Re: [PATCH net v4 1/5] xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()
From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Date: 2026-05-21 12:05:02
Also in:
bpf
On Wed, May 20, 2026 at 08:42:40AM +0800, Jason Xing wrote:
From: Jason Xing <kernelxing@tencent.com> 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/ (local) Fixes: 48eb03dd2630 ("xsk: Add TX timestamp and TX checksum offload support") Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
quoted hunk ↗ jump to hunk
--- 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 5e5786cd9af5..f8c8a8c9dfba 100644 --- a/net/xdp/xsk.c +++ b/net/xdp/xsk.c@@ -802,6 +802,7 @@ 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;@@ -811,13 +812,15 @@ 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)) {-- 2.43.7