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

From: Jesper Dangaard Brouer
Date: Wed Aug 02 2023 - 10:14:07 EST

On 02/08/2023 00.07, Andrew Kanner wrote:
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")
Signed-off-by: Andrew Kanner <andrew.kanner@xxxxxxxxx>

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@xxxxxxxxxx> 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@xxxxxxxxxx>. Otherwise extra packets will be dropped
without falling back to tun_alloc_skb(). And in the discussion of v3
Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx> 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@xxxxxxxxxx>
* 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@xxxxxxxxxx>

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)
+ rcu_read_unlock();

Isolated seen, I guess, this is a correct fix to 7df13219d757.

+ if (SKB_DATA_ALIGN(len + pad) +
SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
return false;

Question to Jason Wang:
Why fall back (to e.g. tun_alloc_skb()) when size is above PAGE_SIZE?

AFAIK tun_build_skb() *can* create get larger packets than PAGE_SIZE
from it's page_frag. Is there a reason for this limitation?

(To Andrew, I assume a change in this area is another patch).