netdev - Re: [PATCH net] net/sctp: Always set scope_id in sctp_inet6_skb

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Message-Id: <20171116.230026.45581437273132607.davem@davemloft.net>
Date:   Thu, 16 Nov 2017 23:00:26 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     ebiederm@...ssion.com
Cc:     keescook@...omium.org, eric.dumazet@...il.com, kcc@...gle.com,
        andreyknvl@...gle.com, edumazet@...gle.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, security@...nel.org,
        glider@...gle.com, linux-sctp@...r.kernel.org,
        nhorman@...driver.com, vyasevich@...il.com
Subject: Re: [PATCH net] net/sctp: Always set scope_id in
 sctp_inet6_skb_msgname

From: ebiederm@...ssion.com (Eric W. Biederman)
Date: Wed, 15 Nov 2017 22:17:48 -0600

> 
> Alexandar Potapenko while testing the kernel with KMSAN and syzkaller
> discovered that in some configurations sctp would leak 4 bytes of
> kernel stack.
> 
> Working with his reproducer I discovered that those 4 bytes that
> are leaked is the scope id of an ipv6 address returned by recvmsg.
> 
> With a little code inspection and a shrewd guess I discovered that
> sctp_inet6_skb_msgname only initializes the scope_id field for link
> local ipv6 addresses to the interface index the link local address
> pertains to instead of initializing the scope_id field for all ipv6
> addresses.
> 
> That is almost reasonable as scope_id's are meaniningful only for link
> local addresses.  Set the scope_id in all other cases to 0 which is
> not a valid interface index to make it clear there is nothing useful
> in the scope_id field.
> 
> There should be no danger of breaking userspace as the stack leak
> guaranteed that previously meaningless random data was being returned.
> 
> Fixes: 372f525b495c ("SCTP:  Resync with LKSCTP tree.")
> History-tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
> Reported-by: Alexander Potapenko <glider@...gle.com>
> Tested-by: Alexander Potapenko <glider@...gle.com>
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Applied and queued up for -stable, thanks Eric.