[PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[Andrew Kanner <andrew.kanner@gmail.com>]

Using the 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(). This may end up
with exceeding the PAGE_SIZE limit in tun_build_skb().

Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
---

Notes:
    v3 -> v4:
    * fall back to v1, fixing only missing XDP_PACKET_HEADROOM in pad and
      removing bpf_xdp_adjust_tail() check for frame_sz.
    * added rcu read lock, noted by Jason Wang <jasowang@redhat.com> in v1
    * I decided to leave the packet length check in tun_can_build_skb()
      instead of moving to tun_build_skb() suggested by Jason Wang
      <jasowang@redhat.com>. Otherwise extra packets will be dropped
      without falling back to tun_alloc_skb(). And in the discussion of v3
      Jesper Dangaard Brouer <jbrouer@redhat.com> noticed that XDP is ok
      with a higher order pages if it's a contiguous physical memory
      allocation, so falling to tun_alloc_skb() -> do_xdp_generic() should
      be ok.
    
    v2 -> v3:
    * attach the forgotten changelog
    
    v1 -> v2:
    * merged 2 patches in 1, fixing both issues: WARN_ON_ONCE with
      syzkaller repro and missing XDP_PACKET_HEADROOM in pad
    * changed the title and description of the execution path, suggested
      by Jason Wang <jasowang@redhat.com>
    * move the limit check from tun_can_build_skb() to tun_build_skb() to
      remove duplication and locking issue, and also drop the packet in
      case of a failed check - noted by Jason Wang <jasowang@redhat.com>

 drivers/net/tun.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d75456adc62a..a1d04bc9485f 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1582,6 +1582,9 @@ 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)
 {
+	struct bpf_prog *xdp_prog;
+	int pad = TUN_RX_PAD;
+
 	if ((tun->flags & TUN_TYPE_MASK) != IFF_TAP)
 		return false;
 
@@ -1594,7 +1597,13 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
 	if (zerocopy)
 		return false;
 
-	if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
+	rcu_read_lock();
+	xdp_prog = rcu_dereference(tun->xdp_prog);
+	if (xdp_prog)
+		pad += XDP_PACKET_HEADROOM;
+	rcu_read_unlock();
+
+	if (SKB_DATA_ALIGN(len + pad) +
 	    SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
 		return false;
 
-- 
2.39.3