Re: [PATCH net v3] net/rds: ib: zero the unwritten tail of a short receive fragment

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- [Medium] The patch closes the info leak, but its v3 approach rests on a
  false premise, and the commit message leaves out what the new memset…

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net/rds: ib: zero the unwritten tail of a short receive fragment

This patch zeroes the unwritten tail of an RDS/IB receive fragment page in
rds_ib_process_recv() when the received payload is shorter than
RDS_FRAG_SIZE. The goal is to stop rds_ib_inc_copy_to_user() from copying
stale page contents to userspace when a peer sends a fragment shorter than
its h_len implies.

Closes: https://lore.kernel.org/netdev/20261006205204.1322102-1-tjdqudcks0424@naver.com/

> diff --git a/net/rds/ib_recv.c b/net/rds/ib_recv.c
> index bd6cb3ffaa571..9695c7f24eaa7 100644
> --- a/net/rds/ib_recv.c
> +++ b/net/rds/ib_recv.c

[ ... ]

> @@ -949,6 +950,19 @@ static void rds_ib_process_recv(struct rds_connection *conn,
>  		}
>  	}
>  
> +	/* The device wrote only data_len bytes into this recycled fragment
> +	 * page.  Zero the unwritten tail so a later rds_ib_inc_copy_to_user(),
> +	 * which copies up to the peer-declared h_len, cannot hand stale page
> +	 * contents to userspace.
> +	 */
> +	if (data_len < RDS_FRAG_SIZE) {
> +		void *frag = kmap_local_page(sg_page(&recv->r_frag->f_sg));
> +
> +		memset(frag + recv->r_frag->f_sg.offset + data_len, 0,
> +		       RDS_FRAG_SIZE - data_len);
> +		kunmap_local(frag);
> +	}

[Severity: Medium]
Isn't this condition true for the final fragment of almost every well-formed
message? A message whose h_len is not a multiple of RDS_FRAG_SIZE always
ends with a short fragment. So a 100 byte message now pays for a kmap and a
memset of roughly 4KB in the receive completion path.

For well-formed traffic that work doesn't look necessary, because
rds_ib_inc_copy_to_user() already stops at h_len:

net/rds/ib_recv.c:rds_ib_inc_copy_to_user() {
    ...
		to_copy = min_t(unsigned long, to_copy, len - copied);
    ...
}

The stale tail can only be reached when a peer sends a short non-final
fragment, meaning data_len < min(RDS_FRAG_SIZE, ic->i_recv_data_rem).

The v3 changelog gives this reason for dropping the v2 length check:

  A zero-copy send with a small iovec segment can legitimately produce a
  non-final fragment shorter than RDS_FRAG_SIZE

Can that happen on RDS/IB? rds_sendmsg() rejects zero-copy on every
transport except TCP:

net/rds/send.c:rds_sendmsg() {
    ...
	if (zcopy) {
		if (rs->rs_transport->t_type != RDS_TRANS_TCP) {
			ret = -EOPNOTSUPP;
			goto out;
		}
    ...
}

On the copy path, rds_message_copy_from_user() gets its sg entries from
rds_page_remainder_alloc(). Every entry it returns is a full PAGE_SIZE
except the last one. rds_ib_xmit() then splits each entry with:

	len = min(RDS_FRAG_SIZE, sg_dma_len(scat) - rm->data.op_dmaoff);

That means only the last fragment of a message should ever be short.

Would it be better to bound the memset by
min(RDS_FRAG_SIZE, ic->i_recv_data_rem), which is already set at this
point? Rejecting that case as v2 did is another option. Either one should
still close the leak and leave well-formed traffic alone.

As written, a peer that sends short non-final fragments also gets its
message passed to the application with zero-filled holes, as if it were
valid.

The commit message also says:

  Fragments that fill the page are unaffected.

In the common small-message case the final fragment doesn't fill the page.
Could the changelog say that the memset runs on most messages?

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