Re: [PATCH net] ipv6: don't let tb6_root node share routes with other node

[Ido Schimmel <idosch@idosch.org>]

Hi Wei, Martin,

On Thu, Jan 18, 2018 at 03:31:29PM -0800, Wei Wang wrote:
> On Thu, Jan 18, 2018 at 2:47 PM, Martin KaFai Lau <kafai@fb.com> wrote:
> > On Thu, Jan 18, 2018 at 10:40:03AM -0800, Wei Wang wrote:
> >> From: Wei Wang <weiwan@google.com>
> >>
> >> After commit 4512c43eac7e, if we add a route to the subtree of tb6_root
> >> which does not have any route attached to it yet, the current code will
> >> let tb6_root and the node in the subtree share the same route.
> >> This could cause problem cause tb6_root has RTN_INFO flag marked and the
> > You meant the RTN_RTINFO check in fib6_purge_rt()?
> >
> Yes. Exactly.

The check in fib6_purge_rt() is indeed problematic as tb6_root will not
release its reference on the deleted route. I can easily reproduce that
on my system. However, I don't understand how come we end up with a
use-after-free given tb6_root takes a reference on the route?

Thanks

> 
> >> tree repair and clean up code will not work properly.
> >> This commit makes sure tb6_root->leaf points back to null_entry instead
> >> of sharing route with other node.
> >>
> >> It fixes the following syzkaller reported issue:
> >> BUG: KASAN: use-after-free in ipv6_prefix_equal include/net/ipv6.h:540 [inline]
> >> BUG: KASAN: use-after-free in fib6_add_1+0x165f/0x1790 net/ipv6/ip6_fib.c:618
> >> Read of size 8 at addr ffff8801bc043498 by task syz-executor5/19819
> >>
> >> CPU: 1 PID: 19819 Comm: syz-executor5 Not tainted 4.15.0-rc7+ #186
> >> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> >> Call Trace:
> >>  __dump_stack lib/dump_stack.c:17 [inline]
> >>  dump_stack+0x194/0x257 lib/dump_stack.c:53
> >>  print_address_description+0x73/0x250 mm/kasan/report.c:252
> >>  kasan_report_error mm/kasan/report.c:351 [inline]
> >>  kasan_report+0x25b/0x340 mm/kasan/report.c:409
> >>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:430
> >>  ipv6_prefix_equal include/net/ipv6.h:540 [inline]
> >>  fib6_add_1+0x165f/0x1790 net/ipv6/ip6_fib.c:618
> >>  fib6_add+0x5fa/0x1540 net/ipv6/ip6_fib.c:1214
> >>  __ip6_ins_rt+0x6c/0x90 net/ipv6/route.c:1003
> >>  ip6_route_add+0x141/0x190 net/ipv6/route.c:2790
> >>  ipv6_route_ioctl+0x4db/0x6b0 net/ipv6/route.c:3299
> >>  inet6_ioctl+0xef/0x1e0 net/ipv6/af_inet6.c:520
> >>  sock_do_ioctl+0x65/0xb0 net/socket.c:958
> >>  sock_ioctl+0x2c2/0x440 net/socket.c:1055
> >>  vfs_ioctl fs/ioctl.c:46 [inline]
> >>  do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:686
> >>  SYSC_ioctl fs/ioctl.c:701 [inline]
> >>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
> >>  entry_SYSCALL_64_fastpath+0x23/0x9a
> >> RIP: 0033:0x452ac9
> >> RSP: 002b:00007fd42b321c58 EFLAGS: 00000212 ORIG_RAX: 0000000000000010
> >> RAX: ffffffffffffffda RBX: 000000000071bea0 RCX: 0000000000452ac9
> >> RDX: 0000000020fd7000 RSI: 000000000000890b RDI: 0000000000000013
> >> RBP: 000000000000049e R08: 0000000000000000 R09: 0000000000000000
> >> R10: 0000000000000000 R11: 0000000000000212 R12: 00000000006f4f70
> >> R13: 00000000ffffffff R14: 00007fd42b3226d4 R15: 0000000000000000
> >>
> >> Fixes: 4512c43eac7e ("ipv6: remove null_entry before adding default route")
> >> Signed-off-by: Wei Wang <weiwan@google.com>
> >> Acked-by: Eric Dumazet <edumazet@google.com>
> >> ---
> >>  net/ipv6/ip6_fib.c | 10 ++++++++--
> >>  1 file changed, 8 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> >> index 9dcc3924a975..217683d40f12 100644
> >> --- a/net/ipv6/ip6_fib.c
> >> +++ b/net/ipv6/ip6_fib.c
> >> @@ -1226,8 +1226,14 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt,
> >>               }
> >>
> >>               if (!rcu_access_pointer(fn->leaf)) {
> >> -                     atomic_inc(&rt->rt6i_ref);
> >> -                     rcu_assign_pointer(fn->leaf, rt);
> >> +                     if (fn->fn_flags & RTN_TL_ROOT) {
> >> +                             /* put back null_entry for root node */
> >> +                             rcu_assign_pointer(fn->leaf,
> >> +                                         info->nl_net->ipv6.ip6_null_entry);
> >> +                     } else {
> >> +                             atomic_inc(&rt->rt6i_ref);
> >> +                             rcu_assign_pointer(fn->leaf, rt);
> >> +                     }
> >>               }
> >>               fn = sn;
> >>       }
> >> --
> >> 2.16.0.rc1.238.g530d649a79-goog
> >>