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Subject: Re: [PATCH v2] net: fix uninit value error in __sys_sendmmsg
Cc:     linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+09a5d591c1f98cf5efcb@syzkaller.appspotmail.com,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20200913110313.4239-1-anant.thazhemadam@gmail.com>
From:   Anant Thazhemadam <anant.thazhemadam@gmail.com>
Message-ID: <6adeef07-7dcb-3d05-df26-204a9d43301c@gmail.com>
Date:   Mon, 14 Sep 2020 13:44:37 +0530
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On 13/09/20 4:33 pm, Anant Thazhemadam wrote:
> The crash report indicated that there was a local variable;
> ----iovstack.i@__sys_sendmmsg created at:
>  ___sys_sendmsg net/socket.c:2388 [inline]
>  __sys_sendmmsg+0x6db/0xc90 net/socket.c:2480
>
>  that was left uninitialized.
>
> Initializing this stack to 0s prevents this bug from happening.
> Since the memory pointed to by *iov is freed at the end of the function
> call, memory leaks are not likely to be an issue.
>
> syzbot seems to have triggered this error by passing an array of 0's as
> a parameter while making the system call.
>
> Reported-by: syzbot+09a5d591c1f98cf5efcb@syzkaller.appspotmail.com
> Tested-by: syzbot+09a5d591c1f98cf5efcb@syzkaller.appspotmail.com
> Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
> ---
> Changes from v1:
> 	* Fixed the build warning that v1 had introduced
>  net/socket.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/net/socket.c b/net/socket.c
> index 0c0144604f81..1e6f9b54982c 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -2398,6 +2398,7 @@ static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg,
>  	struct iovec iovstack[UIO_FASTIOV], *iov = iovstack;
>  	ssize_t err;
>  
> +	memset(iov, 0, UIO_FASTIOV);
>  	msg_sys->msg_name = &address;
>  
>  	err = sendmsg_copy_msghdr(msg_sys, msg, flags, &iov);
It has since been determined that this patch is incorrect, and even if it were correct,
provides a huge performance overhead, that is not welcome. Kindly ignore this patch.
Sorry.

Thanks,
Anant