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Mon, 20 Apr 2026 12:34:32 -0700 (PDT) Received: from localhost ([2a03:2880:12ff:1::]) by smtp.gmail.com with ESMTPSA id 46e09a7af769-7dc975b057bsm10256033a34.20.2026.04.20.12.34.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2026 12:34:32 -0700 (PDT) Date: Mon, 20 Apr 2026 12:34:31 -0700 Message-ID: <519af099662f2b79eb95ceaabc7f7069.sdf.kernel@gmail.com> From: Stanislav Fomichev To: Jason Xing Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing In-Reply-To: <20260420082805.14844-3-kerneljasonxing@gmail.com> Subject: Re: [PATCH net v2 2/8] xsk: handle NULL dereference of the skb without frags issue Content-Type: text/plain Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: > From: Jason Xing > > When a first descriptor (xs->skb == NULL) triggers -EOVERFLOW in > xsk_build_skb_zerocopy (e.g., MAX_SKB_FRAGS exceeded), the free_err > EOVERFLOW handler unconditionally dereferences xs->skb via > xsk_inc_num_desc(xs->skb) and xsk_drop_skb(xs->skb), causing a NULL > pointer dereference. > > In this series, the skb is already freed by kfree_skb() inside > xsk_build_skb_zerocopy for the first-descriptor case, so we only need > to do the bookkeeping: cancel the one reserved CQ slot and account for > the single invalid descriptor. > > Guard the existing xsk_inc_num_desc/xsk_drop_skb calls with an > xs->skb check (for the continuation case), and add an else branch > for the first-descriptor case that manually cancels the CQ slot and > increments invalid_descs by one. > > Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path") > Signed-off-by: Jason Xing > --- > net/xdp/xsk.c | 11 ++++++++--- > 1 file changed, 8 insertions(+), 3 deletions(-) > > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c > index 6149f6a79897..6521604f8d42 100644 > --- a/net/xdp/xsk.c > +++ b/net/xdp/xsk.c > @@ -893,9 +893,14 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs, > kfree_skb(skb); > > if (err == -EOVERFLOW) { > - /* Drop the packet */ > - xsk_inc_num_desc(xs->skb); > - xsk_drop_skb(xs->skb); > + if (xs->skb) { > + /* Drop the packet */ > + xsk_inc_num_desc(xs->skb); > + xsk_drop_skb(xs->skb); > + } else { > + xsk_cq_cancel_locked(xs->pool, 1); > + xs->tx->invalid_descs++; > + } > xskq_cons_release(xs->tx); > } else { > /* Let application retry */ > -- > 2.41.3 > Acked-by: Stanislav Fomichev