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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , stable@vger.kernel.org, sashiko-bot@kernel.org Subject: Re: [PATCH net v3 1/2] net/sched: sch_teql: unshare the header before writing and fix retry strip Message-ID: <20261109171539.GU18859@horms.kernel.org> References: X-Mailing-List: netdev@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Nov 03, 2026 at 07:39:15AM -0500, Jamal Hadi Salim wrote: > The issue: > teql_master_xmit() writes the slave's link header into the skb in > teql_resolve() and on retry pulls it back with __skb_pull(). Two bugs: > > 1. When the skb's header is cloned (e.g. a tc-mirred action on a > physical device's clsact egress redirects to the teql master), the > write corrupts the other clone. Call skb_cow_head() first. > > 2. The pull always removes skb_network_offset() bytes. For an > AF_PACKET/SOCK_RAW frame with no dst entry, teql_resolve() adds no > header, so the retry strips the user-supplied one. Pull back only > the length this slave added. > > The Fix: > Track the added length per iteration; a header-cloned skb is unshared > before the write and an AF_PACKET frame retries with its header intact. > > Conditions to recreate the bug: > CONFIG_NET_SCH_TEQL=y and CONFIG_VETH=y, run as root. A teql master with > two veth slaves, the first slave returning NETDEV_TX_BUSY without > consuming the skb so that teql retries the frame on the second one: > > ip link add vethBAD type veth peer name vethBADp > ip link add vethGOOD type veth peer name vethGOODp > ip link set vethBAD up; ip link set vethBADp up > ip link set vethGOOD up; ip link set vethGOODp up > tc qdisc add dev vethBAD root teql0 > tc qdisc add dev vethGOOD root teql0 > ip link set teql0 up > > Send a SOCK_RAW frame that carries its own link header on teql0 and > capture it on the second slave's peer. A veth transmit does not return > NETDEV_TX_BUSY on its own; to hold the skb pending this reproduction used > a test-only IFF_DEBUG accelerator in veth_xmit(), which is not part of > this patch and was applied to both the unfixed and the fixed build. > > On the unfixed tree the captured frame arrives with its header stripped > (176 of the 208 bytes sent, starting at the payload) because the retry > pulls skb_network_offset() bytes while this slave added none. With the > patch the frame arrives intact. > > The shared headroom case needs a cloned skb: a PF_PACKET tap on the > master whose socket is never drained keeps the clone alive across the > transmit. A kprobe on eth_header then reports cloned=1 on the unfixed > tree and cloned=0 with the patch. > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") > Reported-by: Sashiko (gemini) > Closes: https://sashiko.dev/#/patchset/20260824115928.4099988-1-victor@mojatatu.com > Link: https://lore.kernel.org/netdev/20260824115928.4099988-1-victor@mojatatu.com/ > Cc: stable@vger.kernel.org > Reviewed-by: Victor Nogueira > Signed-off-by: Jamal Hadi Salim Reviewed-by: Simon Horman