net: GPF in rt6_nexthop_info

[Dmitry Vyukov]

Hello,

The following program triggers GPF in rt6_nexthop_info

// autogenerated by syzkaller (http://github.com/google/syzkaller)
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/sched.h>
#include <unistd.h>
#include <sched.h>

int main()
{
  unshare(CLONE_NEWNET);
  int fd = socket(PF_NETLINK, SOCK_DGRAM|SOCK_NONBLOCK, 0);
  const char* data ="\x22\x00\x00\x00\x1a\x00\x07\x00\x08\x09\x00\x0f"
                    "\x09\x00\x07\x00\x0a\xff\xfc\x03\x1b\x00\xa8\xc6"
                    "\x19\xf3\xff\xff\x05\x00\x10\x00\xbe\x45";
  write(fd, data, 34);
  return 0;
}

general protection fault: 0000 [#1] SMP KASAN
Modules linked in:
CPU: 2 PID: 2965 Comm: a.out Not tainted 4.10.0+ #229
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff8800650fc100 task.stack: ffff880065378000
RIP: 0010:rt6_nexthop_info+0x278/0x3b0 net/ipv6/route.c:3325
RSP: 0018:ffff88006537f350 EFLAGS: 00010203
RAX: dffffc0000000000 RBX: ffff88006865da00 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 000000000000004a RDI: 0000000000000254
RBP: ffff88006537f3f0 R08: ffffed000cc55820 R09: ffffed000cc55820
R10: 0000000000000001 R11: ffffed000cc5581f R12: 1ffff1000ca6fe6d
R13: ffff8800662ac0d8 R14: ffff88006537f3c8 R15: dffffc0000000000
FS:  0000000000c39880(0000) GS:ffff88006d100000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004b20e0 CR3: 000000006cdb7000 CR4: 00000000001406e0
Call Trace:
 rt6_fill_node.isra.61+0xea4/0x1780 net/ipv6/route.c:3513
 inet6_rtm_getroute+0x7da/0xce0 net/ipv6/route.c:3639
 rtnetlink_rcv_msg+0x609/0x860 net/core/rtnetlink.c:4104
 netlink_rcv_skb+0x2ab/0x390 net/netlink/af_netlink.c:2298
 rtnetlink_rcv+0x2a/0x40 net/core/rtnetlink.c:4110
 netlink_unicast_kernel net/netlink/af_netlink.c:1231 [inline]
 netlink_unicast+0x514/0x730 net/netlink/af_netlink.c:1257
 netlink_sendmsg+0xa9f/0xe50 net/netlink/af_netlink.c:1803
 sock_sendmsg_nosec net/socket.c:633 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:643
 sock_write_iter+0x326/0x600 net/socket.c:846
 new_sync_write fs/read_write.c:499 [inline]
 __vfs_write+0x483/0x740 fs/read_write.c:512
 vfs_write+0x187/0x530 fs/read_write.c:560
 SYSC_write fs/read_write.c:607 [inline]
 SyS_write+0xfb/0x230 fs/read_write.c:599
 entry_SYSCALL_64_fastpath+0x1f/0xc2
RIP: 0033:0x433ef0
RSP: 002b:00007ffcea6171b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000401730 RCX: 0000000000433ef0
RDX: 0000000000000022 RSI: 0000000000493548 RDI: 0000000000000003
RBP: 0000000000000000 R08: 000000000000000b R09: 0000000000000004
R10: 000000000000000d R11: 0000000000000246 R12: 00000000004002b0
R13: 00007ffcea6172c8 R14: 0000000000000002 R15: 0000000000000000
Code: ff df 80 3c 01 00 0f 85 3c 01 00 00 48 8b 8b 58 01 00 00 48 b8
00 00 00 00 00 fc ff df 48 8d b9 54 02 00 00 48 89 fe 48 c1 ee 03 <0f>
b6 34 06 48 89 f8 83 e0 07 83 c0 03 40 38 f0 7c 05 40 84 f6
RIP: rt6_nexthop_info+0x278/0x3b0 net/ipv6/route.c:3325 RSP: ffff88006537f350
---[ end trace 6a59074c79adfb5f ]---

On commit e5d56efc97f8240d0b5d66c03949382b6d7e5570