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From:   Kees Cook <keescook@chromium.org>
Date:   Thu, 18 Jan 2018 13:36:03 -0800
Message-ID: <CAGXu5jK_VyEM4-MaQ53o2AS8NNiTyR8X5LUrbwd=KcDWfKhGTg@mail.gmail.com>
Subject: Re: [PATCH 27/38] sctp: Copy struct sctp_sock.autoclose to userspace
 using put_user()
To:     Laura Abbott <labbott@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        David Windsor <dave@nullcore.net>,
        Vlad Yasevich <vyasevich@gmail.com>,
        Neil Horman <nhorman@tuxdriver.com>,
        "David S. Miller" <davem@davemloft.net>,
        linux-sctp@vger.kernel.org,
        Network Development <netdev@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Christoph Lameter <cl@linux.com>,
        Mark Rutland <mark.rutland@arm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Christoffer Dall <christoffer.dall@linaro.org>,
        Dave Kleikamp <dave.kleikamp@oracle.com>,
        Jan Kara <jack@suse.cz>,
        Luis de Bethencourt <luisbg@kernel.org>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Rik van Riel <riel@redhat.com>,
        Matthew Garrett <mjg59@google.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        kernel-hardening@lists.openwall.com
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On Thu, Jan 18, 2018 at 1:31 PM, Laura Abbott <labbott@redhat.com> wrote:
> On 01/10/2018 06:02 PM, Kees Cook wrote:
>>
>> From: David Windsor <dave@nullcore.net>
>>
>> The autoclose field can be copied with put_user(), so there is no need to
>> use copy_to_user(). In both cases, hardened usercopy is being bypassed
>> since the size is constant, and not open to runtime manipulation.
>>
>> This patch is verbatim from Brad Spengler/PaX Team's PAX_USERCOPY
>> whitelisting code in the last public patch of grsecurity/PaX based on my
>> understanding of the code. Changes or omissions from the original code are
>> mine and don't reflect the original grsecurity/PaX code.
>>
>
> Just tried a quick rebase and it looks like this conflicts with
> c76f97c99ae6 ("sctp: make use of pre-calculated len")
> I don't think we can use put_user if we're copying via the full
> len?

It should be fine, since:

        len = sizeof(int);

c76f97c99ae6 just does a swap of sizeof(int) with len, put_user() will
work in either case, since autoclose will always be int sized.

-Kees

>
> Thanks,
> Laura
>
>
>> Signed-off-by: David Windsor <dave@nullcore.net>
>> [kees: adjust commit log]
>> Cc: Vlad Yasevich <vyasevich@gmail.com>
>> Cc: Neil Horman <nhorman@tuxdriver.com>
>> Cc: "David S. Miller" <davem@davemloft.net>
>> Cc: linux-sctp@vger.kernel.org
>> Cc: netdev@vger.kernel.org
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>>   net/sctp/socket.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
>> index efbc8f52c531..15491491ec88 100644
>> --- a/net/sctp/socket.c
>> +++ b/net/sctp/socket.c
>> @@ -5011,7 +5011,7 @@ static int sctp_getsockopt_autoclose(struct sock
>> *sk, int len, char __user *optv
>>         len = sizeof(int);
>>         if (put_user(len, optlen))
>>                 return -EFAULT;
>> -       if (copy_to_user(optval, &sctp_sk(sk)->autoclose, sizeof(int)))
>> +       if (put_user(sctp_sk(sk)->autoclose, (int __user *)optval))
>>                 return -EFAULT;
>>         return 0;
>>   }
>>
>


-- 
Kees Cook
Pixel Security