Re: [PATCH net v2 0/3] net/iucv: fix the recvmsg window update

[Aswin Karuvally <aswin@linux.ibm.com>]

On 21/08/26 16:47, Bryam Vargas via B4 Relay wrote:
> v2 answers Jakub's reviews of v1 with changes rather than replies.
> 
> 1/3 keeps its hunk and states the impact properly. SO_MSGLIMIT accepts 1,
> which makes msglimit / 2 zero, and msg_recv never leaves zero on a classic
> socket, so the NULL read is one recvmsg() away for an unprivileged process
> on a socket of its own. With relocate_lowcore it faults.
> 
> Its Fixes: tag was wrong as well. The unconditional send does date to
> 3881ac441f64, but iucv_send_ctrl() sized the skb with a constant ETH_HLEN
> until 238965b71b96, so before that it just returned -ENODEV. Backport
> window is v5.3, not v3.1.
> 
> 2/3 is new. v1 admitted it widened the msg_recv race and left it open,
> which isn't good enough under Cc: stable. afiucv_hs_send() samples the
> counter and settles it after dev_queue_xmit(); sendmsg reaches that under
> lock_sock() and recvmsg reaches it under no socket lock, so the two don't
> exclude each other and both can subtract the same value. Negative counter,
> WARN_ON, and the same credit advertised twice to a peer whose
> afiucv_hs_callback_win() subtracts the wire value from msg_sent unchecked.
> 
> Taking the socket lock around the deferred send would close it too, since
> recvmsg is the only unlocked side. I didn't: recvmsg has never held that
> lock, and making it do so changes the receive path for every caller. That
> belongs in the locking rework, not in a stable fix.
> 
> The claim sits after the last error exit rather than in the header build,
> so the counter reads zero only while the transmit is in flight, and a
> concurrent sender isn't talked out of its own update.
> 
> 3/3 is v1's patch, and it must not be applied without 2/3: hoisting the
> send out of message_q.lock drops the serialisation that lock gave two
> concurrent recvmsg(). Backported alone it recreates what 2/3 fixes. Both
> changelogs say so.
> 
> Litmus test under LKMM: the counter reaches -2 before 2/3 and cannot after.
> No hardware run; I have no IBM Z, and CONFIG_AFIUCV is s390-only.
> 
> The unlocked hs_dev and sk_shutdown window, the unbounded backlog_skb_q and
> the WARN_ON a flooding peer can reach are pre-existing and wider than this
> series. They belong with the locking rework Alexandra has open.
> 
> Thanks for the reviews.
> 
> ---
> Bryam Vargas (3):
>       net/iucv: only send the window update on HiperSockets sockets
>       net/iucv: claim the receive credit atomically
>       net/iucv: send the window update outside message_q.lock
> 
>  net/iucv/af_iucv.c | 42 +++++++++++++++++++++++++-----------------
>  1 file changed, 25 insertions(+), 17 deletions(-)
> ---
> base-commit: 746fc0787f616da418ffc04a110296fe95d53491
> change-id: 20260821-b4-disp-3a6e8695-c1f4a6069169
> 
> Best regards,
> --  
> Bryam Vargas <hexlabsecurity@proton.me>

Thank you for the fixes Bryam!

Tested-by: Aswin Karuvally <aswin@linux.ibm.com>