linux-kernel - Re: [Linux-kernel-mentees] [PATCH net] AX.25: Fix out-of-bounds read in ax25

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Date:   Fri, 24 Jul 2020 07:41:20 +1000
From:   vk2tv <vk2tv@...mail.com.au>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        Peilin Ye <yepeilin.cs@...il.com>
Cc:     Joerg Reuter <jreuter@...na.de>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        syzkaller-bugs@...glegroups.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-hams@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Linux-kernel-mentees] [PATCH net] AX.25: Fix out-of-bounds read
 in ax25_connect()



On 24/7/20 1:50 am, Dan Carpenter wrote:
> On Thu, Jul 23, 2020 at 11:13:55AM -0400, Peilin Ye wrote:
>> On Thu, Jul 23, 2020 at 05:28:15PM +0300, Dan Carpenter wrote:
>>> On Wed, Jul 22, 2020 at 11:19:01AM -0400, Peilin Ye wrote:
>>>> Checks on `addr_len` and `fsa->fsa_ax25.sax25_ndigis` are insufficient.
>>>> ax25_connect() can go out of bounds when `fsa->fsa_ax25.sax25_ndigis`
>>>> equals to 7 or 8. Fix it.
>>>>
>>>> This issue has been reported as a KMSAN uninit-value bug, because in such
>>>> a case, ax25_connect() reaches into the uninitialized portion of the
>>>> `struct sockaddr_storage` statically allocated in __sys_connect().
>>>>
>>>> It is safe to remove `fsa->fsa_ax25.sax25_ndigis > AX25_MAX_DIGIS` because
>>>> `addr_len` is guaranteed to be less than or equal to
>>>> `sizeof(struct full_sockaddr_ax25)`.
>>>>
>>>> Reported-by: syzbot+c82752228ed975b0a623@...kaller.appspotmail.com
>>>> Link: https://syzkaller.appspot.com/bug?id=55ef9d629f3b3d7d70b69558015b63b48d01af66
>>>> Signed-off-by: Peilin Ye <yepeilin.cs@...il.com>
>>>> ---
>>>>   net/ax25/af_ax25.c | 4 +++-
>>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
>>>> index fd91cd34f25e..ef5bf116157a 100644
>>>> --- a/net/ax25/af_ax25.c
>>>> +++ b/net/ax25/af_ax25.c
>>>> @@ -1187,7 +1187,9 @@ static int __must_check ax25_connect(struct socket *sock,
>>>>   	if (addr_len > sizeof(struct sockaddr_ax25) &&
>>>>   	    fsa->fsa_ax25.sax25_ndigis != 0) {
>>>>   		/* Valid number of digipeaters ? */
>>>> -		if (fsa->fsa_ax25.sax25_ndigis < 1 || fsa->fsa_ax25.sax25_ndigis > AX25_MAX_DIGIS) {
>>>> +		if (fsa->fsa_ax25.sax25_ndigis < 1 ||
>>>> +		    addr_len < sizeof(struct sockaddr_ax25) +
>>>> +		    sizeof(ax25_address) * fsa->fsa_ax25.sax25_ndigis) {
>>> The "sizeof(ax25_address) * fsa->fsa_ax25.sax25_ndigis" can have an
>>> integer overflow so you still need the
>>> "fsa->fsa_ax25.sax25_ndigis > AX25_MAX_DIGIS" check.
>> Thank you for fixing this up! I did some math but I didn't think of
>> that. Will be more careful when removing things.
> No problem.  You had the right approach to look for ways to clean things
> up.
>
> Your patches make me happy because you're trying to fix important bugs.
>
> regards,
> dan carpenter
As a long-term user (25 years) of kernel ax25 I appreciate any and all 
efforts to improve the code (which I mostly don't understand), and I 
applaud those individuals rising to the task.

Thanks guys (and gals).

Ray vk2tv