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

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.

/P