Re: [PATCH] net: sctp, forbid negative length

[Neil Horman]

On Fri, Oct 21, 2016 at 02:13:24PM +0200, Jiri Slaby wrote:
> Most of getsockopt handlers in net/sctp/socket.c check len against
> sizeof some structure like:
>         if (len < sizeof(int))
>                 return -EINVAL;
> 
> On the first look, the check seems to be correct. But since len is int
> and sizeof returns size_t, int gets promoted to unsigned size_t too. So
> the test returns false for negative lengths. Yes, (-1 < sizeof(long)) is
> false.
> 
> Fix this in sctp by explicitly checking len < 0 before any getsockopt
> handler is called.
> 
> Note that sctp_getsockopt_events already handled the negative case.
> Since we added the < 0 check elsewhere, this one can be removed.
> 
> If not checked, this is the result:
> UBSAN: Undefined behaviour in ../mm/page_alloc.c:2722:19
> shift exponent 52 is too large for 32-bit type 'int'
> CPU: 1 PID: 24535 Comm: syz-executor Not tainted 4.8.1-0-syzkaller #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014
>  0000000000000000 ffff88006d99f2a8 ffffffffb2f7bdea 0000000041b58ab3
>  ffffffffb4363c14 ffffffffb2f7bcde ffff88006d99f2d0 ffff88006d99f270
>  0000000000000000 0000000000000000 0000000000000034 ffffffffb5096422
> Call Trace:
>  [<ffffffffb3051498>] ? __ubsan_handle_shift_out_of_bounds+0x29c/0x300
> ...
>  [<ffffffffb273f0e4>] ? kmalloc_order+0x24/0x90
>  [<ffffffffb27416a4>] ? kmalloc_order_trace+0x24/0x220
>  [<ffffffffb2819a30>] ? __kmalloc+0x330/0x540
>  [<ffffffffc18c25f4>] ? sctp_getsockopt_local_addrs+0x174/0xca0 [sctp]
>  [<ffffffffc18d2bcd>] ? sctp_getsockopt+0x10d/0x1b0 [sctp]
>  [<ffffffffb37c1219>] ? sock_common_getsockopt+0xb9/0x150
>  [<ffffffffb37be2f5>] ? SyS_getsockopt+0x1a5/0x270
> 
> Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
> Cc: Vlad Yasevich <vyasevich@xxxxxxxxx>
> Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: linux-sctp@xxxxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx
> ---
>  net/sctp/socket.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index fb02c7033307..9fbb6feb8c27 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -4687,7 +4687,7 @@ static int sctp_getsockopt_disable_fragments(struct sock *sk, int len,
>  static int sctp_getsockopt_events(struct sock *sk, int len, char __user *optval,
>  				  int __user *optlen)
>  {
> -	if (len <= 0)
> +	if (len == 0)
>  		return -EINVAL;
>  	if (len > sizeof(struct sctp_event_subscribe))
>  		len = sizeof(struct sctp_event_subscribe);
> @@ -6430,6 +6430,9 @@ static int sctp_getsockopt(struct sock *sk, int level, int optname,
>  	if (get_user(len, optlen))
>  		return -EFAULT;
>  
> +	if (len < 0)
> +		return -EINVAL;
> +
>  	lock_sock(sk);
>  
>  	switch (optname) {
> -- 
> 2.10.1
> 
> 
Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx>