Re: [PATCH net] can: isotp: isotp_rcv_cf(): fix so->rx race problem

[Oliver Hartkopp]

On 20.01.22 12:28, Ziyang Xuan (William) wrote:
> > On 20.01.22 07:24, Ziyang Xuan (William) wrote:
> >
> > > I have reproduced the syz problem with Marc's commit, the commit can not fix the panic problem.
> > > So I tried to find the root cause for panic and gave my solution.
> > >
> > > Marc's commit just fix the condition that packet size bigger than INT_MAX which trigger
> > > tpcon::{idx,len} integer overflow, but the packet size is 4096 in the syz problem.
> > >
> > > so->rx.len is 0 after the following logic in isotp_rcv_ff():
> > >
> > > /* get the FF_DL */
> > > so->rx.len = (cf->data[ae] & 0x0F) << 8;
> > > so->rx.len += cf->data[ae + 1];
> > >
> > > so->rx.len is 4096 after the following logic in isotp_rcv_ff():
> > >
> > > /* FF_DL = 0 => get real length from next 4 bytes */
> > > so->rx.len = cf->data[ae + 2] << 24;
> > > so->rx.len += cf->data[ae + 3] << 16;
> > > so->rx.len += cf->data[ae + 4] << 8;
> > > so->rx.len += cf->data[ae + 5];
> >
> > In these cases the values 0 could be the minimum value in so->rx.len - but e.g. the value 0 can not show up in isotp_rcv_cf() as this function requires so->rx.state to be ISOTP_WAIT_DATA.
>
> Consider the scenario that isotp_rcv_cf() and isotp_rcv_cf() are concurrent for the same isotp_sock as following sequence:

o_O

Sorry but the receive path is not designed to handle concurrent 
receptions that would run isotp_rcv_cf() and isotp_rcv_ff() simultaneously.

> isotp_rcv_cf()
> if (so->rx.state != ISOTP_WAIT_DATA) [false]
> 						isotp_rcv_ff()
> 						so->rx.state = ISOTP_IDLE
> 						/* get the FF_DL */ [so->rx.len == 0]
> alloc_skb() [so->rx.len == 0]
> 						/* FF_DL = 0 => get real length from next 4 bytes */ [so->rx.len == 4096]
> skb_put(nskb, so->rx.len) [so->rx.len == 4096]
> skb_over_panic()

Even though this case is not possible with a real CAN bus due to the CAN 
frame transmission times we could introduce some locking (or dropping of 
concurrent CAN frames) in isotp_rcv() - but this code runs in net 
softirq context ...

Regards,
Oliver


> > And when so->rx.len is 0 in isotp_rcv_ff() this check
> >
> > if (so->rx.len + ae + off + ff_pci_sz < so->rx.ll_dl)
> >          return 1;
> >
> > will return from isotp_rcv_ff() before ISOTP_WAIT_DATA is set at the end. So after that above check we are still in ISOTP_IDLE state.
> >
> > Or did I miss something here?
> >
> > > so->rx.len is 0 before alloc_skb() and is 4096 after alloc_skb() in isotp_rcv_cf(). The following
> > > skb_put() will trigger panic.
> > >
> > > The following log is my reproducing log with Marc's commit and my debug modification in isotp_rcv_cf().
> > >
> > > [  150.605776][    C6] isotp_rcv_cf: before alloc_skb so->rc.len: 0, after alloc_skb so->rx.len: 4096
> >
> >
> > But so->rx_len is not a value that is modified by alloc_skb():
> >
> >                  nskb = alloc_skb(so->rx.len, gfp_any());
> >                  if (!nskb)
> >                          return 1;
> >
> >                  memcpy(skb_put(nskb, so->rx.len), so->rx.buf,
> >                         so->rx.len);
> >
> >
> > Can you send your debug modification changes please?
>
> My reproducing debug as attachment and following:
>
> diff --git a/net/can/isotp.c b/net/can/isotp.c
> index df6968b28bf4..8b12d63b4d59 100644
> --- a/net/can/isotp.c
> +++ b/net/can/isotp.c
> @@ -119,8 +119,8 @@ enum {
>   };
>
>   struct tpcon {
> -       int idx;
> -       int len;
> +       unsigned int idx;
> +       unsigned int len;
>          u32 state;
>          u8 bs;
>          u8 sn;
> @@ -505,6 +505,7 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
>          struct isotp_sock *so = isotp_sk(sk);
>          struct sk_buff *nskb;
>          int i;
> +       bool unexpection = false;
>
>          if (so->rx.state != ISOTP_WAIT_DATA)
>                  return 0;
> @@ -562,11 +563,13 @@ static int isotp_rcv_cf(struct sock *sk, struct canfd_frame *cf, int ae,
>                                  sk_error_report(sk);
>                          return 1;
>                  }
> -
> +               if (so->rx.len == 0)
> +                       unexpection = true;
>                  nskb = alloc_skb(so->rx.len, gfp_any());
>                  if (!nskb)
>                          return 1;
> -
> +               if (unexpection)
> +                       printk("%s: before alloc_skb so->rc.len: 0, after alloc_skb so->rx.len: %u\n", __func__, so->rx.len);
>                  memcpy(skb_put(nskb, so->rx.len), so->rx.buf,
>                         so->rx.len);
>
>
> > Best regards,
> > Oliver
> >
> > > [  150.611477][    C6] skbuff: skb_over_panic: text:ffffffff881ff7be len:4096 put:4096 head:ffff88807f93a800 data:ffff88807f93a800 tail:0x1000 end:0xc0 dev:<NULL>
> > > [  150.615837][    C6] ------------[ cut here ]------------
> > > [  150.617238][    C6] kernel BUG at net/core/skbuff.c:113!
> >
> > .