Re: [PATCH net v2] sctp: verify size of a new chunk in _sctp_make_chunk()

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Fri, Feb 09, 2018 at 05:35:23PM +0300, Alexey Kodanev wrote:
> When SCTP makes INIT or INIT_ACK packet the total chunk length
> can exceed SCTP_MAX_CHUNK_LEN which leads to kernel panic when
> transmitting these packets, e.g. the crash on sending INIT_ACK:
> 
> [  597.804948] skbuff: skb_over_panic: text:00000000ffae06e4 len:120168
>                put:120156 head:000000007aa47635 data:00000000d991c2de
>                tail:0x1d640 end:0xfec0 dev:<NULL>
> ...
> [  597.976970] ------------[ cut here ]------------
> [  598.033408] kernel BUG at net/core/skbuff.c:104!
> [  600.314841] Call Trace:
> [  600.345829]  <IRQ>
> [  600.371639]  ? sctp_packet_transmit+0x2095/0x26d0 [sctp]
> [  600.436934]  skb_put+0x16c/0x200
> [  600.477295]  sctp_packet_transmit+0x2095/0x26d0 [sctp]
> [  600.540630]  ? sctp_packet_config+0x890/0x890 [sctp]
> [  600.601781]  ? __sctp_packet_append_chunk+0x3b4/0xd00 [sctp]
> [  600.671356]  ? sctp_cmp_addr_exact+0x3f/0x90 [sctp]
> [  600.731482]  sctp_outq_flush+0x663/0x30d0 [sctp]
> [  600.788565]  ? sctp_make_init+0xbf0/0xbf0 [sctp]
> [  600.845555]  ? sctp_check_transmitted+0x18f0/0x18f0 [sctp]
> [  600.912945]  ? sctp_outq_tail+0x631/0x9d0 [sctp]
> [  600.969936]  sctp_cmd_interpreter.isra.22+0x3be1/0x5cb0 [sctp]
> [  601.041593]  ? sctp_sf_do_5_1B_init+0x85f/0xc30 [sctp]
> [  601.104837]  ? sctp_generate_t1_cookie_event+0x20/0x20 [sctp]
> [  601.175436]  ? sctp_eat_data+0x1710/0x1710 [sctp]
> [  601.233575]  sctp_do_sm+0x182/0x560 [sctp]
> [  601.284328]  ? sctp_has_association+0x70/0x70 [sctp]
> [  601.345586]  ? sctp_rcv+0xef4/0x32f0 [sctp]
> [  601.397478]  ? sctp6_rcv+0xa/0x20 [sctp]
> ...
> 
> Here the chunk size for INIT_ACK packet becomes too big, mostly
> because of the state cookie (INIT packet has large size with
> many address parameters), plus additional server parameters.
> 
> Later this chunk causes the panic in skb_put_data():
> 
>   skb_packet_transmit()
>       sctp_packet_pack()
>           skb_put_data(nskb, chunk->skb->data, chunk->skb->len);
> 
> 'nskb' (head skb) was previously allocated with packet->size
> from u16 'chunk->chunk_hdr->length'.
> 
> As suggested by Marcelo we should check the chunk's length in
> _sctp_make_chunk() before trying to allocate skb for it and
> discard a chunk if its size bigger than SCTP_MAX_CHUNK_LEN.
> 
> Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>

Acked-by: Marcelo Ricardo Leitner <marcelo.leinter@gmail.com>

Thanks Alexey.

> ---
> 
> v2: account for padding before checking chunklen
> 
>  net/sctp/sm_make_chunk.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
> index 793b05e..d01475f 100644
> --- a/net/sctp/sm_make_chunk.c
> +++ b/net/sctp/sm_make_chunk.c
> @@ -1380,9 +1380,14 @@ static struct sctp_chunk *_sctp_make_chunk(const struct sctp_association *asoc,
>  	struct sctp_chunk *retval;
>  	struct sk_buff *skb;
>  	struct sock *sk;
> +	int chunklen;
> +
> +	chunklen = SCTP_PAD4(sizeof(*chunk_hdr) + paylen);
> +	if (chunklen > SCTP_MAX_CHUNK_LEN)
> +		goto nodata;
>  
>  	/* No need to allocate LL here, as this is only a chunk. */
> -	skb = alloc_skb(SCTP_PAD4(sizeof(*chunk_hdr) + paylen), gfp);
> +	skb = alloc_skb(chunklen, gfp);
>  	if (!skb)
>  		goto nodata;
>  
> -- 
> 1.8.3.1
>