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[82.53.135.12]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-490424aa561sm118548825e9.5.2026.05.25.06.09.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 25 May 2026 06:10:00 -0700 (PDT) Date: Mon, 25 May 2026 15:09:54 +0200 From: Stefano Garzarella To: "Michael S. Tsirkin" Cc: David Laight , patchwork-bot+netdevbpf@kernel.org, netdev@vger.kernel.org, xuanzhuo@linux.alibaba.com, horms@kernel.org, virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, kuba@kernel.org, eperezma@redhat.com, pabeni@redhat.com, davem@davemloft.net, jasowang@redhat.com, stefanha@redhat.com, edumazet@google.com, stable@vger.kernel.org Subject: Re: [PATCH net] vsock/virtio: fix skb overhead overflow on 32-bit builds Message-ID: References: <20260521124732.125771-1-sgarzare@redhat.com> <177950282964.1445071.6600517211632117224.git-patchwork-notify@kernel.org> <20260523173557.5cc4f4f6@pumpkin> <20260525115314.3cf310e6@pumpkin> <20260525083859-mutt-send-email-mst@kernel.org> 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; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260525083859-mutt-send-email-mst@kernel.org> On Mon, May 25, 2026 at 08:42:01AM -0400, Michael S. Tsirkin wrote: >On Mon, May 25, 2026 at 11:53:14AM +0100, David Laight wrote: >> On Mon, 25 May 2026 11:57:45 +0200 >> Stefano Garzarella wrote: >> >> > On Sat, May 23, 2026 at 05:35:57PM +0100, David Laight wrote: >> > >On Sat, 23 May 2026 02:20:29 +0000 >> > >patchwork-bot+netdevbpf@kernel.org wrote: >> > > >> > >> Hello: >> > >> >> > >> This patch was applied to netdev/net.git (main) >> > >> by Jakub Kicinski : >> > > >> > >Did anyone else notice that is isn't a bug? >> > > >> > >There is no way that a 'count of bytes of kernel memory' can overflow >> > >the size of 'long'. >> > >> > It's more of an estimate than an actual calculation of memory usage if >> > we queue the incoming packet. In theory, an overflow could occur if the >> > user sets `buf_alloc` to 4GB. In practice, though, I think you're right: >> > the memory should run out before we get to that check. >> >> The calculation is: >> >> u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0); >> >> skb_queue_len() will be the number of items on the queue. >> SKB_TRUESIZE(0) is the memory taken up by a zero length skb (basically sizeof(skb)). >> >> Unless you either corrupt the queue length or manage to allocate skb that use >> less than the minimum about of memory that product can't overflow 'unsigned long'. >> >> The later calculations might wrap - but the multiply can't. >> >> -- David > > >Indeed, I wasn't thinking. For this to even get close to overflowing >we'd have to have almost all of 4G available to the 32 bit kernel taken >up by this single queue. > >Revert, I'd say. I also blindly added the cast to silence sashiko :-( I see now that it could never actually happen, but semantically it’s correct, so maybe we can avoid the revert. Thanks, Stefano > >> > >> > Thanks, >> > Stefano >> > >> > > >> > >-- David >> > > >> > >> >> > >> On Thu, 21 May 2026 14:47:32 +0200 you wrote: >> > >> > From: Stefano Garzarella >> > >> > >> > >> > On 32-bit architectures, both skb_queue_len() and SKB_TRUESIZE(0) evaluate >> > >> > to 32-bit values. The multiplication can overflow before being assigned to >> > >> > the u64 skb_overhead variable, making the skb overhead check ineffective. >> > >> > >> > >> > Cast skb_queue_len() to u64 so the multiplication is always performed in >> > >> > 64-bit arithmetic. >> > >> > >> > >> > [...] >> > >> >> > >> Here is the summary with links: >> > >> - [net] vsock/virtio: fix skb overhead overflow on 32-bit builds >> > >> https://git.kernel.org/netdev/net/c/4157501b9a8f >> > >> >> > >> You are awesome, thank you! >> > > >> > >