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Subject: Re: [PATCH] net: recvmsg: Unconditionally zero struct
 sockaddr_storage
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Kees Cook <keescook@chromium.org>
Cc: "David S. Miller" <davem@davemloft.net>,
        Alexander Potapenko <glider@google.com>,
        Kostya Serebryany <kcc@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Eric Dumazet <edumazet@google.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, security@kernel.org
Date: Tue, 31 Oct 2017 10:31:34 -0700
In-Reply-To: <20171031161445.GA140874@beast>
References: <20171031161445.GA140874@beast>
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On Tue, 2017-10-31 at 09:14 -0700, Kees Cook wrote:
> Some protocols do not correctly wipe the contents of the on-stack
> struct sockaddr_storage sent down into recvmsg() (e.g. SCTP), and leak
> kernel stack contents to userspace. This wipes it unconditionally before
> per-protocol handlers run.
> 
> Note that leaks like this are mitigated by building with
> CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL=y
> 
> Reported-by: Alexander Potapenko <glider@google.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: netdev@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  net/socket.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/socket.c b/net/socket.c
> index c729625eb5d3..34183f4fbdf8 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -2188,6 +2188,7 @@ static int ___sys_recvmsg(struct socket *sock, struct user_msghdr __user *msg,
>  	struct sockaddr __user *uaddr;
>  	int __user *uaddr_len = COMPAT_NAMELEN(msg);
>  
> +	memset(&addr, 0, sizeof(addr));
>  	msg_sys->msg_name = &addr;
>  

This kind of patch comes every year.

Standard answer is : We fix the buggy protocol, we do not make
everything slower just because we are lazy.

struct sockaddr is 128 bytes, but IPV4 only uses a fraction of it.

Also memset() is using long word stores, so next 4-byte or 2-byte stores
on same location hit a performance problem on x86.

By adding all these defensive programming, we would give strong
incentives to bypass the kernel for networking. That would be bad.