linux-kernel - Re: [PATCH net] net: 9p: initialize sun_server.sun_path to have addr's value only when addr is valid

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Date:   Mon, 12 Oct 2020 09:59:10 +0200
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Anant Thazhemadam <anant.thazhemadam@...il.com>
Cc:     ericvh@...il.com, lucho@...kov.net, davem@...emloft.net,
        kuba@...nel.org, v9fs-developer@...ts.sourceforge.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+75d51fe5bf4ebe988518@...kaller.appspotmail.com
Subject: Re: [PATCH net] net: 9p: initialize sun_server.sun_path to have
 addr's value only when addr is valid

Anant Thazhemadam wrote on Mon, Oct 12, 2020:
> In p9_fd_create_unix, checking is performed to see if the addr (passed
> as an argument) is NULL or not.
> However, no check is performed to see if addr is a valid address, i.e.,
> it doesn't entirely consist of only 0's.
> The initialization of sun_server.sun_path to be equal to this faulty
> addr value leads to an uninitialized variable, as detected by KMSAN.
> Checking for this (faulty addr) and returning a negative error number
> appropriately, resolves this issue.

I'm not sure I agree a fully zeroed address is faulty but I agree we can
probably refuse it given userspace can't pass useful abstract addresses
here.

Just one nitpick but this is otherwise fine - good catch!

> Reported-by: syzbot+75d51fe5bf4ebe988518@...kaller.appspotmail.com
> Tested-by: syzbot+75d51fe5bf4ebe988518@...kaller.appspotmail.com
> Signed-off-by: Anant Thazhemadam <anant.thazhemadam@...il.com>
> ---
>  net/9p/trans_fd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
> index c0762a302162..8f528e783a6c 100644
> --- a/net/9p/trans_fd.c
> +++ b/net/9p/trans_fd.c
> @@ -1023,7 +1023,7 @@ p9_fd_create_unix(struct p9_client *client, const char *addr, char *args)
>  
>  	csocket = NULL;
>  
> -	if (addr == NULL)
> +	if (!addr || !strlen(addr))

Since we don't care about the actual length here, how about checking for
addr[0] directly?
That'll spare a strlen() call in the valid case.

Well, I guess it doesn't really matter -- I'll queue this up anyway and
update if you resend.


Thanks,
-- 
Dominique