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Mon, 3 Nov 2025 11:39:27 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.202]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4A9ED180057D; Mon, 3 Nov 2025 11:39:25 +0000 (UTC) Date: Mon, 3 Nov 2025 11:39:21 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Cc: Peter Maydell , qemu-devel@nongnu.org, Jason Wang , Bin Meng Subject: Re: [PATCH] net: pad packets to minimum length in qemu_receive_packet() Message-ID: References: <20251028160042.3321933-1-peter.maydell@linaro.org> <1a8ea87f-41b2-40a5-8511-df019b5833c5@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1a8ea87f-41b2-40a5-8511-df019b5833c5@linaro.org> User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, Nov 03, 2025 at 12:35:48PM +0100, Philippe Mathieu-Daudé wrote: > On 28/10/25 17:00, Peter Maydell wrote: > > In commits like 969e50b61a28 ("net: Pad short frames to minimum size > > before sending from SLiRP/TAP") we switched away from requiring > > network devices to handle short frames to instead having the net core > > code do the padding of short frames out to the ETH_ZLEN minimum size. > > We then dropped the code for handling short frames from the network > > devices in a series of commits like 140eae9c8f7 ("hw/net: e1000: > > Remove the logic of padding short frames in the receive path"). > > > > This missed one route where the device's receive code can still see a > > short frame: if the device is in loopback mode and it transmits a > > short frame via the qemu_receive_packet() function, this will be fed > > back into its own receive code without being padded. > > > > Add the padding logic to qemu_receive_packet(). > > > > This fixes a buffer overrun which can be triggered in the > > e1000_receive_iov() logic via the loopback code path. > > > > Other devices that use qemu_receive_packet() to implement loopback > > are cadence_gem, dp8393x, lan9118, msf2-emac, pcnet, rtl8139 > > and sungem. > > > > Cc: qemu-stable@nongnu.org > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3043 > > Signed-off-by: Peter Maydell > > --- > > I think this is the right fix, but I'm not very familiar > > with the net internals... > > --- > > net/net.c | 10 ++++++++++ > > 1 file changed, 10 insertions(+) > > > > diff --git a/net/net.c b/net/net.c > > index 27e0d278071..8aefdb3424f 100644 > > --- a/net/net.c > > +++ b/net/net.c > > @@ -775,10 +775,20 @@ ssize_t qemu_send_packet(NetClientState *nc, const uint8_t *buf, int size) > > ssize_t qemu_receive_packet(NetClientState *nc, const uint8_t *buf, int size) > > { > > + uint8_t min_pkt[ETH_ZLEN]; > > + size_t min_pktsz = sizeof(min_pkt); > > + > > if (!qemu_can_receive_packet(nc)) { > > return 0; > > } > > + if (net_peer_needs_padding(nc)) { > > + if (eth_pad_short_frame(min_pkt, &min_pktsz, buf, size)) { > > + buf = min_pkt; > > + size = min_pktsz; > > + } > > + } > > + > > return qemu_net_queue_receive(nc->incoming_queue, buf, size); > > } > > Nitpicking, variables scope can be reduced: > > -- >8 -- > @@ -777,5 +777,2 @@ ssize_t qemu_receive_packet(NetClientState *nc, const > uint8_t *buf, int size) > { > - uint8_t min_pkt[ETH_ZLEN]; > - size_t min_pktsz = sizeof(min_pkt); > - > if (!qemu_can_receive_packet(nc)) { > @@ -785,2 +782,5 @@ ssize_t qemu_receive_packet(NetClientState *nc, const > uint8_t *buf, int size) > if (net_peer_needs_padding(nc)) { > + uint8_t min_pkt[ETH_ZLEN]; > + size_t min_pktsz = sizeof(min_pkt); > + > if (eth_pad_short_frame(min_pkt, &min_pktsz, buf, size)) { > --- That isn't desirable, as then 'buf' would be holding a stack pointer that has gone out of scope when qemu_net_queue_receive is called. > Reviewed-by: Philippe Mathieu-Daudé > > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|