Re: [PATCH net v2] net/smc: validate peer CDC cursor against RMBE size before accepting it

[Dust Li <dust.li@linux.alibaba.com>]

On 2026-07-28 12:04:32, Paolo Abeni wrote:
>On 7/22/26 12:17 PM, Ibrahim Hashimov wrote:
>> smc_cdc_cursor_to_host() converts the wire-format producer/consumer
>> cursor of an incoming CDC message into a host smc_host_cursor. It rejects
>> a cursor that goes backwards, but never checks that the cursor stays
>> inside the buffer it indexes. Per smc_host_cursor ("an offset in an
>> RMBE") and the invariant smc_curs_add() enforces for every local advance
>> (0 <= count < size), a valid cursor count must be < size and a single
>> advance can be at most one bufferful; the wire cursor is peer-controlled
>> and was never checked against either.
>> 
>> smcr_cdc_msg_to_host() accepts the producer and consumer cursors, and the
>> unbounded value feeds smc_curs_diff() in smc_cdc_msg_recv_action(), which
>> computes the advance without clamping against size. A peer can inflate it
>> two ways: an out-of-range prod.count (e.g. 0x7fffffff), or -- since on a
>> wrap increment smc_curs_diff() returns (size - old.count) + new.count --
>> a cursor with a bumped wrap and new.count above old.count. Either drives
>> bytes_to_rcv far past rmb_desc->len, the very invariant the comment above
>> the atomic_add() claims but does not enforce.
>> 
>> smc_rx_recvmsg() then trusts bytes_to_rcv as the amount of valid RMB
>> data. Its first copy chunk is safely bounded by rmb_desc->len -
>> cons.count, but the second chunk copies (copylen - chunk_len) bytes from
>> offset 0, and copylen came from the inflated readable -- an out-of-bounds
>> read of the RMB's backing (v)malloc allocation, copied straight to the
>> receiving process via _copy_to_iter(). This is a remote kernel-memory
>> disclosure driven by a malicious SMC-R peer, needing no local privilege
>> on the victim. The same unbounded count also reaches
>> smc_cdc_handle_urg_data_arrival() (base + urg_curs.count - 1) -- a
>> second, narrower OOB read.
>> 
>> Reject the bad cursor at ingress, mirroring the cursor-sanity idiom two
>> lines above: drop any incoming cursor whose count is outside the buffer
>> (>= size), and any whose advance from the last accepted cursor exceeds
>> one bufferful (smc_curs_diff() > size), which catches the wrap case.
>> smc_cdc_cursor_to_host() gains a size parameter; smcr_cdc_msg_to_host()
>> passes conn->rmb_desc->len for the producer cursor and
>> conn->peer_rmbe_size for the consumer cursor, bounding it at its single
>> origin instead of patching every downstream consumer. SMC-D
>> (smcd_cdc_msg_to_host()) does not use this helper and is a separate,
>> out-of-scope gap.
>
>@SMC crew: please review.
>
>The above looks so correlated and quite simple to address that possibly
>it makes sense to address it in the same series.

Hi Paolo,

This patch was discussed in the v1 thread. Bryan had already sent a the
v5 addressing the issue, so I think this patch can be dropped.

https://lore.kernel.org/netdev/20260723-b4-disp-0d07164f-v5-0-6a9e235dbc4e@proton.me

Best regards,
Dust