netdev - Re: [PATCH v3] net: don't return uninitialized addresses on concurrent socket shutdown

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Date:	Sat, 16 Nov 2013 12:46:42 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	mpb <mpb.mail@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v3] net: don't return uninitialized addresses on
 concurrent socket shutdown

On Sat, 2013-11-16 at 20:19 +0100, Hannes Frederic Sowa wrote:
> If a blocking read waits on a socket which gets concurrently shut down we
> return 0 as error and so indicate success to the socket functions which
> thus copies an uninitialized stack allocated address back to the user.
> 
> Fix this by introducing a new AF_INVALID sa_family marker and check if the
> recvmsg function overwrote it. In case it was not overwritten, clear the
> address with zeros (AF_UNSPEC) before returning it to the user. IMHO we
> should only increase msg.msg_namelen (if we have to truncate the address),
> so don't clear msg.msg_namelen.
> 
> This patch fixes the problem for recvfrom, recvmsg and recvmmsg.
> 
> Reported-by: mpb <mpb.mail@...il.com>
> Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
> ---
>  include/linux/socket.h | 2 ++
>  net/socket.c           | 6 ++++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index 445ef75..bdf9205 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -182,6 +182,8 @@ struct ucred {
>  #define AF_VSOCK	40	/* vSockets			*/
>  #define AF_MAX		41	/* For now.. */
>  
> +#define AF_INVALID	((__kernel_sa_family_t)(~0U))
> +
>  /* Protocol families, same as address families. */
>  #define PF_UNSPEC	AF_UNSPEC
>  #define PF_UNIX		AF_UNIX
> diff --git a/net/socket.c b/net/socket.c
> index c226ace..8361e15 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -1834,6 +1834,7 @@ SYSCALL_DEFINE6(recvfrom, int, fd, void __user *, ubuf, size_t, size,
>  	if (!sock)
>  		goto out;
>  
> +	address.ss_family = AF_INVALID;


	address.ss_family = AF_UNSPEC;

>  	msg.msg_control = NULL;
>  	msg.msg_controllen = 0;
>  	msg.msg_iovlen = 1;
> @@ -1847,6 +1848,8 @@ SYSCALL_DEFINE6(recvfrom, int, fd, void __user *, ubuf, size_t, size,
>  	err = sock_recvmsg(sock, &msg, size, flags);
>  
>  	if (err >= 0 && addr != NULL) {
> +		if (unlikely(address.ss_family == AF_INVALID))
> +			memset(&address, 0, sizeof(address));

Why clearing 128 bytes, and return msg.msg_namelen null bytes to the
user ?

What useful information will the user get from this ? Is this even
documented ?


>  		err2 = move_addr_to_user(&address,
>  					 msg.msg_namelen, addr, addr_len);
>  		if (err2 < 0)

I really don't think userland should expect to read 128 null bytes if it
asked 128 bytes.

We can certainly return 2 null bytes (AF_UNSPEC) and comply with the
documentation.

if (unlikely(address.ss_family == AF_UNSPEC))
	msg.msg_namelen = sizeof(address.ss_family);



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