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Mon, 20 Jul 2026 13:24:37 -0700 (PDT) Date: Mon, 20 Jul 2026 13:24:32 -0700 From: Stanislav Fomichev To: "Cen Zhang (Microsoft)" Cc: magnus.karlsson@intel.com, maciej.fijalkowski@intel.com, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, sdf@fomichev.me, horms@kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, AutonomousCodeSecurity@microsoft.com, tgopinath@linux.microsoft.com, kys@microsoft.com Subject: Re: [PATCH net] xsk: reject tx_metadata_len smaller than struct xsk_tx_metadata Message-ID: References: <20260720155210.34229-1-blbllhy@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260720155210.34229-1-blbllhy@gmail.com> On 07/20, Cen Zhang (Microsoft) wrote: > xdp_umem_reg() validates tx_metadata_len for upper bound (<256) and > alignment (%8) but not a lower bound. xsk_skb_metadata() computes > meta = buffer - pool->tx_metadata_len then unconditionally accesses > the full 24-byte struct xsk_tx_metadata, so any value less than > sizeof(struct xsk_tx_metadata) allows an out-of-bounds read. > > KASAN reports this as: > > BUG: KASAN: vmalloc-out-of-bounds in xsk_skb_metadata+0x4b2/0x500 > Read of size 8 at addr ffffc90000f11000 by task exploit/148 > > xsk_skb_metadata (net/xdp/xsk.c:837) > xsk_build_skb (net/xdp/xsk.c) > __xsk_generic_xmit (net/xdp/xsk.c) > xsk_sendmsg (net/xdp/xsk.c) > > Add a lower-bound check in xdp_umem_reg() to reject tx_metadata_len > values that cannot cover the full metadata struct. > > Fixes: 341ac980eab9 ("xsk: Support tx_metadata_len") > Reported-by: AutonomousCodeSecurity@microsoft.com > Signed-off-by: Cen Zhang (Microsoft) > --- > net/xdp/xdp_umem.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c > index 58da2f4f4397..d16ad9d8f919 100644 > --- a/net/xdp/xdp_umem.c > +++ b/net/xdp/xdp_umem.c > @@ -208,7 +208,8 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr) > return -EINVAL; > > if (mr->flags & XDP_UMEM_TX_METADATA_LEN) { > - if (mr->tx_metadata_len >= 256 || mr->tx_metadata_len % 8) > + if (mr->tx_metadata_len < sizeof(struct xsk_tx_metadata) || > + mr->tx_metadata_len >= 256 || mr->tx_metadata_len % 8) > return -EINVAL; > umem->tx_metadata_len = mr->tx_metadata_len; > } > -- > 2.53.0 > This will make adding new tx metadata types harder (and will require all userspace to be updated whenever we do so), will the following be a bit nicer? (but, obviously, paying more per-packet at runtime) (untested) diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h index 46797645a0c2..b55b878949d5 100644 --- a/include/net/xdp_sock_drv.h +++ b/include/net/xdp_sock_drv.h @@ -260,9 +260,21 @@ xsk_buff_raw_get_ctx(const struct xsk_buff_pool *pool, u64 addr) 0) static inline bool -xsk_buff_valid_tx_metadata(const struct xsk_tx_metadata *meta) +xsk_buff_valid_tx_metadata(const struct xsk_buff_pool *pool, + const struct xsk_tx_metadata *meta) { - return !(meta->flags & ~XDP_TXMD_FLAGS_VALID); + /* covers flags, XDP_TXMD_FLAGS_CHECKSUM & XDP_TXMD_FLAGS_TIMESTAMP */ + if (unlikely(pool->tx_metadata_len < 16)) + return false; + + if (unlikely(meta->flags & ~XDP_TXMD_FLAGS_VALID)) + return false; + + if (meta->flags & XDP_TXMD_FLAGS_LAUNCH_TIME) + if (unlikely(pool->tx_metadata_len < offsetofend(struct xsk_tx_metadata, launch_time))) + return false; + + return true; } static inline struct xsk_tx_metadata * @@ -274,7 +286,7 @@ __xsk_buff_get_metadata(const struct xsk_buff_pool *pool, void *data) return NULL; meta = data - pool->tx_metadata_len; - if (unlikely(!xsk_buff_valid_tx_metadata(meta))) + if (unlikely(!xsk_buff_valid_tx_metadata(pool, meta))) return NULL; /* no way to signal the error to the user */ return meta; @@ -469,7 +481,8 @@ xsk_buff_raw_get_ctx(const struct xsk_buff_pool *pool, u64 addr) return (struct xdp_desc_ctx){ }; } -static inline bool xsk_buff_valid_tx_metadata(struct xsk_tx_metadata *meta) +static inline bool xsk_buff_valid_tx_metadata(const struct xsk_buff_pool *pool, + struct xsk_tx_metadata *meta) { return false; } diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c index b970f30ea9b9..75b2c97e41e4 100644 --- a/net/xdp/xsk.c +++ b/net/xdp/xsk.c @@ -809,7 +809,7 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer, return -EINVAL; meta = buffer - pool->tx_metadata_len; - if (unlikely(!xsk_buff_valid_tx_metadata(meta))) + if (unlikely(!xsk_buff_valid_tx_metadata(pool, meta))) return -EINVAL; if (meta->flags & XDP_TXMD_FLAGS_CHECKSUM) {