irqsave(&conf->device_lock, flags);
> + plug = raid1_check_plugged(mddev);
> if (plug) {
> bio_list_add(&plug->pending, mbio);
> plug->pending_cnt++;
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index e3fd725..d7d4397 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1052,6 +1052,18 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
> kfree(plug);
> }
>
> +static struct raid10_plug_cb *raid10_check_plugged(struct mddev *mddev)
> +{
> + /* return (struct raid1_plug_cb*)blk_check_plugged(...); */
> + struct blk_plug_cb *cb;
> + struct raid10_plug_cb *plug = NULL;
> +
> + cb = blk_check_plugged(raid10_unplug, mddev, sizeof(*plug));
> + if (cb)
> + plug = container_of(cb, struct raid10_plug_cb, cb);
> + return plug;
> +}
> +
> static void __make_request(struct mddev *mddev, struct bio *bio)
> {
> struct r10conf *conf = mddev->private;
> @@ -1066,7 +1078,6 @@ static void __make_request(struct mddev *mddev, struct bio *bio)
> const unsigned long do_same = (bio->bi_rw & REQ_WRITE_SAME);
> unsigned long flags;
> struct md_rdev *blocked_rdev;
> - struct blk_plug_cb *cb;
> struct raid10_plug_cb *plug = NULL;
> int sectors_handled;
> int max_sectors;
> @@ -1369,14 +1380,8 @@ retry_write:
>
> atomic_inc(&r10_bio->remaining);
>
> - cb = blk_check_plugged(raid10_unplug, mddev,
> - sizeof(*plug));
> - if (cb)
> - plug = container_of(cb, struct raid10_plug_cb,
> - cb);
> - else
> - plug = NULL;
> spin_lock_irqsave(&conf->device_lock, flags);
> + plug = raid10_check_plugged(mddev);
> if (plug) {
> bio_list_add(&plug->pending, mbio);
> plug->pending_cnt++;
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 8ab8b65..4e3b02b 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -5034,6 +5034,11 @@ static void release_stripe_plug(struct mddev *mddev,
> }
>
> cb = container_of(blk_cb, struct raid5_plug_cb, cb);
> +/* FIXME
> + * Nothing protects current from being scheduled, which means cb, aka plug,
> + * may implicitly be "unplugged" any time now, before it even is initialized,
> + * and will then be a pointer to free()d space.
> + */
>
> if (cb->list.next == NULL) {
> int i;
> diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
> index 0b7792e..17757d4 100644
> --- a/fs/btrfs/raid56.c
> +++ b/fs/btrfs/raid56.c
> @@ -1774,6 +1774,11 @@ int raid56_parity_write(struct btrfs_root *root, struct bio *bio,
> cb = blk_check_plugged(btrfs_raid_unplug, root->fs_info,
> sizeof(*plug));
> if (cb) {
> +/* FIXME
> + * Nothing protects current from being scheduled, which means cb, aka plug,
> + * may implicitly be "unplugged" any time now, before it even is initialized,
> + * and will then be a pointer to free()d space.
> + */
> plug = container_of(cb, struct btrfs_plug_cb, cb);
> if (!plug->info) {
> plug->info = root->fs_info;
> --
> 1.9.1
On Tue, Apr 05, 2016 at 03:36:57PM +0200, Lars Ellenberg wrote:
> blk_check_plugged() will return a pointer
> to an object linked on current->plug->cb_list.
>
> That list may "at any time" be implicitly cleared by
> blk_flush_plug_list()
> flush_plug_callbacks()
> either as a result of blk_finish_plug(),
> or implicitly by schedule() [and maybe other implicit mechanisms?]
>
> If there is no protection against an implicit unplug
> between the call to blk_check_plug() and using its return value,
> that implicit unplug may have already happened,
> even before the plug is actually initialized or populated,
> and we may be using a pointer to already free()d data.
>
> I suggest that both raid1 and raid10 can easily be fixed
> by moving the call to blk_check_plugged() inside the spinlock.
>
> For md/raid5 and btrfs/raid56,
> I'm unsure how (if) this needs to be fixed.
I think you're right, digging in to see if there's something I missed.
But as Neil said, it looks like we just got saved by preemption being
off by default.
-chris
On Tue, Apr 05, 2016 at 03:36:57PM +0200, Lars Ellenberg wrote:
> blk_check_plugged() will return a pointer
> to an object linked on current->plug->cb_list.
>
> That list may "at any time" be implicitly cleared by
> blk_flush_plug_list()
> flush_plug_callbacks()
> either as a result of blk_finish_plug(),
> or implicitly by schedule() [and maybe other implicit mechanisms?]
>
> If there is no protection against an implicit unplug
> between the call to blk_check_plug() and using its return value,
> that implicit unplug may have already happened,
> even before the plug is actually initialized or populated,
> and we may be using a pointer to already free()d data.
This isn't correct. flush plug is never called in preemption, which is designed
only called when the task is going to sleep. See sched_submit_work. Am I
missing anything?
Thanks,
Shaohua
On Wed, Apr 06 2016, Shaohua Li wrote:
> On Tue, Apr 05, 2016 at 03:36:57PM +0200, Lars Ellenberg wrote:
>> blk_check_plugged() will return a pointer
>> to an object linked on current->plug->cb_list.
>>
>> That list may "at any time" be implicitly cleared by
>> blk_flush_plug_list()
>> flush_plug_callbacks()
>> either as a result of blk_finish_plug(),
>> or implicitly by schedule() [and maybe other implicit mechanisms?]
>>
>> If there is no protection against an implicit unplug
>> between the call to blk_check_plug() and using its return value,
>> that implicit unplug may have already happened,
>> even before the plug is actually initialized or populated,
>> and we may be using a pointer to already free()d data.
>
> This isn't correct. flush plug is never called in preemption, which is designed
> only called when the task is going to sleep. See sched_submit_work. Am I
> missing anything?
Ahh yes, thanks.
Only two places call blk_schedule_flush_plug().
One is io_schedule_timeout() which must be called explicitly.
There other is, as you say, sched_submit_work(). It starts:
static inline void sched_submit_work(struct task_struct *tsk)
{
if (!tsk->state || tsk_is_pi_blocked(tsk))
return;
so if the task is runnable, then as
include/linux/sched.h:#define TASK_RUNNING 0
it will never call blk_schedule_flush_plug().
So I don't think you are missing anything, we were.
Lars: have your concerns been relieved or do you still have reason to
think there is a problem?
Thanks,
NeilBrown
On Wed, Apr 06, 2016 at 01:10:57PM +1000, NeilBrown wrote:
> On Wed, Apr 06 2016, Shaohua Li wrote:
>
> > On Tue, Apr 05, 2016 at 03:36:57PM +0200, Lars Ellenberg wrote:
> >> blk_check_plugged() will return a pointer
> >> to an object linked on current->plug->cb_list.
> >>
> >> That list may "at any time" be implicitly cleared by
> >> blk_flush_plug_list()
> >> flush_plug_callbacks()
> >> either as a result of blk_finish_plug(),
> >> or implicitly by schedule() [and maybe other implicit mechanisms?]
> >>
> >> If there is no protection against an implicit unplug
> >> between the call to blk_check_plug() and using its return value,
> >> that implicit unplug may have already happened,
> >> even before the plug is actually initialized or populated,
> >> and we may be using a pointer to already free()d data.
> >
> > This isn't correct. flush plug is never called in preemption, which is designed
> > only called when the task is going to sleep. See sched_submit_work. Am I
> > missing anything?
>
> Ahh yes, thanks.
>
> Only two places call blk_schedule_flush_plug().
> One is io_schedule_timeout() which must be called explicitly.
> There other is, as you say, sched_submit_work(). It starts:
>
> static inline void sched_submit_work(struct task_struct *tsk)
> {
> if (!tsk->state || tsk_is_pi_blocked(tsk))
> return;
>
> so if the task is runnable, then as
> include/linux/sched.h:#define TASK_RUNNING 0
>
> it will never call blk_schedule_flush_plug().
>
> So I don't think you are missing anything, we were.
>
> Lars: have your concerns been relieved or do you still have reason to
> think there is a problem?
So just don't call anything that might_sleep() between
blk_check_plug() and using its return value.
All good.
I thought I must have overlooked something.
Thanks,
Lars