[Linux-kernel-mentees] [PATCH net] AX.25: Fix out-of-bounds read in ax25_connect()

[Peilin Ye]

On Thu, Jul 23, 2020 at 06:50:58PM +0300, 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 at syzkaller.appspotmail.com
> > > > Link: https://syzkaller.appspot.com/bug?id=55ef9d629f3b3d7d70b69558015b63b48d01af66
> > > > Signed-off-by: Peilin Ye <yepeilin.cs at gmail.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.

It is very encouraging to hear that! I will try to do what I can do.

Thank you,
Peilin Ye