2023-07-24 22:58:33

by Andrew Kanner

Subject: [PATCH 2/2] drivers: net: prevent tun_can_build_skb() to exceed xdp size limits

Tested with syzkaller repro with reduced packet size. It was
discovered that XDP_PACKET_HEADROOM is not checked in
tun_can_build_skb(), although pad may be incremented in
tun_build_skb().

Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
Link: https://syzkaller.appspot.com/text?tag=ReproC&x=12b2593ea80000
Signed-off-by: Andrew Kanner <[email protected]>
---
drivers/net/tun.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 18ccbbe9830a..cdf2bd85b383 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1582,7 +1582,13 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
int len, int noblock, bool zerocopy, int *skb_xdp)
{
- if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
+ int pad = TUN_RX_PAD;
+ struct bpf_prog *xdp_prog = rcu_dereference(tun->xdp_prog);
+
+ if (xdp_prog)
+ pad += XDP_PACKET_HEADROOM;
+
+ if (SKB_DATA_ALIGN(len + pad) +
SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE) {
*skb_xdp = 0;
return false;
--
2.39.3

2023-07-25 05:13:11

by Jason Wang

Subject: Re: [PATCH 2/2] drivers: net: prevent tun_can_build_skb() to exceed xdp size limits

On Tue, Jul 25, 2023 at 6:15 AM Andrew Kanner <[email protected]> wrote:
>
> Tested with syzkaller repro with reduced packet size. It was
> discovered that XDP_PACKET_HEADROOM is not checked in
> tun_can_build_skb(), although pad may be incremented in
> tun_build_skb().
>
> Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
> Link: https://syzkaller.appspot.com/text?tag=ReproC&x=12b2593ea80000
> Signed-off-by: Andrew Kanner <[email protected]>
> ---
> drivers/net/tun.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 18ccbbe9830a..cdf2bd85b383 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1582,7 +1582,13 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
> static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
> int len, int noblock, bool zerocopy, int *skb_xdp)
> {
> - if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> + int pad = TUN_RX_PAD;
> + struct bpf_prog *xdp_prog = rcu_dereference(tun->xdp_prog);

This misses rcu read lock.

I wonder if things could be simpler if we move the limit check from
tun_can_build_skb() to tun_build_skb():

rcu_read_lock();
xdp_prog = rcu_dereference(tun->xdp_prog);
if (xdp_prog)
pad += XDP_PACKET_HEADROOM;
buflen += SKB_DATA_ALIGN(len + pad);
rcu_read_unlock();

Thanks

> +
> + if (xdp_prog)
> + pad += XDP_PACKET_HEADROOM;
> +
> + if (SKB_DATA_ALIGN(len + pad) +
> SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE) {
> *skb_xdp = 0;
> return false;
> --
> 2.39.3
>

2023-07-25 12:02:40

by Andrew Kanner

Subject: Re: [PATCH 2/2] drivers: net: prevent tun_can_build_skb() to exceed xdp size limits

On Tue, Jul 25, 2023 at 11:39:46AM +0800, Jason Wang wrote:
> On Tue, Jul 25, 2023 at 6:15 AM Andrew Kanner <[email protected]> wrote:
> >
> > Tested with syzkaller repro with reduced packet size. It was
> > discovered that XDP_PACKET_HEADROOM is not checked in
> > tun_can_build_skb(), although pad may be incremented in
> > tun_build_skb().
> >
> > Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
> > Link: https://syzkaller.appspot.com/text?tag=ReproC&x=12b2593ea80000
> > Signed-off-by: Andrew Kanner <[email protected]>
> > ---
> > drivers/net/tun.c | 8 +++++++-
> > 1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index 18ccbbe9830a..cdf2bd85b383 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1582,7 +1582,13 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
> > static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
> > int len, int noblock, bool zerocopy, int *skb_xdp)
> > {
> > - if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> > + int pad = TUN_RX_PAD;
> > + struct bpf_prog *xdp_prog = rcu_dereference(tun->xdp_prog);
>
> This misses rcu read lock.
>
> I wonder if things could be simpler if we move the limit check from
> tun_can_build_skb() to tun_build_skb():
>
> rcu_read_lock();
> xdp_prog = rcu_dereference(tun->xdp_prog);
> if (xdp_prog)
> pad += XDP_PACKET_HEADROOM;
> buflen += SKB_DATA_ALIGN(len + pad);
> rcu_read_unlock();
>
> Thanks
>

Thanks, I missed the part with rcu read lock for some reason.

It's a good idea to move / reduce duplication. Let me think and try to
fix according to your comments, I will resend it as v2.

--
Andrew Kanner