netdev - Re: [PATCH] sctp: verify size of a new chunk in _sctp_make

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Date:   Fri, 9 Feb 2018 11:27:32 -0200
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Alexey Kodanev <alexey.kodanev@...cle.com>
Cc:     netdev@...r.kernel.org, Neil Horman <nhorman@...driver.com>,
        Vlad Yasevich <vyasevich@...il.com>,
        David Miller <davem@...emloft.net>, linux-sctp@...r.kernel.org
Subject: Re: [PATCH] sctp: verify size of a new chunk in _sctp_make_chunk()

On Fri, Feb 09, 2018 at 04:02:31PM +0300, Alexey Kodanev wrote:
> When SCTP makes INIT or INIT_ACK packets 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@...cle.com>
> ---
>  net/sctp/sm_make_chunk.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
> index 793b05e..95618eb 100644
> --- a/net/sctp/sm_make_chunk.c
> +++ b/net/sctp/sm_make_chunk.c
> @@ -1380,9 +1380,14 @@ void sctp_init_addrs(struct sctp_chunk *chunk, union sctp_addr *src,

Weird how it identified sctp_init_addrs as the context here o.O
Line numbers above and the rest below matches _sctp_make_chunk.

>  	struct sctp_chunk *retval;
>  	struct sk_buff *skb;
>  	struct sock *sk;
> +	int chunklen;
> +
> +	chunklen = sizeof(*chunk_hdr) + paylen;

It's better to do the padding here too, as it may grow the length by 3
bytes.

  Marcelo

> +	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(SCTP_PAD4(chunklen), gfp);
>  	if (!skb)
>  		goto nodata;
>  
> -- 
> 1.7.1
> 
> --
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