Accessing buf->flags after pipe_buf_release(pipe, buf) is unsafe, because
the 'buf' memory maybe freed.

In fact, pipe->note_loss does not need the protection of spinlock
pipe->rd_wait.lock, it only needs the protection of __pipe_lock(pipe). So
make the assignment of pipe->note_loss complete before releasing 'buf' to
eliminate the risk.

Fixes: e7d553d69cf6 ("pipe: Add notification lossage handling")
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 fs/pipe.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 42c7ff41c2dba29..0f873949337ed28 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -321,12 +321,12 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 			}
 
 			if (!buf->len) {
-				pipe_buf_release(pipe, buf);
-				spin_lock_irq(&pipe->rd_wait.lock);
 #ifdef CONFIG_WATCH_QUEUE
 				if (buf->flags & PIPE_BUF_FLAG_LOSS)
 					pipe->note_loss = true;
 #endif
+				pipe_buf_release(pipe, buf);
+				spin_lock_irq(&pipe->rd_wait.lock);
 				tail++;
 				pipe->tail = tail;
 				spin_unlock_irq(&pipe->rd_wait.lock);
-- 
2.25.1

On Thu, Nov 17, 2022 at 07:53:23PM +0800, Zhen Lei wrote:
> Accessing buf->flags after pipe_buf_release(pipe, buf) is unsafe, because
> the 'buf' memory maybe freed.

Huh?  What are you talking about?
                        struct pipe_buffer *buf = &pipe->bufs[tail & mask];
To free *buf you would need to free the entire damn array, which is
obviously not going to be possible here; if you are talking about reuse
of *buf - that's controlled by pipe->tail, and we do not assign it until
later.

Fetching any fields of *buf is safe; what can get freed is buf->page, not
buf itself.  So that buf->flags access is fine.

On 2022/11/25 14:33, Al Viro wrote:
> On Thu, Nov 17, 2022 at 07:53:23PM +0800, Zhen Lei wrote:
>> Accessing buf->flags after pipe_buf_release(pipe, buf) is unsafe, because
>> the 'buf' memory maybe freed.
> 
> Huh?  What are you talking about?
>                         struct pipe_buffer *buf = &pipe->bufs[tail & mask];
> To free *buf you would need to free the entire damn array, which is
> obviously not going to be possible here; if you are talking about reuse
> of *buf - that's controlled by pipe->tail, and we do not assign it until
> later.
> 
> Fetching any fields of *buf is safe; what can get freed is buf->page, not
> buf itself.  So that buf->flags access is fine.

Right. Thank you for explaining clearly. Sorry, I misunderstood in the
course of learning.

> 
> .
> 

-- 
Regards,
  Zhen Lei