Re: [PATCH net v2] rds: tcp: hold the RCU lock across ipv6_chk_addr() in rds_tcp_laddr_check()

[Xiang Mei]

On Fri, Jul 10, 2026 at 8:53 PM Allison Henderson <achender@xxxxxxxxxx> wrote:
>
> On Fri, 2026-07-10 at 15:30 -0700, Xiang Mei wrote:
> > rds_tcp_laddr_check() looks up a scoped IPv6 interface with
> > dev_get_by_index_rcu(), drops the RCU read-side lock, and only then
> > passes the bare struct net_device * into ipv6_chk_addr().
> >
> > dev_get_by_index_rcu() only keeps the device alive within the same RCU
> > read-side section. After rcu_read_unlock(), a concurrent RTM_DELLINK can
> > free the net_device; ipv6_chk_addr() then dereferences the stale pointer
> > in __ipv6_chk_addr_and_flags() (e.g. l3mdev_master_dev_rcu(dev)), reading
> > freed memory.
> >
> > Keep the RCU read-side lock held across the ipv6_chk_addr() call instead
> > of dropping it right after the lookup, so the device cannot be freed
> > while it is in use.
> >
> >   BUG: KASAN: slab-use-after-free in __ipv6_chk_addr_and_flags (... net/ipv6/addrconf.c:1998)
> >   Read of size 8 at addr ffff8880106ec000 by task exploit/153
> >   Call Trace:
> >    ...
> >    kasan_report (mm/kasan/report.c:595)
> >    __ipv6_chk_addr_and_flags (... net/ipv6/addrconf.c:1998)
> >    ipv6_chk_addr (net/ipv6/addrconf.c:2031 net/ipv6/addrconf.c:1972)
> >    rds_tcp_laddr_check (net/rds/tcp.c:370)
> >    rds_bind (net/rds/bind.c:248)
> >    __sys_bind (net/socket.c:1920)
> >    __x64_sys_bind (net/socket.c:1956)
> >    do_syscall_64 (arch/x86/entry/syscall_64.c:63)
> >    entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
> >
>
>
>
> Thanks Xiang!  Just more thing to make sure your patches gets through. Add a change log here like this:
>
> Changes since v1:
>   Use rcu_read_locks instead of dev_hold/put
>   Rebased on [PATCH net v2] rds: Fix inet6_addr_lst NULL dereference when IPv6 is disabled
>
> Changes since v2:
>   Add change log
>
>
> Otherwise, it might get bounced if they happen to try your patch first.
> With that fixed:
> Reviewed-by: Allison Henderson <achender@xxxxxxxxxx>
>
> Thanks!
> Allison
>
Thanks for the tips; I learned it!

v3 is ready:
https://lore.kernel.org/netdev/20260714212239.1511269-1-xmei5@xxxxxxx/T/#u

Xiang
> > Thanks Xiang!  Just more thing to make sure your patches gets through. Add a change log here like this:
> >
> > Changes since v1:
> >   Use rcu_read_locks instead of dev_hold/put
> >   Rebased on [PATCH net v2] rds: Fix inet6_addr_lst NULL dereference when IPv6 is disabled
> >
> > Changes since v2:
> >   Add change log
> >
> > Reviewed-by: Allison Henderson <achender@xxxxxxxxxx>
> > Fixes: eee2fa6ab322 ("rds: Changing IP address internal representation to struct in6_addr")
> > Reported-by: Weiming Shi <bestswngs@xxxxxxxxx>
> > Assisted-by: Claude:claude-opus-4-8
> > Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
> > ---
> >  net/rds/tcp.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/net/rds/tcp.c b/net/rds/tcp.c
> > index 955d92277d5a..30cfb0087f2c 100644
> > --- a/net/rds/tcp.c
> > +++ b/net/rds/tcp.c
> > @@ -355,23 +355,25 @@ int rds_tcp_laddr_check(struct net *net, const struct in6_addr *addr,
> >       /* If the scope_id is specified, check only those addresses
> >        * hosted on the specified interface.
> >        */
> > +     rcu_read_lock();
> >       if (scope_id != 0) {
> > -             rcu_read_lock();
> >               dev = dev_get_by_index_rcu(net, scope_id);
> >               /* scope_id is not valid... */
> >               if (!dev) {
> >                       rcu_read_unlock();
> >                       return -EADDRNOTAVAIL;
> >               }
> > -             rcu_read_unlock();
> >       }
> >  #if IS_ENABLED(CONFIG_IPV6)
> >       if (ipv6_mod_enabled()) {
> >               ret = ipv6_chk_addr(net, addr, dev, 0);
> > -             if (ret)
> > +             if (ret) {
> > +                     rcu_read_unlock();
> >                       return 0;
> > +             }
> >       }
> >  #endif
> > +     rcu_read_unlock();
> >       return -EADDRNOTAVAIL;
> >  }
> >
>