Re: [PATCH bpf v2 1/2] bpf, veth: xdp: fix page_pool page leak on skb-backed XDP

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Sun Aug 23, 2026 at 11:06 PM EDT, Jiayuan Chen wrote:
> bpf_xdp_shrink_data() frees a released frag via __xdp_return() using
> xdp->rxq->mem.type, but that type is wrong for skb-backed XDP: the skb is
> cow'd into page_pool memory while the rxq still says MEM_TYPE_PAGE_SHARED,
> so the page_pool page is freed with page_frag_free() and we hit
> "Bad page state ... page_pool leak".
>
> Both generic XDP and veth are affected. A non-linear skb is cow'd into
> page_pool memory (skb_cow_data_for_xdp() -> skb_pp_cow_data() for generic
> XDP, veth_convert_skb_to_xdp_buff() for veth), so its frags become
> page_pool pages while the rxq keeps MEM_TYPE_PAGE_SHARED.
>
> We can't just fix rxq->mem.type in place:
> - generic XDP: xdp->rxq is dev->_rx[queue].xdp_rxq (see
>   bpf_prog_run_generic_xdp()), a shared rxq that other CPUs may access in
>   parallel, so we must not write to it.
> - veth: rq->xdp_rxq.mem is shared per-queue state that veth resets on XDP
>   teardown, and with GRO that reset runs without stopping in-flight NAPI,
>   so a type stashed there can be clobbered under a packet still in flight.
>
> Adding a check in __xdp_return() or bpf_xdp_shrink_data() itself is not an
> option either: without recording it somewhere, both can only guess the
> frag's memory type, which quickly gets confusing.
>
> So record it in the xdp_buff. Add a XDP_FLAGS_FRAGS_PAGE_POOL flag; the two
> skb-cow sites set it, and bpf_xdp_shrink_data() frees the frag to the
> page_pool when it is set, otherwise it keeps falling back to
> xdp->rxq->mem.type unchanged. No other path changes behaviour.
>
> Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
> Fixes: 0ebab78cbcbf ("net: veth: add page_pool for page recycling")
> Reported-by: syzbot+237bbeed8dfe0699b7f5@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=237bbeed8dfe0699b7f5
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

Hi Jiayuan,

> ---
>  drivers/net/veth.c |  5 +++++
>  include/net/xdp.h  | 14 ++++++++++++++
>  net/core/dev.c     |  5 +++++
>  net/core/filter.c  |  7 +++++++
>  4 files changed, 31 insertions(+)
>
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 6ed3ee81153f..0afa0661ada1 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -775,6 +775,11 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
>  	if (skb_shinfo(skb)->nr_frags) {
>  		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
>  		xdp_buff_set_frags_flag(xdp);
> +		/* A nonlinear skb was cow'd into rq->page_pool above, so the
> +		 * frags must be freed to that pool, not via the rxq's
> +		 * MEM_TYPE_PAGE_SHARED.
> +		 */
> +		xdp_buff_set_frag_pp(xdp);
>  	} else {
>  		xdp_buff_clear_frags_flag(xdp);
>  	}
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index aa742f413c35..b389dc527adc 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -81,6 +81,10 @@ enum xdp_buff_flags {
>  	 * XDP program is not attached.
>  	 */
>  	XDP_FLAGS_FRAGS_UNREADABLE	= BIT(2),
> +	/* frags are page_pool memory even though rxq->mem.type is not: a
> +	 * skb-backed XDP buff (generic XDP, veth) is cow'd into a page_pool.
> +	 */
> +	XDP_FLAGS_FRAGS_PAGE_POOL	= BIT(3),
>  };
>  
>  struct xdp_buff {
> @@ -131,6 +135,16 @@ static __always_inline void xdp_buff_set_frag_unreadable(struct xdp_buff *xdp)
>  	xdp->flags |= XDP_FLAGS_FRAGS_UNREADABLE;
>  }
>  
> +static __always_inline void xdp_buff_set_frag_pp(struct xdp_buff *xdp)
> +{
> +	xdp->flags |= XDP_FLAGS_FRAGS_PAGE_POOL;
> +}

For veth_xdp_rcv_skb() we also need to clear the flag in XDP_TX and XDP_REDIRECT.
The veth_xdp_get() call takes page refs but not page pool refs, while consume_skb() 
drops the page pool reference. We should be clearing the new flag so that __xdp_return()
doesn't try to free it back to the page pool, otherwise we get a similar mismatch-related 
splat to the original bug's.

pw-bot: cr



> +
> +static __always_inline bool xdp_buff_is_frag_pp(const struct xdp_buff *xdp)
> +{
> +	return !!(xdp->flags & XDP_FLAGS_FRAGS_PAGE_POOL);
> +}
> +
>  static __always_inline u32 xdp_buff_get_skb_flags(const struct xdp_buff *xdp)
>  {
>  	return xdp->flags;
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 38336858c168..be36020484b6 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5532,6 +5532,11 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
>  	if (skb_is_nonlinear(skb)) {
>  		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
>  		xdp_buff_set_frags_flag(xdp);
> +		/* A nonlinear skb was cow'd into page_pool memory by
> +		 * skb_cow_data_for_xdp() before we got here, so the frags must
> +		 * be freed to that pool, not via the rxq's MEM_TYPE_PAGE_SHARED.
> +		 */
> +		xdp_buff_set_frag_pp(xdp);
>  	} else {
>  		xdp_buff_clear_frags_flag(xdp);
>  	}
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 61940e753552..d34ba56d79d8 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4378,6 +4378,13 @@ static bool bpf_xdp_shrink_data(struct xdp_buff *xdp, skb_frag_t *frag,
>  	if (mem_type == MEM_TYPE_XSK_BUFF_POOL) {
>  		netmem = 0;
>  		zc_frag = bpf_xdp_shrink_data_zc(xdp, shrink, tail, release);
> +	} else if (xdp_buff_is_frag_pp(xdp)) {
> +		/*
> +		 * Skb-backed XDP (generic XDP, veth) cow's the frags into a
> +		 * page_pool while the rxq stays MEM_TYPE_PAGE_SHARED, so free
> +		 * the frag to the pool, not via page_frag_free().
> +		 */
> +		mem_type = MEM_TYPE_PAGE_POOL;
>  	}
>  
>  	if (release) {