Re: [PATCH net 1/4] xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 05/10, 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/
> Fixes: 48eb03dd2630 ("xsk: Add TX timestamp and TX checksum offload support")
> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---
>  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..cd039e397018 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 = meta->request.csum_start;
> +		csum_offset = meta->request.csum_offset;

Wondering if it's better to READ_ONCE(x) these?