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[34.85.235.172]) by smtp.gmail.com with ESMTPSA id 00721157ae682-81ef42be12csm59727467b3.36.2026.07.21.00.59.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 Jul 2026 00:59:55 -0700 (PDT) Date: Tue, 21 Jul 2026 03:59:55 -0400 From: Willem de Bruijn To: Asim Viladi Oglu Manizada , netdev@vger.kernel.org Cc: Willem de Bruijn , Jason Wang , Andrew Lunn , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , daniel@iogearbox.net Message-ID: In-Reply-To: <20260721014117.2234892-1-manizada@pm.me> References: <20260721014117.2234892-1-manizada@pm.me> Subject: Re: [PATCH net] tun: prevent underflow in rx headroom calculation Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Asim Viladi Oglu Manizada wrote: > tun_get_user() calculates good_linear with SKB_MAX_HEAD(align). The align > value comes from ndo_set_rx_headroom() and can be larger than the linear > space available in a one-page skb head. > > OVS can reach this case by carrying headroom from a netkit/VXLAN port to a > TUN port. SKB_MAX_HEAD() then underflows, leaving good_linear negative. Separate from protecting tun from such huge headroom, which is good: Is cooking paths with such huge headroom itself incorrect? Should netkit have some reasonable upper bound on configurable headroom? Netkit is not the only path that can reach this, right? > Assigning that value to the size_t linear variable in tun_get_user() > converts it to a large positive value. The wrapped value is passed to > tun_alloc_skb(), where prepad + linear and len - linear wrap. skb->data can > then end up past the allocated head, and later packet processing can access > memory outside the skb. > > Clamp good_linear to zero when SKB_MAX_HEAD() returns a negative value. > This lets tun_alloc_skb() allocate the requested headroom and place the > packet data linearly or in fragments without wrapping. > > Fixes: eaea34b23c46 ("net/tun: implement ndo_set_rx_headroom") > Cc: stable@vger.kernel.org > Assisted-by: avom-custom-harness:gpt-5.5-qwen3.6-mod-mix > Signed-off-by: Asim Viladi Oglu Manizada Reviewed-by: Willem de Bruijn