Message-ID: <CADvbK_dCuuaBSkOeO=utQZcmf0H4WKSi5ZrUG+UggbxbjWAm2g@mail.gmail.com>
Date: Thu, 11 Dec 2025 09:21:02 -0500
From: Xin Long <lucien.xin@...il.com>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>, "David S . Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org, 
	syzbot+ec33a1a006ed5abe7309@...kaller.appspotmail.com
Subject: Re: [PATCH v2 net 2/2] sctp: Clear inet_opt in sctp_v6_copy_ip_options().

On Wed, Dec 10, 2025 at 5:45 PM Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
>
> On Wed, Dec 10, 2025 at 11:32 AM Xin Long <lucien.xin@...il.com> wrote:
> >
> > On Wed, Dec 10, 2025 at 3:12 AM Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
> > >
> > > syzbot reported the splat below. [0]
> > >
> > > Since the cited commit, the child socket inherits all fields
> > > of its parent socket unless explicitly cleared.
> > >
> > > syzbot set IP_OPTIONS to AF_INET6 socket and created a child
> > > socket inheriting inet_sk(sk)->inet_opt.
> > >
> > > sctp_v6_copy_ip_options() only clones np->opt, and leaving
> > > inet_opt results in double-free.
> > >
> > > Let's clear inet_opt in sctp_v6_copy_ip_options().
> > >
> > > [0]:
> > > BUG: KASAN: double-free in inet_sock_destruct+0x538/0x740 net/ipv4/af_inet.c:159
> > > Free of addr ffff8880304b6d40 by task ksoftirqd/0/15
> > >
> > > CPU: 0 UID: 0 PID: 15 Comm: ksoftirqd/0 Not tainted syzkaller #0 PREEMPT(full)
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
> > > Call Trace:
> > >  <TASK>
> > >  dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
> > >  print_address_description mm/kasan/report.c:378 [inline]
> > >  print_report+0xca/0x240 mm/kasan/report.c:482
> > >  kasan_report_invalid_free+0xea/0x110 mm/kasan/report.c:557
> > >  check_slab_allocation+0xe1/0x130 include/linux/page-flags.h:-1
> > >  kasan_slab_pre_free include/linux/kasan.h:198 [inline]
> > >  slab_free_hook mm/slub.c:2484 [inline]
> > >  slab_free mm/slub.c:6630 [inline]
> > >  kfree+0x148/0x6d0 mm/slub.c:6837
> > >  inet_sock_destruct+0x538/0x740 net/ipv4/af_inet.c:159
> > >  __sk_destruct+0x89/0x660 net/core/sock.c:2350
> > >  sock_put include/net/sock.h:1991 [inline]
> > >  sctp_endpoint_destroy_rcu+0xa1/0xf0 net/sctp/endpointola.c:197
> > >  rcu_do_batch kernel/rcu/tree.c:2605 [inline]
> > >  rcu_core+0xcab/0x1770 kernel/rcu/tree.c:2861
> > >  handle_softirqs+0x286/0x870 kernel/softirq.c:622
> > >  run_ksoftirqd+0x9b/0x100 kernel/softirq.c:1063
> > >  smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
> > >  kthread+0x711/0x8a0 kernel/kthread.c:463
> > >  ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
> > >  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> > >  </TASK>
> > >
> > > Allocated by task 6003:
> > >  kasan_save_stack mm/kasan/common.c:56 [inline]
> > >  kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
> > >  poison_kmalloc_redzone mm/kasan/common.c:400 [inline]
> > >  __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:417
> > >  kasan_kmalloc include/linux/kasan.h:262 [inline]
> > >  __do_kmalloc_node mm/slub.c:5642 [inline]
> > >  __kmalloc_noprof+0x411/0x7f0 mm/slub.c:5654
> > >  kmalloc_noprof include/linux/slab.h:961 [inline]
> > >  kzalloc_noprof include/linux/slab.h:1094 [inline]
> > >  ip_options_get+0x51/0x4c0 net/ipv4/ip_options.c:517
> > >  do_ip_setsockopt+0x1d9b/0x2d00 net/ipv4/ip_sockglue.c:1087
> > >  ip_setsockopt+0x66/0x110 net/ipv4/ip_sockglue.c:1417
> > >  do_sock_setsockopt+0x17c/0x1b0 net/socket.c:2360
> > >  __sys_setsockopt net/socket.c:2385 [inline]
> > >  __do_sys_setsockopt net/socket.c:2391 [inline]
> > >  __se_sys_setsockopt net/socket.c:2388 [inline]
> > >  __x64_sys_setsockopt+0x13f/0x1b0 net/socket.c:2388
> > >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> > >  do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
> > >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > >
> > > Freed by task 15:
> > >  kasan_save_stack mm/kasan/common.c:56 [inline]
> > >  kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
> > >  __kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:587
> > >  kasan_save_free_info mm/kasan/kasan.h:406 [inline]
> > >  poison_slab_object mm/kasan/common.c:252 [inline]
> > >  __kasan_slab_free+0x5c/0x80 mm/kasan/common.c:284
> > >  kasan_slab_free include/linux/kasan.h:234 [inline]
> > >  slab_free_hook mm/slub.c:2539 [inline]
> > >  slab_free mm/slub.c:6630 [inline]
> > >  kfree+0x19a/0x6d0 mm/slub.c:6837
> > >  inet_sock_destruct+0x538/0x740 net/ipv4/af_inet.c:159
> > >  __sk_destruct+0x89/0x660 net/core/sock.c:2350
> > >  sock_put include/net/sock.h:1991 [inline]
> > >  sctp_endpoint_destroy_rcu+0xa1/0xf0 net/sctp/endpointola.c:197
> > >  rcu_do_batch kernel/rcu/tree.c:2605 [inline]
> > >  rcu_core+0xcab/0x1770 kernel/rcu/tree.c:2861
> > >  handle_softirqs+0x286/0x870 kernel/softirq.c:622
> > >  run_ksoftirqd+0x9b/0x100 kernel/softirq.c:1063
> > >  smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
> > >  kthread+0x711/0x8a0 kernel/kthread.c:463
> > >  ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
> > >  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> > >
> > > Fixes: 16942cf4d3e31 ("sctp: Use sk_clone() in sctp_accept().")
> > > Reported-by: syzbot+ec33a1a006ed5abe7309@...kaller.appspotmail.com
> > > Closes: https://lore.kernel.org/netdev/6936d112.a70a0220.38f243.00a8.GAE@google.com/
> > > Signed-off-by: Kuniyuki Iwashima <kuniyu@...gle.com>
> > > ---
> > >  net/sctp/ipv6.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
> > > index 069b7e45d8bda..531cb0690007a 100644
> > > --- a/net/sctp/ipv6.c
> > > +++ b/net/sctp/ipv6.c
> > > @@ -492,6 +492,8 @@ static void sctp_v6_copy_ip_options(struct sock *sk, struct sock *newsk)
> > >         struct ipv6_pinfo *newnp, *np = inet6_sk(sk);
> > >         struct ipv6_txoptions *opt;
> > >
> > > +       inet_sk(newsk)->inet_opt = NULL;
> > > +
> > newinet->pinet6 = inet6_sk_generic(newsk);
> > newinet->ipv6_fl_list = NULL;
> > newinet->inet_opt = NULL;
> >
> > newnp->ipv6_mc_list = NULL;
> > newnp->ipv6_ac_list = NULL;
> >
> > I noticed these fields are reset after sk_clone() for both SCTP and TCP.
> > I believe the same applies to MPTCP.
> >
> > If that's the case, is it possible to move their initialization up into
> > sk_clone()? Doing so would address both issues in this patchset.
>
> Yes, but moving unrelated ipv6_mc_list etc would be net-next
> material, so I can follow up after net-next reopens next year.
>
>
> >
> > Also, memcpy(newnp, inet6_sk(sk), sizeof(struct ipv6_pinfo)) might be
> > redundant, since sock_copy() already copies this with prot->obj_size.
>
> Exactly, I should've removed it when switching to sk_clone().
> I'll follow up on this too in net-next.
Fair enough. It would be nice to also set newinet->inet_opt = NULL for IPv4,
and newnp->opt = NULL for IPv6 sockets.  That way, any sk_clone() users that
forget to re-initialize these fields won’t end up crashing the kernel.

Thanks.

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