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Subject: Re: [Linux-kernel-mentees] [PATCH] net: fix uninit value error in
 __sys_sendmmsg
To:     Greg KH <greg@kroah.com>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jakub Kicinski <kuba@kernel.org>,
        syzbot+09a5d591c1f98cf5efcb@syzkaller.appspotmail.com,
        "David S. Miller" <davem@davemloft.net>,
        linux-kernel-mentees@lists.linuxfoundation.org
References: <20200913055639.15639-1-anant.thazhemadam@gmail.com>
 <20200913061351.GA585618@kroah.com>
From:   Anant Thazhemadam <anant.thazhemadam@gmail.com>
Message-ID: <89526337-9657-8f4d-3022-9f2ad830fbe9@gmail.com>
Date:   Sun, 13 Sep 2020 11:50:52 +0530
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On 13/09/20 11:43 am, Greg KH wrote:
> 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@__sys_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@syzkaller.appspotmail.com
>> Tested-by: syzbot+09a5d591c1f98cf5efcb@syzkaller.appspotmail.com
>> Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.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?
>
> :(
My apologies. I think I ended up overlooking the build warning. Thank you for pointing that out.
If everything else looks good, I'd be happy to send in a v2 that fixes this build warning.

Thanks,
Anant