CVE-2024-56646: ipv6: avoid possible NULL deref in modify_prefix_route()

In the Linux kernel, the following vulnerability has been resolved:

ipv6: avoid possible NULL deref in modifyprefixroute()

syzbot found a NULL deref [1] in modifyprefixroute(), caused by one fib6info without a fib6table pointer set.

This can happen for net->ipv6.fib6nullentry

[1] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000006: 0000 [#1] PREEMPT SMP KASAN NOPTI KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037] CPU: 1 UID: 0 PID: 5837 Comm: syz-executor888 Not tainted 6.12.0-syzkaller-09567-g7eef7e306d3c #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024 RIP: 0010:lockacquire+0xe4/0x3c40 kernel/locking/lockdep.c:5089 Code: 08 84 d2 0f 85 15 14 00 00 44 8b 0d ca 98 f5 0e 45 85 c9 0f 84 b4 0e 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 e2 48 c1 ea 03 <80> 3c 02 00 0f 85 96 2c 00 00 49 8b 04 24 48 3d a0 07 7f 93 0f 84 RSP: 0018:ffffc900035d7268 EFLAGS: 00010006 RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000 RDX: 0000000000000006 RSI: 1ffff920006bae5f RDI: 0000000000000030 RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000001 R10: ffffffff90608e17 R11: 0000000000000001 R12: 0000000000000030 R13: ffff888036334880 R14: 0000000000000000 R15: 0000000000000000 FS: 0000555579e90380(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007ffc59cc4278 CR3: 0000000072b54000 CR4: 00000000003526f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <TASK> lockacquire.part.0+0x11b/0x380 kernel/locking/lockdep.c:5849 rawspinlockbh include/linux/spinlockapismp.h:126 [inline] rawspinlockbh+0x33/0x40 kernel/locking/spinlock.c:178 spinlockbh include/linux/spinlock.h:356 [inline] modifyprefixroute+0x30b/0x8b0 net/ipv6/addrconf.c:4831 inet6addrmodify net/ipv6/addrconf.c:4923 [inline] inet6rtmnewaddr+0x12c7/0x1ab0 net/ipv6/addrconf.c:5055 rtnetlinkrcvmsg+0x3c7/0xea0 net/core/rtnetlink.c:6920 netlinkrcvskb+0x16b/0x440 net/netlink/afnetlink.c:2541 netlinkunicastkernel net/netlink/afnetlink.c:1321 [inline] netlinkunicast+0x53c/0x7f0 net/netlink/afnetlink.c:1347 netlinksendmsg+0x8b8/0xd70 net/netlink/afnetlink.c:1891 socksendmsgnosec net/socket.c:711 [inline] socksendmsg net/socket.c:726 [inline] syssendmsg+0xaaf/0xc90 net/socket.c:2583 syssendmsg+0x135/0x1e0 net/socket.c:2637 syssendmsg+0x16e/0x220 net/socket.c:2669 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0xcd/0x250 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x77/0x7f RIP: 0033:0x7fd1dcef8b79 Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 c1 17 00 00 90 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 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007ffc59cc4378 EFLAGS: 00000246 ORIGRAX: 000000000000002e RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fd1dcef8b79 RDX: 0000000000040040 RSI: 0000000020000140 RDI: 0000000000000004 RBP: 00000000000113fd R08: 0000000000000006 R09: 0000000000000006 R10: 0000000000000006 R11: 0000000000000246 R12: 00007ffc59cc438c R13: 431bde82d7b634db R14: 0000000000000001 R15: 0000000000000001 </TASK>