[net] l2tp: use sk_dst_check() to avoid race on sk->sk_dst_cache

Message ID 20180809185405.38744-1-tracywwnj@gmail.com
State Superseded
Delegated to: David Miller
Headers show
Series
  • [net] l2tp: use sk_dst_check() to avoid race on sk->sk_dst_cache
Related show

Commit Message

Wei Wang Aug. 9, 2018, 6:54 p.m.
From: Wei Wang <weiwan@google.com>

In l2tp code, if it is a L2TP_UDP_ENCAP tunnel, tunnel->sk points to a
UDP socket. User could call sendmsg() on both this tunnel and the UDP
socket itself concurrently. As l2tp_xmit_skb() holds socket lock and call
__sk_dst_check() to refresh sk->sk_dst_cache, while udpv6_sendmsg() is
lockless and call sk_dst_check() to refresh sk->sk_dst_cache, there
could be a race and cause the dst cache to be freed multiple times.
So we fix l2tp side code to always call sk_dst_check() to garantee
xchg() is called when refreshing sk->sk_dst_cache to avoid race
conditions.

Syzkaller reported stack trace:
BUG: KASAN: use-after-free in atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
BUG: KASAN: use-after-free in atomic_fetch_add_unless include/linux/atomic.h:575 [inline]
BUG: KASAN: use-after-free in atomic_add_unless include/linux/atomic.h:597 [inline]
BUG: KASAN: use-after-free in dst_hold_safe include/net/dst.h:308 [inline]
BUG: KASAN: use-after-free in ip6_hold_safe+0xe6/0x670 net/ipv6/route.c:1029
Read of size 4 at addr ffff8801aea9a880 by task syz-executor129/4829

CPU: 0 PID: 4829 Comm: syz-executor129 Not tainted 4.18.0-rc7-next-20180802+ #30
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
 print_address_description+0x6c/0x20b mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report.cold.7+0x242/0x30d mm/kasan/report.c:412
 check_memory_region_inline mm/kasan/kasan.c:260 [inline]
 check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
 kasan_check_read+0x11/0x20 mm/kasan/kasan.c:272
 atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
 atomic_fetch_add_unless include/linux/atomic.h:575 [inline]
 atomic_add_unless include/linux/atomic.h:597 [inline]
 dst_hold_safe include/net/dst.h:308 [inline]
 ip6_hold_safe+0xe6/0x670 net/ipv6/route.c:1029
 rt6_get_pcpu_route net/ipv6/route.c:1249 [inline]
 ip6_pol_route+0x354/0xd20 net/ipv6/route.c:1922
 ip6_pol_route_output+0x54/0x70 net/ipv6/route.c:2098
 fib6_rule_lookup+0x283/0x890 net/ipv6/fib6_rules.c:122
 ip6_route_output_flags+0x2c5/0x350 net/ipv6/route.c:2126
 ip6_dst_lookup_tail+0x1278/0x1da0 net/ipv6/ip6_output.c:978
 ip6_dst_lookup_flow+0xc8/0x270 net/ipv6/ip6_output.c:1079
 ip6_sk_dst_lookup_flow+0x5ed/0xc50 net/ipv6/ip6_output.c:1117
 udpv6_sendmsg+0x2163/0x36b0 net/ipv6/udp.c:1354
 inet_sendmsg+0x1a1/0x690 net/ipv4/af_inet.c:798
 sock_sendmsg_nosec net/socket.c:622 [inline]
 sock_sendmsg+0xd5/0x120 net/socket.c:632
 ___sys_sendmsg+0x51d/0x930 net/socket.c:2115
 __sys_sendmmsg+0x240/0x6f0 net/socket.c:2210
 __do_sys_sendmmsg net/socket.c:2239 [inline]
 __se_sys_sendmmsg net/socket.c:2236 [inline]
 __x64_sys_sendmmsg+0x9d/0x100 net/socket.c:2236
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x446a29
Code: e8 ac b8 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 eb 08 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f4de5532db8 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00000000006dcc38 RCX: 0000000000446a29
RDX: 00000000000000b8 RSI: 0000000020001b00 RDI: 0000000000000003
RBP: 00000000006dcc30 R08: 00007f4de5533700 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dcc3c
R13: 00007ffe2b830fdf R14: 00007f4de55339c0 R15: 0000000000000001

Fixes: 71b1391a4128 ("l2tp: ensure sk->dst is still valid")
Reported-by: syzbot+05f840f3b04f211bad55@syzkaller.appspotmail.com
Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
---
 net/l2tp/l2tp_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Guillaume Nault Aug. 10, 2018, 3:59 p.m. | #1
On Thu, Aug 09, 2018 at 11:54:05AM -0700, Wei Wang wrote:
> From: Wei Wang <weiwan@google.com>
> 
> In l2tp code, if it is a L2TP_UDP_ENCAP tunnel, tunnel->sk points to a
> UDP socket. User could call sendmsg() on both this tunnel and the UDP
> socket itself concurrently. As l2tp_xmit_skb() holds socket lock and call
> __sk_dst_check() to refresh sk->sk_dst_cache, while udpv6_sendmsg() is
> lockless and call sk_dst_check() to refresh sk->sk_dst_cache, there
> could be a race and cause the dst cache to be freed multiple times.
> So we fix l2tp side code to always call sk_dst_check() to garantee
> xchg() is called when refreshing sk->sk_dst_cache to avoid race
> conditions.
> 
> Syzkaller reported stack trace:
> BUG: KASAN: use-after-free in atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
> BUG: KASAN: use-after-free in atomic_fetch_add_unless include/linux/atomic.h:575 [inline]
> BUG: KASAN: use-after-free in atomic_add_unless include/linux/atomic.h:597 [inline]
> BUG: KASAN: use-after-free in dst_hold_safe include/net/dst.h:308 [inline]
> BUG: KASAN: use-after-free in ip6_hold_safe+0xe6/0x670 net/ipv6/route.c:1029
> Read of size 4 at addr ffff8801aea9a880 by task syz-executor129/4829
> 
> CPU: 0 PID: 4829 Comm: syz-executor129 Not tainted 4.18.0-rc7-next-20180802+ #30
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
>  print_address_description+0x6c/0x20b mm/kasan/report.c:256
>  kasan_report_error mm/kasan/report.c:354 [inline]
>  kasan_report.cold.7+0x242/0x30d mm/kasan/report.c:412
>  check_memory_region_inline mm/kasan/kasan.c:260 [inline]
>  check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
>  kasan_check_read+0x11/0x20 mm/kasan/kasan.c:272
>  atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
>  atomic_fetch_add_unless include/linux/atomic.h:575 [inline]
>  atomic_add_unless include/linux/atomic.h:597 [inline]
>  dst_hold_safe include/net/dst.h:308 [inline]
>  ip6_hold_safe+0xe6/0x670 net/ipv6/route.c:1029
>  rt6_get_pcpu_route net/ipv6/route.c:1249 [inline]
>  ip6_pol_route+0x354/0xd20 net/ipv6/route.c:1922
>  ip6_pol_route_output+0x54/0x70 net/ipv6/route.c:2098
>  fib6_rule_lookup+0x283/0x890 net/ipv6/fib6_rules.c:122
>  ip6_route_output_flags+0x2c5/0x350 net/ipv6/route.c:2126
>  ip6_dst_lookup_tail+0x1278/0x1da0 net/ipv6/ip6_output.c:978
>  ip6_dst_lookup_flow+0xc8/0x270 net/ipv6/ip6_output.c:1079
>  ip6_sk_dst_lookup_flow+0x5ed/0xc50 net/ipv6/ip6_output.c:1117
>  udpv6_sendmsg+0x2163/0x36b0 net/ipv6/udp.c:1354
>  inet_sendmsg+0x1a1/0x690 net/ipv4/af_inet.c:798
>  sock_sendmsg_nosec net/socket.c:622 [inline]
>  sock_sendmsg+0xd5/0x120 net/socket.c:632
>  ___sys_sendmsg+0x51d/0x930 net/socket.c:2115
>  __sys_sendmmsg+0x240/0x6f0 net/socket.c:2210
>  __do_sys_sendmmsg net/socket.c:2239 [inline]
>  __se_sys_sendmmsg net/socket.c:2236 [inline]
>  __x64_sys_sendmmsg+0x9d/0x100 net/socket.c:2236
>  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x446a29
> Code: e8 ac b8 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 eb 08 fc ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007f4de5532db8 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
> RAX: ffffffffffffffda RBX: 00000000006dcc38 RCX: 0000000000446a29
> RDX: 00000000000000b8 RSI: 0000000020001b00 RDI: 0000000000000003
> RBP: 00000000006dcc30 R08: 00007f4de5533700 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dcc3c
> R13: 00007ffe2b830fdf R14: 00007f4de55339c0 R15: 0000000000000001
> 
> Fixes: 71b1391a4128 ("l2tp: ensure sk->dst is still valid")
> Reported-by: syzbot+05f840f3b04f211bad55@syzkaller.appspotmail.com
> Signed-off-by: Wei Wang <weiwan@google.com>
> Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> Cc: David Ahern <dsahern@gmail.com>
> Cc: Cong Wang <xiyou.wangcong@gmail.com>
> ---
>  net/l2tp/l2tp_core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index 40261cb68e83..7166b61338d4 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -1110,7 +1110,7 @@ int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb, int hdr_len
>  
>  	/* Get routing info from the tunnel socket */
>  	skb_dst_drop(skb);
> -	skb_dst_set(skb, dst_clone(__sk_dst_check(sk, 0)));
> +	skb_dst_set(skb, dst_clone(sk_dst_check(sk, 0)));
>  
With sk_dst_check(), we're now holding an extra reference on the cached
dst. How is it dropped? Shouldn't we remove dst_clone()?

Also, does syzbot have a reproducer? I haven't found the original
report on the mailing list.
Wei Wang Aug. 10, 2018, 4:54 p.m. | #2
On Fri, Aug 10, 2018 at 8:59 AM Guillaume Nault <g.nault@alphalink.fr> wrote:
>
> On Thu, Aug 09, 2018 at 11:54:05AM -0700, Wei Wang wrote:
> > From: Wei Wang <weiwan@google.com>
> >
> > In l2tp code, if it is a L2TP_UDP_ENCAP tunnel, tunnel->sk points to a
> > UDP socket. User could call sendmsg() on both this tunnel and the UDP
> > socket itself concurrently. As l2tp_xmit_skb() holds socket lock and call
> > __sk_dst_check() to refresh sk->sk_dst_cache, while udpv6_sendmsg() is
> > lockless and call sk_dst_check() to refresh sk->sk_dst_cache, there
> > could be a race and cause the dst cache to be freed multiple times.
> > So we fix l2tp side code to always call sk_dst_check() to garantee
> > xchg() is called when refreshing sk->sk_dst_cache to avoid race
> > conditions.
> >
> > Syzkaller reported stack trace:
> > BUG: KASAN: use-after-free in atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
> > BUG: KASAN: use-after-free in atomic_fetch_add_unless include/linux/atomic.h:575 [inline]
> > BUG: KASAN: use-after-free in atomic_add_unless include/linux/atomic.h:597 [inline]
> > BUG: KASAN: use-after-free in dst_hold_safe include/net/dst.h:308 [inline]
> > BUG: KASAN: use-after-free in ip6_hold_safe+0xe6/0x670 net/ipv6/route.c:1029
> > Read of size 4 at addr ffff8801aea9a880 by task syz-executor129/4829
> >
> > CPU: 0 PID: 4829 Comm: syz-executor129 Not tainted 4.18.0-rc7-next-20180802+ #30
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
> >  print_address_description+0x6c/0x20b mm/kasan/report.c:256
> >  kasan_report_error mm/kasan/report.c:354 [inline]
> >  kasan_report.cold.7+0x242/0x30d mm/kasan/report.c:412
> >  check_memory_region_inline mm/kasan/kasan.c:260 [inline]
> >  check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
> >  kasan_check_read+0x11/0x20 mm/kasan/kasan.c:272
> >  atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
> >  atomic_fetch_add_unless include/linux/atomic.h:575 [inline]
> >  atomic_add_unless include/linux/atomic.h:597 [inline]
> >  dst_hold_safe include/net/dst.h:308 [inline]
> >  ip6_hold_safe+0xe6/0x670 net/ipv6/route.c:1029
> >  rt6_get_pcpu_route net/ipv6/route.c:1249 [inline]
> >  ip6_pol_route+0x354/0xd20 net/ipv6/route.c:1922
> >  ip6_pol_route_output+0x54/0x70 net/ipv6/route.c:2098
> >  fib6_rule_lookup+0x283/0x890 net/ipv6/fib6_rules.c:122
> >  ip6_route_output_flags+0x2c5/0x350 net/ipv6/route.c:2126
> >  ip6_dst_lookup_tail+0x1278/0x1da0 net/ipv6/ip6_output.c:978
> >  ip6_dst_lookup_flow+0xc8/0x270 net/ipv6/ip6_output.c:1079
> >  ip6_sk_dst_lookup_flow+0x5ed/0xc50 net/ipv6/ip6_output.c:1117
> >  udpv6_sendmsg+0x2163/0x36b0 net/ipv6/udp.c:1354
> >  inet_sendmsg+0x1a1/0x690 net/ipv4/af_inet.c:798
> >  sock_sendmsg_nosec net/socket.c:622 [inline]
> >  sock_sendmsg+0xd5/0x120 net/socket.c:632
> >  ___sys_sendmsg+0x51d/0x930 net/socket.c:2115
> >  __sys_sendmmsg+0x240/0x6f0 net/socket.c:2210
> >  __do_sys_sendmmsg net/socket.c:2239 [inline]
> >  __se_sys_sendmmsg net/socket.c:2236 [inline]
> >  __x64_sys_sendmmsg+0x9d/0x100 net/socket.c:2236
> >  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > RIP: 0033:0x446a29
> > Code: e8 ac b8 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 eb 08 fc ff c3 66 2e 0f 1f 84 00 00 00 00
> > RSP: 002b:00007f4de5532db8 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
> > RAX: ffffffffffffffda RBX: 00000000006dcc38 RCX: 0000000000446a29
> > RDX: 00000000000000b8 RSI: 0000000020001b00 RDI: 0000000000000003
> > RBP: 00000000006dcc30 R08: 00007f4de5533700 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dcc3c
> > R13: 00007ffe2b830fdf R14: 00007f4de55339c0 R15: 0000000000000001
> >
> > Fixes: 71b1391a4128 ("l2tp: ensure sk->dst is still valid")
> > Reported-by: syzbot+05f840f3b04f211bad55@syzkaller.appspotmail.com
> > Signed-off-by: Wei Wang <weiwan@google.com>
> > Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> > Cc: David Ahern <dsahern@gmail.com>
> > Cc: Cong Wang <xiyou.wangcong@gmail.com>
> > ---
> >  net/l2tp/l2tp_core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> > index 40261cb68e83..7166b61338d4 100644
> > --- a/net/l2tp/l2tp_core.c
> > +++ b/net/l2tp/l2tp_core.c
> > @@ -1110,7 +1110,7 @@ int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb, int hdr_len
> >
> >       /* Get routing info from the tunnel socket */
> >       skb_dst_drop(skb);
> > -     skb_dst_set(skb, dst_clone(__sk_dst_check(sk, 0)));
> > +     skb_dst_set(skb, dst_clone(sk_dst_check(sk, 0)));
> >
> With sk_dst_check(), we're now holding an extra reference on the cached
> dst. How is it dropped? Shouldn't we remove dst_clone()?
>
Yes. You are right. dst_clone() should get removed because
sk_dst_check() already gets a reference. I will send a v2 to correct
this.

> Also, does syzbot have a reproducer? I haven't found the original
> report on the mailing list.
This bug is reported in google internally.
I attached the .config and repro.c here.
// autogenerated by syzkaller (https://github.com/google/syzkaller)

#define _GNU_SOURCE

#include <arpa/inet.h>
#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <net/if_arp.h>
#include <pthread.h>
#include <sched.h>
#include <signal.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/prctl.h>
#include <sys/resource.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

#include <linux/futex.h>
#include <linux/if.h>
#include <linux/if_ether.h>
#include <linux/if_tun.h>
#include <linux/ip.h>
#include <linux/tcp.h>

static void sleep_ms(uint64_t ms)
{
  usleep(ms * 1000);
}

static uint64_t current_time_ms()
{
  struct timespec ts;
  if (clock_gettime(CLOCK_MONOTONIC, &ts))
    exit(1);
  return (uint64_t)ts.tv_sec * 1000 + (uint64_t)ts.tv_nsec / 1000000;
}

static void thread_start(void* (*fn)(void*), void* arg)
{
  pthread_t th;
  pthread_attr_t attr;
  pthread_attr_init(&attr);
  pthread_attr_setstacksize(&attr, 128 << 10);
  if (pthread_create(&th, &attr, fn, arg))
    exit(1);
  pthread_attr_destroy(&attr);
}

typedef struct {
  int state;
} event_t;

static void event_init(event_t* ev)
{
  ev->state = 0;
}

static void event_reset(event_t* ev)
{
  ev->state = 0;
}

static void event_set(event_t* ev)
{
  if (ev->state)
    exit(1);
  __atomic_store_n(&ev->state, 1, __ATOMIC_RELEASE);
  syscall(SYS_futex, &ev->state, FUTEX_WAKE);
}

static void event_wait(event_t* ev)
{
  while (!__atomic_load_n(&ev->state, __ATOMIC_ACQUIRE))
    syscall(SYS_futex, &ev->state, FUTEX_WAIT, 0, 0);
}

static int event_isset(event_t* ev)
{
  return __atomic_load_n(&ev->state, __ATOMIC_ACQUIRE);
}

static int event_timedwait(event_t* ev, uint64_t timeout)
{
  uint64_t start = current_time_ms();
  uint64_t now = start;
  for (;;) {
    uint64_t remain = timeout - (now - start);
    struct timespec ts;
    ts.tv_sec = remain / 1000;
    ts.tv_nsec = (remain % 1000) * 1000 * 1000;
    syscall(SYS_futex, &ev->state, FUTEX_WAIT, 0, &ts);
    if (__atomic_load_n(&ev->state, __ATOMIC_RELAXED))
      return 1;
    now = current_time_ms();
    if (now - start > timeout)
      return 0;
  }
}

static void vsnprintf_check(char* str, size_t size, const char* format,
                            va_list args)
{
  int rv;
  rv = vsnprintf(str, size, format, args);
  if (rv < 0)
    exit(1);
  if ((size_t)rv >= size)
    exit(1);
}

#define COMMAND_MAX_LEN 128
#define PATH_PREFIX                                                            \
  "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin "
#define PATH_PREFIX_LEN (sizeof(PATH_PREFIX) - 1)

static void execute_command(bool panic, const char* format, ...)
{
  va_list args;
  char command[PATH_PREFIX_LEN + COMMAND_MAX_LEN];
  int rv;
  va_start(args, format);
  memcpy(command, PATH_PREFIX, PATH_PREFIX_LEN);
  vsnprintf_check(command + PATH_PREFIX_LEN, COMMAND_MAX_LEN, format, args);
  va_end(args);
  rv = system(command);
  if (rv) {
    if (panic)
      exit(1);
  }
}

#define DEV_IPV4 "172.20.20.%d"
#define DEV_IPV6 "fe80::%02hx"
#define DEV_MAC "aa:aa:aa:aa:aa:%02hx"

static void snprintf_check(char* str, size_t size, const char* format, ...)
{
  va_list args;
  va_start(args, format);
  vsnprintf_check(str, size, format, args);
  va_end(args);
}
static void initialize_netdevices(void)
{
  unsigned i;
  const char* devtypes[] = {"ip6gretap", "bridge", "vcan", "bond", "team"};
  const char* devnames[] = {"lo",
                            "sit0",
                            "bridge0",
                            "vcan0",
                            "tunl0",
                            "gre0",
                            "gretap0",
                            "ip_vti0",
                            "ip6_vti0",
                            "ip6tnl0",
                            "ip6gre0",
                            "ip6gretap0",
                            "erspan0",
                            "bond0",
                            "veth0",
                            "veth1",
                            "team0",
                            "veth0_to_bridge",
                            "veth1_to_bridge",
                            "veth0_to_bond",
                            "veth1_to_bond",
                            "veth0_to_team",
                            "veth1_to_team"};
  const char* devmasters[] = {"bridge", "bond", "team"};
  for (i = 0; i < sizeof(devtypes) / (sizeof(devtypes[0])); i++)
    execute_command(0, "ip link add dev %s0 type %s", devtypes[i], devtypes[i]);
  execute_command(0, "ip link add type veth");
  for (i = 0; i < sizeof(devmasters) / (sizeof(devmasters[0])); i++) {
    execute_command(
        0, "ip link add name %s_slave_0 type veth peer name veth0_to_%s",
        devmasters[i], devmasters[i]);
    execute_command(
        0, "ip link add name %s_slave_1 type veth peer name veth1_to_%s",
        devmasters[i], devmasters[i]);
    execute_command(0, "ip link set %s_slave_0 master %s0", devmasters[i],
                    devmasters[i]);
    execute_command(0, "ip link set %s_slave_1 master %s0", devmasters[i],
                    devmasters[i]);
    execute_command(0, "ip link set veth0_to_%s up", devmasters[i]);
    execute_command(0, "ip link set veth1_to_%s up", devmasters[i]);
  }
  execute_command(0, "ip link set bridge_slave_0 up");
  execute_command(0, "ip link set bridge_slave_1 up");
  for (i = 0; i < sizeof(devnames) / (sizeof(devnames[0])); i++) {
    char addr[32];
    snprintf_check(addr, sizeof(addr), DEV_IPV4, i + 10);
    execute_command(0, "ip -4 addr add %s/24 dev %s", addr, devnames[i]);
    snprintf_check(addr, sizeof(addr), DEV_IPV6, i + 10);
    execute_command(0, "ip -6 addr add %s/120 dev %s", addr, devnames[i]);
    snprintf_check(addr, sizeof(addr), DEV_MAC, i + 10);
    execute_command(0, "ip link set dev %s address %s", devnames[i], addr);
    execute_command(0, "ip link set dev %s up", devnames[i]);
  }
}

static void setup_common()
{
}

static void loop();

static void sandbox_common()
{
  prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
  setpgrp();
  setsid();
  struct rlimit rlim;
  rlim.rlim_cur = rlim.rlim_max = 160 << 20;
  setrlimit(RLIMIT_AS, &rlim);
  rlim.rlim_cur = rlim.rlim_max = 8 << 20;
  setrlimit(RLIMIT_MEMLOCK, &rlim);
  rlim.rlim_cur = rlim.rlim_max = 136 << 20;
  setrlimit(RLIMIT_FSIZE, &rlim);
  rlim.rlim_cur = rlim.rlim_max = 1 << 20;
  setrlimit(RLIMIT_STACK, &rlim);
  rlim.rlim_cur = rlim.rlim_max = 0;
  setrlimit(RLIMIT_CORE, &rlim);
  rlim.rlim_cur = rlim.rlim_max = 256;
  setrlimit(RLIMIT_NOFILE, &rlim);
  if (unshare(CLONE_NEWNS)) {
  }
  if (unshare(CLONE_NEWIPC)) {
  }
  if (unshare(0x02000000)) {
  }
  if (unshare(CLONE_NEWUTS)) {
  }
  if (unshare(CLONE_SYSVSEM)) {
  }
}

int wait_for_loop(int pid)
{
  if (pid < 0)
    exit(1);
  int status = 0;
  while (waitpid(-1, &status, __WALL) != pid) {
  }
  return WEXITSTATUS(status);
}

static int do_sandbox_none(void)
{
  if (unshare(CLONE_NEWPID)) {
  }
  int pid = fork();
  if (pid != 0)
    return wait_for_loop(pid);
  setup_common();
  sandbox_common();
  if (unshare(CLONE_NEWNET)) {
  }
  initialize_netdevices();
  loop();
  exit(1);
}

#define SYZ_HAVE_SETUP_TEST 1
static void setup_test()
{
  prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
  setpgrp();
}

#define SYZ_HAVE_RESET_TEST 1
static void reset_test()
{
  int fd;
  for (fd = 3; fd < 30; fd++)
    close(fd);
}

struct thread_t {
  int created, call;
  event_t ready, done;
};

static struct thread_t threads[16];
static void execute_call(int call);
static int running;

static void* thr(void* arg)
{
  struct thread_t* th = (struct thread_t*)arg;
  for (;;) {
    event_wait(&th->ready);
    event_reset(&th->ready);
    execute_call(th->call);
    __atomic_fetch_sub(&running, 1, __ATOMIC_RELAXED);
    event_set(&th->done);
  }
  return 0;
}

static void execute_one()
{
  int call, thread;
  for (call = 0; call < 6; call++) {
    for (thread = 0; thread < sizeof(threads) / sizeof(threads[0]); thread++) {
      struct thread_t* th = &threads[thread];
      if (!th->created) {
        th->created = 1;
        event_init(&th->ready);
        event_init(&th->done);
        event_set(&th->done);
        thread_start(thr, th);
      }
      if (!event_isset(&th->done))
        continue;
      event_reset(&th->done);
      th->call = call;
      __atomic_fetch_add(&running, 1, __ATOMIC_RELAXED);
      event_set(&th->ready);
      event_timedwait(&th->done, 25);
      if (__atomic_load_n(&running, __ATOMIC_RELAXED))
        sleep_ms((call == 6 - 1) ? 10 : 2);
      break;
    }
  }
}

static void execute_one();

#define WAIT_FLAGS __WALL

static void loop()
{
  int iter;
  for (iter = 0;; iter++) {
    int pid = fork();
    if (pid < 0)
      exit(1);
    if (pid == 0) {
      setup_test();
      execute_one();
      reset_test();
      exit(0);
    }
    int status = 0;
    uint64_t start = current_time_ms();
    for (;;) {
      if (waitpid(-1, &status, WNOHANG | WAIT_FLAGS) == pid)
        break;
      sleep_ms(1);
      if (current_time_ms() - start < 5 * 1000)
        continue;
      kill(-pid, SIGKILL);
      kill(pid, SIGKILL);
      while (waitpid(-1, &status, WAIT_FLAGS) != pid) {
      }
      break;
    }
  }
}

uint64_t r[2] = {0xffffffffffffffff, 0xffffffffffffffff};

void execute_call(int call)
{
  long res;
  switch (call) {
  case 0:
    res = syscall(__NR_socket, 0xa, 2, 0);
    if (res != -1)
      r[0] = res;
    break;
  case 1:
    *(uint16_t*)0x20000080 = 0xa;
    *(uint16_t*)0x20000082 = htobe16(0);
    *(uint32_t*)0x20000084 = 0;
    *(uint8_t*)0x20000088 = 0xfe;
    *(uint8_t*)0x20000089 = 0x80;
    *(uint8_t*)0x2000008a = 0;
    *(uint8_t*)0x2000008b = 0;
    *(uint8_t*)0x2000008c = 0;
    *(uint8_t*)0x2000008d = 0;
    *(uint8_t*)0x2000008e = 0;
    *(uint8_t*)0x2000008f = 0;
    *(uint8_t*)0x20000090 = 0;
    *(uint8_t*)0x20000091 = 0;
    *(uint8_t*)0x20000092 = 0;
    *(uint8_t*)0x20000093 = 0;
    *(uint8_t*)0x20000094 = 0;
    *(uint8_t*)0x20000095 = 0;
    *(uint8_t*)0x20000096 = 0;
    *(uint8_t*)0x20000097 = 0xbb;
    *(uint32_t*)0x20000098 = 0xb;
    syscall(__NR_connect, r[0], 0x20000080, 0x1c);
    break;
  case 2:
    res = syscall(__NR_socket, 0x18, 1, 1);
    if (res != -1)
      r[1] = res;
    break;
  case 3:
    *(uint16_t*)0x205fafd2 = 0x18;
    *(uint32_t*)0x205fafd4 = 1;
    *(uint32_t*)0x205fafd8 = 0;
    *(uint32_t*)0x205fafdc = r[0];
    *(uint16_t*)0x205fafe0 = 2;
    *(uint16_t*)0x205fafe2 = htobe16(0);
    *(uint32_t*)0x205fafe4 = htobe32(0xe0000002);
    *(uint8_t*)0x205fafe8 = 0;
    *(uint8_t*)0x205fafe9 = 0;
    *(uint8_t*)0x205fafea = 0;
    *(uint8_t*)0x205fafeb = 0;
    *(uint8_t*)0x205fafec = 0;
    *(uint8_t*)0x205fafed = 0;
    *(uint8_t*)0x205fafee = 0;
    *(uint8_t*)0x205fafef = 0;
    *(uint32_t*)0x205faff0 = 4;
    *(uint32_t*)0x205faff4 = 0;
    *(uint32_t*)0x205faff8 = 0;
    *(uint32_t*)0x205faffc = 0;
    syscall(__NR_connect, r[1], 0x205fafd2, 0x2e);
    break;
  case 4:
    *(uint64_t*)0x20005fc0 = 0x20005680;
    *(uint16_t*)0x20005680 = 0x1f;
    *(uint8_t*)0x20005682 = 0;
    *(uint8_t*)0x20005683 = 0;
    *(uint8_t*)0x20005684 = 0;
    *(uint8_t*)0x20005685 = 0;
    *(uint8_t*)0x20005686 = 0;
    *(uint8_t*)0x20005687 = 0;
    *(uint32_t*)0x20005fc8 = 0x80;
    *(uint64_t*)0x20005fd0 = 0x20005b00;
    *(uint64_t*)0x20005fd8 = 0;
    *(uint64_t*)0x20005fe0 = 0;
    *(uint64_t*)0x20005fe8 = 0;
    *(uint32_t*)0x20005ff0 = 0;
    *(uint32_t*)0x20005ff8 = 0;
    *(uint64_t*)0x20006000 = 0x20005b80;
    *(uint16_t*)0x20005b80 = 0x1f;
    *(uint16_t*)0x20005b82 = 0;
    *(uint8_t*)0x20005b84 = 0;
    *(uint8_t*)0x20005b85 = 0;
    *(uint8_t*)0x20005b86 = 0;
    *(uint8_t*)0x20005b87 = 0;
    *(uint8_t*)0x20005b88 = 0;
    *(uint8_t*)0x20005b89 = 0;
    *(uint16_t*)0x20005b8a = 0;
    *(uint8_t*)0x20005b8c = 0;
    *(uint32_t*)0x20006008 = 0x80;
    *(uint64_t*)0x20006010 = 0x20005c40;
    *(uint64_t*)0x20006018 = 0x1f4;
    *(uint64_t*)0x20006020 = 0x20005c80;
    *(uint64_t*)0x20006028 = 0x3a00;
    *(uint32_t*)0x20006030 = 0;
    *(uint32_t*)0x20006038 = 0;
    syscall(__NR_sendmmsg, r[1], 0x20005fc0, 0x3e8, 0);
    break;
  case 5:
    syscall(__NR_sendmmsg, r[0], 0x20001b00, 0xb8, 0);
    break;
  }
}
int main()
{
  syscall(__NR_mmap, 0x20000000, 0x1000000, 3, 0x32, -1, 0);
  do_sandbox_none();
  return 0;
}

Patch

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 40261cb68e83..7166b61338d4 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1110,7 +1110,7 @@  int l2tp_xmit_skb(struct l2tp_session *session, struct sk_buff *skb, int hdr_len
 
 	/* Get routing info from the tunnel socket */
 	skb_dst_drop(skb);
-	skb_dst_set(skb, dst_clone(__sk_dst_check(sk, 0)));
+	skb_dst_set(skb, dst_clone(sk_dst_check(sk, 0)));
 
 	inet = inet_sk(sk);
 	fl = &inet->cork.fl;