Re: [PATCH net v2 1/2] net: core: propagate unreadable flag in skb_zerocopy

[Ilya Maximets <i.maximets@ovn.org>]

On 8/10/26 8:09 PM, Mina Almasry wrote:
> skb_zerocopy() fails to propagate the unreadable flag when copying
> devmem fragments, causing target skbs to appear as readable memory.
> 
> This patch fixes the flag propagation. Additionally, it returns -EFAULT
> if standard payload is mixed with unreadable devmem fragments during
> extraction, and clamps unreadable skb lengths in openvswitch
> queue_userspace_packet() to avert truncated invalid payloads.
> 
> Fixes: 65249feb6b3d ("net: add support for skbs with unreadable frags")
> Cc: Pavel Begunkov <asml.silence@gmail.com>
> Cc: Stanislav Fomichev <sdf@fomichev.me>
> Cc: Bobby Eshleman <bobbyeshleman@gmail.com>
> Cc: Florian Westphal <fw@strlen.de>
> Cc: Aaron Conole <aconole@redhat.com>
> Cc: Eelco Chaudron <echaudro@redhat.com>
> Cc: Ilya Maximets <i.maximets@ovn.org>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
> 
> ---
> v2:
> - Return -EFAULT when mixing unreadable and readable frags (Pavel).
> - Clamp unreadable skb lengths for openvswitch queue drops (sashiko).
> v1: https://lore.kernel.org/r/20260801125308.1342897-1-almasrymina@google.com
> 
> Openvswitch maintainers: PTAL at the openvswitch changes closely. They
> are reported by sashiko as an also-need part of this fix:
> https://sashiko.dev/#/patchset/20260706155219.23757-1-fw%40strlen.de

Hmm.  FWIW, I do not see anything about openvswitch at that page.

> diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
> index ae69b2cabab9e..7c663d7846174 100644
> --- a/net/openvswitch/datapath.c
> +++ b/net/openvswitch/datapath.c
> @@ -480,6 +480,8 @@ static int queue_userspace_packet(struct datapath *dp, struct sk_buff *skb,
>  	}
>  
>  	skb_len = min(skb->len, cutlen);
> +	if (!skb_frags_readable(skb))
> +		skb_len = min_t(size_t, skb_len, skb_headlen(skb));

I'm not very familiar with the devmem and the unreadable frags, but if
there is really no way to read 'skb_len' bytes of the packet, it must not
be delivered to userspace.  Delivering truncated packet will confuse
ovs-vswitchd and the packet will be dropped or delivered truncated to the
destination.  We should return something like -EFAULT here and the caller
will drop the packet (MISS upcall) or continue processing if the failure
is not fatal (ACTION upcall).

This practically makes devmem incompatible with OVS, I suppose, as upcalls
are the primary mechanism for initial packet processing, before the datapath
flows are installed.

If there is a way to read this memory, we should make a full copy here.

Best regards, Ilya Maximets.