linux-kernel - Re: [PATCH net v4] tcp: devmem: don't write truncated dmabuf CMSGs to userspace

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Message-ID: <CANn89iKSxQrEPs6nzuQ-4psxHnJODg0TLCHqbj2P0F9VfLtSUg@mail.gmail.com>
Date: Mon, 24 Feb 2025 18:55:57 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Stanislav Fomichev <sdf@...ichev.me>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, linux-kernel@...r.kernel.org, kuniyu@...zon.com, 
	willemb@...gle.com, horms@...nel.org, ncardwell@...gle.com, 
	dsahern@...nel.org, kaiyuanz@...gle.com, asml.silence@...il.com, 
	Mina Almasry <almasrymina@...gle.com>
Subject: Re: [PATCH net v4] tcp: devmem: don't write truncated dmabuf CMSGs to userspace

On Mon, Feb 24, 2025 at 6:44 PM Stanislav Fomichev <sdf@...ichev.me> wrote:
>
> Currently, we report -ETOOSMALL (err) only on the first iteration
> (!sent). When we get put_cmsg error after a bunch of successful
> put_cmsg calls, we don't signal the error at all. This might be
> confusing on the userspace side which will see truncated CMSGs
> but no MSG_CTRUNC signal.
>
> Consider the following case:
> - sizeof(struct cmsghdr) = 16
> - sizeof(struct dmabuf_cmsg) = 24
> - total cmsg size (CMSG_LEN) = 40 (16+24)
>
> When calling recvmsg with msg_controllen=60, the userspace
> will receive two(!) dmabuf_cmsg(s), the first one will
> be a valid one and the second one will be silently truncated. There is no
> easy way to discover the truncation besides doing something like
> "cm->cmsg_len != CMSG_LEN(sizeof(dmabuf_cmsg))".
>
> Introduce new put_devmem_cmsg wrapper that reports an error instead
> of doing the truncation. Mina suggests that it's the intended way
> this API should work.
>
> Note that we might now report MSG_CTRUNC when the users (incorrectly)
> call us with msg_control == NULL.
>
> Fixes: 8f0b3cc9a4c1 ("tcp: RX path for devmem TCP")
> Reviewed-by: Mina Almasry <almasrymina@...gle.com>
> Signed-off-by: Stanislav Fomichev <sdf@...ichev.me>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>