diff mbox series

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

Message ID 20230724221326.384-2-andrew.kanner@gmail.com
State New
Headers
Series [1/2] drivers: net: use xdp only inside tun_build_skb() |

Commit Message

Andrew Kanner July 24, 2023, 10:13 p.m. UTC 
  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 <andrew.kanner@gmail.com>
---
 drivers/net/tun.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

Comments

Jason Wang July 25, 2023, 3:39 a.m. UTC | #1 
On Tue, Jul 25, 2023 at 6:15 AM Andrew Kanner <andrew.kanner@gmail.com> 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 <andrew.kanner@gmail.com>
> ---
>  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
>
Andrew Kanner July 25, 2023, 10:49 a.m. UTC | #2 
On Tue, Jul 25, 2023 at 11:39:46AM +0800, Jason Wang wrote:
> On Tue, Jul 25, 2023 at 6:15 AM Andrew Kanner <andrew.kanner@gmail.com> 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 <andrew.kanner@gmail.com>
> > ---
> >  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.
diff mbox series

Patch

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;