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Fri, 15 May 2026 05:30:39 -0700 (PDT) From: Jason Xing To: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, bjorn@kernel.org, magnus.karlsson@intel.com, maciej.fijalkowski@intel.com, jonathan.lemon@gmail.com, sdf@fomichev.me, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, horms@kernel.org, andrew+netdev@lunn.ch Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing Subject: [PATCH net v2 1/5] xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata() Date: Fri, 15 May 2026 20:30:14 +0800 Message-Id: <20260515123018.80147-2-kerneljasonxing@gmail.com> X-Mailer: git-send-email 2.33.0 In-Reply-To: <20260515123018.80147-1-kerneljasonxing@gmail.com> References: <20260515123018.80147-1-kerneljasonxing@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Jason Xing The TX metadata area resides in the UMEM buffer which is memory-mapped and concurrently writable by userspace. In xsk_skb_metadata(), csum_start and csum_offset are read from shared memory for bounds validation, then read again for skb assignment. A malicious userspace application can race to overwrite these values between the two reads, bypassing the bounds check and causing out-of-bounds memory access during checksum computation in the transmit path. Fix this by reading csum_start and csum_offset into local variables once, then using the local copies for both validation and assignment. Note that other metadata fields (flags, launch_time) and the cached csum fields may be mutually inconsistent due to concurrent userspace writes, but this is benign: the only security-critical invariant is that each field's validated value is the same one used, which local caching guarantees. Closes: https://lore.kernel.org/all/20260503200927.73EA1C2BCB4@smtp.kernel.org/ Fixes: 48eb03dd2630 ("xsk: Add TX timestamp and TX checksum offload support") Signed-off-by: Jason Xing --- net/xdp/xsk.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c index 6bcd77068e52..adcec1d22e8b 100644 --- a/net/xdp/xsk.c +++ b/net/xdp/xsk.c @@ -722,6 +722,7 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer, u32 hr) { struct xsk_tx_metadata *meta = NULL; + u16 csum_start, csum_offset; if (unlikely(pool->tx_metadata_len == 0)) return -EINVAL; @@ -731,13 +732,15 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer, return -EINVAL; if (meta->flags & XDP_TXMD_FLAGS_CHECKSUM) { - if (unlikely(meta->request.csum_start + - meta->request.csum_offset + + csum_start = READ_ONCE(meta->request.csum_start); + csum_offset = READ_ONCE(meta->request.csum_offset); + + if (unlikely(csum_start + csum_offset + sizeof(__sum16) > desc->len)) return -EINVAL; - skb->csum_start = hr + meta->request.csum_start; - skb->csum_offset = meta->request.csum_offset; + skb->csum_start = hr + csum_start; + skb->csum_offset = csum_offset; skb->ip_summed = CHECKSUM_PARTIAL; if (unlikely(pool->tx_sw_csum)) { -- 2.41.3