Re: [PATCH] openvswitch: fix flow actions reallocation

[Andrea Righi <andrea.righi@canonical.com>]

On Wed, Mar 27, 2019 at 08:43:54PM -0700, Pravin Shelar wrote:
> On Wed, Mar 27, 2019 at 3:11 PM Andrea Righi <andrea.righi@canonical.com> wrote:
> >
> > The flow action buffer can be resized if it's not big enough to contain
> > all the requested flow actions. However, this resize doesn't take into
> > account the new requested size, the buffer is only increased by a factor
> > of 2x. This might be not enough to contain the new data, causing a
> > buffer overflow, for example:
> >
> > [   42.044472] =============================================================================
> > [   42.045608] BUG kmalloc-96 (Not tainted): Redzone overwritten
> > [   42.046415] -----------------------------------------------------------------------------
> >
> > [   42.047715] Disabling lock debugging due to kernel taint
> > [   42.047716] INFO: 0x8bf2c4a5-0x720c0928. First byte 0x0 instead of 0xcc
> > [   42.048677] INFO: Slab 0xbc6d2040 objects=29 used=18 fp=0xdc07dec4 flags=0x2808101
> > [   42.049743] INFO: Object 0xd53a3464 @offset=2528 fp=0xccdcdebb
> >
> > [   42.050747] Redzone 76f1b237: cc cc cc cc cc cc cc cc                          ........
> > [   42.051839] Object d53a3464: 6b 6b 6b 6b 6b 6b 6b 6b 0c 00 00 00 6c 00 00 00  kkkkkkkk....l...
> > [   42.053015] Object f49a30cc: 6c 00 0c 00 00 00 00 00 00 00 00 03 78 a3 15 f6  l...........x...
> > [   42.054203] Object acfe4220: 20 00 02 00 ff ff ff ff 00 00 00 00 00 00 00 00   ...............
> > [   42.055370] Object 21024e91: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> > [   42.056541] Object 070e04c3: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> > [   42.057797] Object 948a777a: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> > [   42.059061] Redzone 8bf2c4a5: 00 00 00 00                                      ....
> > [   42.060189] Padding a681b46e: 5a 5a 5a 5a 5a 5a 5a 5a                          ZZZZZZZZ
> >
> > Fix by making sure the new buffer is properly resized to contain all the
> > requested data.
> >
> > BugLink: https://bugs.launchpad.net/bugs/1813244
> > Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> 
> This must be rare combination of action that trigger this case.

It is pretty rare indeed, but the test case reported in the BugLink can
trigger it 100% of the times.

> 
> > ---
> >  net/openvswitch/flow_netlink.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/openvswitch/flow_netlink.c b/net/openvswitch/flow_netlink.c
> > index 691da853bef5..e6f789badaa3 100644
> > --- a/net/openvswitch/flow_netlink.c
> > +++ b/net/openvswitch/flow_netlink.c
> > @@ -2306,14 +2306,14 @@ static struct nlattr *reserve_sfa_size(struct sw_flow_actions **sfa,
> >
> >         struct sw_flow_actions *acts;
> >         int new_acts_size;
> > -       int req_size = NLA_ALIGN(attr_len);
> > +       size_t req_size = NLA_ALIGN(attr_len);
> >         int next_offset = offsetof(struct sw_flow_actions, actions) +
> >                                         (*sfa)->actions_len;
> >
> >         if (req_size <= (ksize(*sfa) - next_offset))
> >                 goto out;
> >
> > -       new_acts_size = ksize(*sfa) * 2;
> > +       new_acts_size = max(req_size, ksize(*sfa) * 2);
> >
> Don't we need to compare current_action_size+req_size and
> current_action_size*2 here ?

You are right! We should compare next_offset+req_size and
ksize(*sfa)*2.

Will send a new patch soon, thanks!

-Andrea

> 
> >         if (new_acts_size > MAX_ACTIONS_BUFSIZE) {
> >                 if ((MAX_ACTIONS_BUFSIZE - next_offset) < req_size) {
> > --
> > 2.19.1
> >