Re: [PATCH net] RDS/IB: validate receive completion payload length

[netdev-bot+sashiko@kernel.org]

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- [High] The new length check can be bypassed, so the stale/uninitialized
  receive-fragment disclosure that this commit claims to close can still…
- [Medium] When the new length check in rds_ib_process_recv()
  (net/rds/ib_recv.c:952-959) rejects a fragment, it jumps to `done` and…
- [Low] The commit message understates the impact.

--

RDS/IB: validate receive completion payload length

This patch adds a check to rds_ib_process_recv(). The payload length
reported by the receive completion must match the expected length of the
current fragment, min(i_recv_data_rem, RDS_FRAG_SIZE). On mismatch the
connection is dropped and reconnected, and the receive fragment is not
attached to the incoming message.

> An RDS/RDMA peer can declare a fragment length larger than the payload
> reported by the receive completion. The receive path attaches the recycled
> receive fragment without validating those lengths, allowing recvmsg() to
> return stale bytes beyond the actual payload.

[Severity: Low]
Is "recycled receive fragment" the full picture here? When
ic->i_cache_frags is empty, rds_ib_refill_one_frag() allocates a new
page:

net/rds/ib_recv.c:rds_ib_refill_one_frag() {
    ...
		ret = rds_page_remainder_alloc(&frag->f_sg,
					       RDS_FRAG_SIZE, page_mask);
    ...
}

page_mask is either GFP_HIGHUSER or the caller's gfp.
rds_page_remainder_alloc() then calls alloc_page() without __GFP_ZERO:

net/rds/page.c:rds_page_remainder_alloc() {
    ...
	if (bytes >= PAGE_SIZE) {
		page = alloc_page(gfp);
    ...
}

On a fresh frag, the bytes after a short payload would be uninitialized
kernel page contents, not only tails from earlier RDS messages.

The code change covers this case too. Could the commit message also
describe it as a kernel memory disclosure?

> diff --git a/net/rds/ib_recv.c b/net/rds/ib_recv.c
> index bd6cb3ffaa571..0daddb108c8a7 100644
> --- a/net/rds/ib_recv.c
> +++ b/net/rds/ib_recv.c
> @@ -949,6 +949,15 @@ static void rds_ib_process_recv(struct rds_connection *conn,
>  		}
>  	}
>  
> +	if (data_len != min_t(u32, ic->i_recv_data_rem, RDS_FRAG_SIZE)) {

[Severity: High]
Can an RDMA write with immediate get past this check?

rds_ib_recv_cqe_handler() passes every successful completion to
rds_ib_process_recv() without checking wc->opcode:

net/rds/ib_recv.c:rds_ib_recv_cqe_handler() {
    ...
	if (wc->status == IB_WC_SUCCESS) {
		rds_ib_process_recv(conn, recv, wc->byte_len, state);
    ...
}

A write with immediate uses up the next receive WQE but does not write
to its scatter list (ic->i_recv_hdrs[slot] and recv->r_frag). For
IB_WC_RECV_RDMA_WITH_IMM, byte_len is the RDMA write length. In rxe,
for example:

drivers/infiniband/sw/rxe/rxe_resp.c:do_complete() {
    ...
		wc->byte_len = (pkt->mask & RXE_IMMDT_MASK &&
				pkt->mask & RXE_WRITE_MASK) ?
					qp->resp.length : wqe->dma.length - wqe->dma.resid;
    ...
}

The header buffer for a ring slot is never cleared between uses. It
still holds the last normal header received into that slot, and that
header's checksum still verifies.

recv->r_frag is either a recycled frag from the cache or a fresh
alloc_page() page.

rds_ib_post_reg_frmr() registers RDS MRs with IB_ACCESS_REMOTE_WRITE. A
peer holding an rkey from a local RDS user could then do this:

  1. send a normal message into a slot with h_len=4096, a bound dport,
     and RDS_FLAG_RETRANSMITTED clear
  2. when that slot comes around again, issue a write with immediate of
     sizeof(struct rds_header) + 4096 bytes to a remote-writable MR

The new check sees data_len == min(i_recv_data_rem, RDS_FRAG_SIZE) and
passes, so the unwritten frag is attached.

rds_recv_incoming() only drops old sequence numbers when
RDS_FLAG_RETRANSMITTED is set:

net/rds/recv.c:rds_recv_incoming() {
    ...
	if (be64_to_cpu(inc->i_hdr.h_sequence) < cp->cp_next_rx_seq &&
	    (inc->i_hdr.h_flags & RDS_FLAG_RETRANSMITTED)) {
    ...
}

rds_ib_inc_copy_to_user() would then copy h_len bytes of stale or
uninitialized frag data to userspace.

Should completions whose opcode is not IB_WC_RECV be rejected before the
receive buffers are parsed or byte_len is trusted?

> +		rds_ib_conn_error(conn,
> +				  "incoming fragment payload length %u, expected %u; "
> +				  "disconnecting and reconnecting\n",
> +				  data_len,
> +				  min_t(u32, ic->i_recv_data_rem, RDS_FRAG_SIZE));
> +		goto done;
> +	}

[Severity: Medium]
Does this goto done leave the partial message in place for later
fragments to complete?

ic->i_ibinc stays set (it was just installed if this fragment started
the message), and ic->i_recv_data_rem is not reduced.

rds_ib_conn_error() ends up in rds_conn_path_drop(). That function only
sets RDS_CONN_ERROR and queues the shutdown work:

net/rds/connection.c:rds_conn_path_drop() {
    ...
	atomic_set(&cp->cp_state, RDS_CONN_ERROR);
    ...
	queue_work(cp->cp_wq, &cp->cp_down_w);
    ...
}

In the meantime poll_rcq() keeps calling rds_ib_recv_cqe_handler() for
the rest of the batch. Later tasklet runs do the same until shutdown
sets i_cq_quiesce.

A later fragment with the same h_sequence, h_len and ports, and the
expected length, passes both the header comparison and the new check.
For example, with h_len=8192 and these completions:

  A: 4096 bytes  -> attached, rem = 4096
  B: 16 bytes    -> rejected here, rem still 4096
  C: 4096 bytes  -> attached, rem = 0

The message is delivered as A+C, with C's payload where B's belongs.

rds_recv_incoming() does not check the connection error state. The
message is queued to the socket and acked through state->ack_next, and
cp_next_rx_seq advances. A later retransmission flagged
RDS_FLAG_RETRANSMITTED would then be dropped as old.

This seems to contradict the commit message:

  "With this change, the malformed message was not delivered"

The existing fragment header mismatch path has the same weakness, and
this patch adds a second rejection path that shares it.

Could the in-progress inc be dropped here instead? That would mean
calling rds_inc_put(&ic->i_ibinc->ii_inc) and clearing ic->i_ibinc and
ic->i_recv_data_rem. Another option is to stop reassembly once the
connection is in error.

>  	list_add_tail(&recv->r_frag->f_item, &ibinc->ii_frags);
>  	recv->r_frag = NULL;
>

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261006205204.1322102-1-tjdqudcks0424%40naver.com