Re: [PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

Re: [PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant]

On Sun, 24 Jul 2022 10:54:26 +0530  Hillf Danton <hdanton@sina.com> wrote:
> According to sysbot's report [1], what is proposed fails to fix the
> reported uaf in the scenario below because of no wqueue->lock acquired
> in the post path. Despite of defunct checked, it is checked after taking
> pipe's wait lock - a bit too late.
> 
> Hillf
> 
> 	post_one_notification	watch_queue_clear	
> 	===			===
> 	pipe = wqueue->pipe;
> 	if (!pipe)
> 		return false;
> 				spin_lock_bh(&wqueue->lock);
> 				wqueue->pipe = NULL;
> 				spin_lock_bh(&wqueue->lock);
> 
> 				pipe is freed
> 
> 	spin_lock_irq(&pipe->rd_wait.lock); // uaf reported
> 
> 	if (wqueue->defunct)
> 		goto out;
> 
> [1] https://lore.kernel.org/lkml/000000000000aa07b205daba6d49@google.com/
> 

Before post_one_notification(), in __post_watch_notification(), on line 228
wqueue->lock is acquired (using lock_wqueue()).

The comment on post_one_notification() also tells that it should be called
with wqueue->lock held. So we do acquire the lock in the post path.

The pipe is freed after the execution of watch_queue_clear() in
free_pipe_info().

A side note: Your emails don't seem to pop up on lore's LKML archive. Maybe
check if the email isn't being blocked?

Thanks,
Siddh

Re: [PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant via Linux-kernel-mentees]

On Sun, 24 Jul 2022 10:54:26 +0530  Hillf Danton <hdanton@sina.com> wrote:
> According to sysbot's report [1], what is proposed fails to fix the
> reported uaf in the scenario below because of no wqueue->lock acquired
> in the post path. Despite of defunct checked, it is checked after taking
> pipe's wait lock - a bit too late.
> 
> Hillf
> 
> 	post_one_notification	watch_queue_clear	
> 	===			===
> 	pipe = wqueue->pipe;
> 	if (!pipe)
> 		return false;
> 				spin_lock_bh(&wqueue->lock);
> 				wqueue->pipe = NULL;
> 				spin_lock_bh(&wqueue->lock);
> 
> 				pipe is freed
> 
> 	spin_lock_irq(&pipe->rd_wait.lock); // uaf reported
> 
> 	if (wqueue->defunct)
> 		goto out;
> 
> [1] https://lore.kernel.org/lkml/000000000000aa07b205daba6d49@google.com/
> 

Before post_one_notification(), in __post_watch_notification(), on line 228
wqueue->lock is acquired (using lock_wqueue()).

The comment on post_one_notification() also tells that it should be called
with wqueue->lock held. So we do acquire the lock in the post path.

The pipe is freed after the execution of watch_queue_clear() in
free_pipe_info().

A side note: Your emails don't seem to pop up on lore's LKML archive. Maybe
check if the email isn't being blocked?

Thanks,
Siddh
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Re: [PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Hillf Danton]

On Sun, 24 Jul 2022 11:56:30 +0530 Siddh Raman Pant wrote:
> 
> Before post_one_notification(), in __post_watch_notification(), on line 228
> wqueue->lock is acquired (using lock_wqueue()).

Yes, you are right. I missed 353f7988dd84 ("watchqueue: make sure to serialize
'wqueue->defunct' properly"). Sorry for my noise.

Given defunct serialized, still need to clear wqueue->pipe in the clear
path as proposed?

Hillf
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Re: [PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant]

On Sun, 24 Jul 2022 12:49:58 +0530  Hillf Danton <hdanton@sina.com> wrote:
> Given defunct serialized, still need to clear wqueue->pipe in the clear
> path as proposed?

I am not sure what you mean by this...

If you mean freeing the pipe, it is done by free_pipe_info(), which is
the caller of watch_queue_clear().

If you mean making wqueue->pipe NULL, it is being done so in the patch.

Thanks,
Siddh

Re: [PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant via Linux-kernel-mentees]

On Sun, 24 Jul 2022 12:49:58 +0530  Hillf Danton <hdanton@sina.com> wrote:
> Given defunct serialized, still need to clear wqueue->pipe in the clear
> path as proposed?

I am not sure what you mean by this...

If you mean freeing the pipe, it is done by free_pipe_info(), which is
the caller of watch_queue_clear().

If you mean making wqueue->pipe NULL, it is being done so in the patch.

Thanks,
Siddh
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Re: [PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[David Howells]

Hillf Danton <hdanton@sina.com> wrote:

> Yes, you are right. I missed 353f7988dd84 ("watchqueue: make sure to serialize
> 'wqueue->defunct' properly"). Sorry for my noise.
> 
> Given defunct serialized, still need to clear wqueue->pipe in the clear
> path as proposed?

In fact, with the locking, is wqueue->defunct even still needed, I wonder?

David

Re: [PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[David Howells]

Hillf Danton <hdanton@sina.com> wrote:

> Yes, you are right. I missed 353f7988dd84 ("watchqueue: make sure to serialize
> 'wqueue->defunct' properly"). Sorry for my noise.
> 
> Given defunct serialized, still need to clear wqueue->pipe in the clear
> path as proposed?

In fact, with the locking, is wqueue->defunct even still needed, I wonder?

David

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[PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant via Linux-kernel-mentees]

If not done, a reference to a freed pipe remains in the watch_queue,
as this function is called before freeing a pipe in free_pipe_info()
(see line 834 of fs/pipe.c).

This causes a UAF when post_one_notification tries to access the pipe on
a key update, which is reported by syzbot.

Bug report: https://syzkaller.appspot.com/bug?id=1870dd7791ba05f2ea7f47f7cbdde701173973fc
Reported-and-tested-by: syzbot+c70d87ac1d001f29a058@syzkaller.appspotmail.com

Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes since v1:
- Removed the superfluous ifdef guard.

 kernel/watch_queue.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index bb9962b33f95..0357e5c6cf18 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -637,8 +637,15 @@ void watch_queue_clear(struct watch_queue *wqueue)
 		spin_lock_bh(&wqueue->lock);
 	}
 
-	spin_unlock_bh(&wqueue->lock);
 	rcu_read_unlock();
+
+	/* Clearing the watch queue, so we should clean the associated pipe. */
+	if (wqueue->pipe) {
+		wqueue->pipe->watch_queue = NULL;
+		wqueue->pipe = NULL;
+	}
+
+	spin_unlock_bh(&wqueue->lock);
 }
 
 /**
-- 
2.35.1


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[PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant]

If not done, a reference to a freed pipe remains in the watch_queue,
as this function is called before freeing a pipe in free_pipe_info()
(see line 834 of fs/pipe.c).

This causes a UAF when post_one_notification tries to access the pipe on
a key update, which is reported by syzbot.

Bug report: https://syzkaller.appspot.com/bug?id=1870dd7791ba05f2ea7f47f7cbdde701173973fc
Reported-and-tested-by: syzbot+c70d87ac1d001f29a058@syzkaller.appspotmail.com

Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes since v1:
- Removed the superfluous ifdef guard.

 kernel/watch_queue.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index bb9962b33f95..0357e5c6cf18 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -637,8 +637,15 @@ void watch_queue_clear(struct watch_queue *wqueue)
 		spin_lock_bh(&wqueue->lock);
 	}
 
-	spin_unlock_bh(&wqueue->lock);
 	rcu_read_unlock();
+
+	/* Clearing the watch queue, so we should clean the associated pipe. */
+	if (wqueue->pipe) {
+		wqueue->pipe->watch_queue = NULL;
+		wqueue->pipe = NULL;
+	}
+
+	spin_unlock_bh(&wqueue->lock);
 }
 
 /**
-- 
2.35.1