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Date:   Sun, 13 Sep 2020 08:13:51 +0200
From:   Greg KH <greg@...ah.com>
To:     Anant Thazhemadam <anant.thazhemadam@...il.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jakub Kicinski <kuba@...nel.org>,
        syzbot+09a5d591c1f98cf5efcb@...kaller.appspotmail.com,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [Linux-kernel-mentees] [PATCH] net: fix uninit value error in
 __sys_sendmmsg

On Sun, Sep 13, 2020 at 11:26:39AM +0530, Anant Thazhemadam wrote:
> The crash report showed that there was a local variable;
> 
> ----iovstack.i@...ys_sendmmsg created at:
>  ___sys_sendmsg net/socket.c:2388 [inline]
>  __sys_sendmmsg+0x6db/0xc90 net/socket.c:2480
>  
>  that was left uninitialized.
> 
> The contents of iovstack are of interest, since the respective pointer
> is passed down as an argument to sendmsg_copy_msghdr as well.
> Initializing this contents of this stack prevents this bug from happening.
> 
> Since the memory that was initialized 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 initial system call.
> 
> Reported-by: syzbot+09a5d591c1f98cf5efcb@...kaller.appspotmail.com
> Tested-by: syzbot+09a5d591c1f98cf5efcb@...kaller.appspotmail.com
> Signed-off-by: Anant Thazhemadam <anant.thazhemadam@...il.com>
> ---
>  net/socket.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/socket.c b/net/socket.c
> index 0c0144604f81..d74443dfd73b 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -2396,6 +2396,7 @@ static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg,
>  {
>  	struct sockaddr_storage address;
>  	struct iovec iovstack[UIO_FASTIOV], *iov = iovstack;
> +	memset(iov, 0, UIO_FASTIOV);
>  	ssize_t err;
>  
>  	msg_sys->msg_name = &address;

I don't think you built this code change, otherwise you would have seen
that it adds a build warning to the system, right?

:(

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