Re: [PATCH v2 1/4] fs/pipe: Limit the slots in pipe_resize_ring()

[K Prateek Nayak]

Hello Oleg,

Thank you for the review.

On 3/7/2025 8:21 PM, Oleg Nesterov wrote:
> On 03/07, K Prateek Nayak wrote:
> > --- a/fs/pipe.c
> > +++ b/fs/pipe.c
> > @@ -1271,6 +1271,10 @@ int pipe_resize_ring(struct pipe_inode_info *pipe, unsigned int nr_slots)
> >   	struct pipe_buffer *bufs;
> >   	unsigned int head, tail, mask, n;
> >
> > +	/* nr_slots larger than limits of pipe->{head,tail} */
> > +	if (unlikely(nr_slots > (pipe_index_t)-1u))
> > +		return -EINVAL;
>
> The whole series look "obviously" good to me,
>
> Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>
>
> -------------------------------------------------------------------------------
> But damn ;) lets look at round_pipe_size(),
>
> 	unsigned int round_pipe_size(unsigned int size)
> 	{
> 		if (size > (1U << 31))
> 			return 0;
>
> 		/* Minimum pipe size, as required by POSIX */
> 		if (size < PAGE_SIZE)
> 			return PAGE_SIZE;
>
> 		return roundup_pow_of_two(size);
> 	}
>
> it is a bit silly to allow the maximum size == 1U << 31 in pipe_set_size()
> or (more importantly) in /proc/sys/fs/pipe-max-size, and then nack nr_slots
> in pipe_resize_ring().
>
> So perhaps this check should go into round_pipe_size() ? Although I can't
> suggest a simple/clear check without unnecesary restrictions for the case
> when pipe_index_t is u16.
>
> pipe_resize_ring() has another caller, watch_queue_set_size(), but it has
> its own hard limits...

"nr_notes" for watch queues cannot cross 512 so we should be covered there.
As for round_pipe_size(), we can do:

diff --git a/fs/pipe.c b/fs/pipe.c
index ce1af7592780..f82098aaa510 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -1253,6 +1253,8 @@ const struct file_operations pipefifo_fops = {
  */
 unsigned int round_pipe_size(unsigned int size)
 {
+	unsigned int max_slots;
+
 	if (size > (1U << 31))
 		return 0;
 
@@ -1260,7 +1262,14 @@ unsigned int round_pipe_size(unsigned int size)
 	if (size < PAGE_SIZE)
 		return PAGE_SIZE;
 
-	return roundup_pow_of_two(size);
+	size = roundup_pow_of_two(size);
+	max_slots = size >> PAGE_SHIFT;
+
+	/* Max slots cannot be covered pipe->{head,tail} limits */
+	if (max_slots > (pipe_index_t)-1U)
+		return 0;
+
+	return size;
 }
 
 /*
--

Since pipe_resize_ring() can be called without actually looking at
"pipe_max_size" as is the case with watch queues, we can either keep the
check in pipe_resize_ring() as well out of paranoia or get rid of it
since the current users are within the bounds.

Thoughts?

> Oleg.

--
Thanks and Regards,
Prateek