Re: [PATCH net] net: prevent NULL pointer dereference in rt_fibinfo_free() and rt_fibinfo_free_cpus()

[Jeongjun Park]

> Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> On Mon, Sep 9, 2024 at 8:48 PM Jeongjun Park <aha310510@xxxxxxxxx> wrote:
>> 
>> rt_fibinfo_free() and rt_fibinfo_free_cpus() only check for rt and do not
>> verify rt->dst and use it, which will result in NULL pointer dereference.
>> 
>> Therefore, to prevent this, we need to add a check for rt->dst.
>> 
>> Fixes: 0830106c5390 ("ipv4: take dst->__refcnt when caching dst in fib")
>> Fixes: c5038a8327b9 ("ipv4: Cache routes in nexthop exception entries.")
>> Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
>> ---
> 
> As far as I can tell, your patch is a NOP, and these Fixes: tags seem
> random to me.

I somewhat agree with the opinion that the fixes tag is random. 
However, I think it is absolutely necessary to add a check for 
&rt->dst , because the existence of rt does not guarantee that 
&rt->dst will not be NULL.

> 
> Also, I am guessing this is based on a syzbot report ?

Yes, but it's not a bug reported to syzbot, it's a bug that 
I accidentally found in my syzkaller fuzzer. The report is too long
to be included in the patch notes, so I'll attach it to this email.

Report:

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 UID: 0 PID: 4694 Comm: systemd-udevd Not tainted 6.11.0-rc6-00326-gd1f2d51b711a #16
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
RIP: 0010:dst_dev_put+0x26/0x330 net/core/dst.c:149
Code: 90 90 90 90 f3 0f 1e fa 41 57 41 56 49 89 fe 41 55 41 54 55 e8 0b 90 af f8 4c 89 f2 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 da 02 00 00 49 8d 7e 3a 4d 8b 26 48 b8 00 00 00
RSP: 0018:ffffc90000007d68 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: 0000000000000001 RCX: ffffffff8976519a
RDX: 0000000000000000 RSI: ffffffff88d97a95 RDI: 0000000000000001
RBP: dffffc0000000000 R08: 0000000000000001 R09: ffffed100c8020c3
R10: 0000000000000000 R11: 0000000000000000 R12: fffffbfff1ab5a81
R13: 0000607f8106c5c8 R14: 0000000000000001 R15: 0000000000000000
FS:  00007f235c5fd8c0(0000) GS:ffff88802c400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f39c90e5168 CR3: 0000000019688000 CR4: 0000000000750ef0
PKRU: 55555554
Call Trace:
 <IRQ>
 rt_fibinfo_free_cpus.part.0+0xf4/0x1d0 net/ipv4/fib_semantics.c:206
 rt_fibinfo_free_cpus net/ipv4/fib_semantics.c:198 [inline]
 fib_nh_common_release+0x121/0x360 net/ipv4/fib_semantics.c:217
 fib_nh_release net/ipv4/fib_semantics.c:229 [inline]
 free_fib_info_rcu+0x18f/0x4b0 net/ipv4/fib_semantics.c:241
 rcu_do_batch kernel/rcu/tree.c:2569 [inline]
 rcu_core+0x826/0x16d0 kernel/rcu/tree.c:2843
 handle_softirqs+0x1d4/0x870 kernel/softirq.c:554
 __do_softirq kernel/softirq.c:588 [inline]
 invoke_softirq kernel/softirq.c:428 [inline]
 __irq_exit_rcu kernel/softirq.c:637 [inline]
 irq_exit_rcu+0xbb/0x120 kernel/softirq.c:649
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1043 [inline]
 sysvec_apic_timer_interrupt+0x99/0xb0 arch/x86/kernel/apic/apic.c:1043
 </IRQ>
 <TASK>
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702
RIP: 0010:__raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:152 [inline]
RIP: 0010:_raw_spin_unlock_irqrestore+0x3c/0x70 kernel/locking/spinlock.c:194
Code: 74 24 10 e8 d6 03 6f f6 48 89 ef e8 8e 77 6f f6 81 e3 00 02 00 00 75 29 9c 58 f6 c4 02 75 35 48 85 db 74 01 fb bf 01 00 00 00 e8 bf 24 61 f6 65 8b 05 c0 79 0b 75 85 c0 74 0e 5b 5d c3 cc cc cc
RSP: 0018:ffffc90001f978f8 EFLAGS: 00000206
RAX: 0000000000000006 RBX: 0000000000000200 RCX: 1ffffffff1fe4be9
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000001
RBP: ffffffff8dbc0ac0 R08: 0000000000000001 R09: 0000000000000001
R10: ffffffff8ff2a39f R11: ffffffff815d29ca R12: 0000000000000246
R13: ffff88801e866100 R14: 0000000000000200 R15: 0000000000000000
 rcu_read_unlock_special kernel/rcu/tree_plugin.h:691 [inline]
 __rcu_read_unlock+0x2d9/0x580 kernel/rcu/tree_plugin.h:436
 __netlink_sendskb net/netlink/af_netlink.c:1278 [inline]
 netlink_broadcast_deliver net/netlink/af_netlink.c:1408 [inline]
 do_one_broadcast net/netlink/af_netlink.c:1495 [inline]
 netlink_broadcast_filtered+0x8ec/0xe00 net/netlink/af_netlink.c:1540
 netlink_broadcast net/netlink/af_netlink.c:1564 [inline]
 netlink_sendmsg+0x9ee/0xd80 net/netlink/af_netlink.c:1899
 sock_sendmsg_nosec net/socket.c:730 [inline]
 __sock_sendmsg net/socket.c:745 [inline]
 ____sys_sendmsg+0xabe/0xc80 net/socket.c:2597
 ___sys_sendmsg+0x11d/0x1c0 net/socket.c:2651
 __sys_sendmsg+0xfe/0x1d0 net/socket.c:2680
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcb/0x250 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f235c8b0e13
Code: 8b 15 b9 a1 00 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b8 0f 1f 00 64 8b 04 25 18 00 00 00 85 c0 75 14 b8 2e 00 00 00 0f 05 48 3d 00 f0 ff ff 77 55 c3 0f 1f 40 00 48 83 ec 28 89 54 24 1c 48
RSP: 002b:00007ffe93910d38 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000556a6858ba60 RCX: 00007f235c8b0e13
RDX: 0000000000000000 RSI: 00007ffe93910d60 RDI: 000000000000000e
RBP: 0000556a6858bdc0 R08: 00000000ffffffff R09: 0000556a685488e0
R10: 0000556a6858be38 R11: 0000000000000246 R12: 0000000000008010
R13: 0000556a6856fc30 R14: 0000000000000000 R15: 00007ffe93910df0
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:dst_dev_put+0x26/0x330 net/core/dst.c:149
Code: 90 90 90 90 f3 0f 1e fa 41 57 41 56 49 89 fe 41 55 41 54 55 e8 0b 90 af f8 4c 89 f2 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 da 02 00 00 49 8d 7e 3a 4d 8b 26 48 b8 00 00 00
RSP: 0018:ffffc90000007d68 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: 0000000000000001 RCX: ffffffff8976519a
RDX: 0000000000000000 RSI: ffffffff88d97a95 RDI: 0000000000000001
RBP: dffffc0000000000 R08: 0000000000000001 R09: ffffed100c8020c3
R10: 0000000000000000 R11: 0000000000000000 R12: fffffbfff1ab5a81
R13: 0000607f8106c5c8 R14: 0000000000000001 R15: 0000000000000000
FS:  00007f235c5fd8c0(0000) GS:ffff88802c400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f39c90e5168 CR3: 0000000019688000 CR4: 0000000000750ef0
PKRU: 55555554
----------------
Code disassembly (best guess):
   0:	90                   	nop
   1:	90                   	nop
   2:	90                   	nop
   3:	90                   	nop
   4:	f3 0f 1e fa          	endbr64
   8:	41 57                	push   %r15
   a:	41 56                	push   %r14
   c:	49 89 fe             	mov    %rdi,%r14
   f:	41 55                	push   %r13
  11:	41 54                	push   %r12
  13:	55                   	push   %rbp
  14:	e8 0b 90 af f8       	call   0xf8af9024
  19:	4c 89 f2             	mov    %r14,%rdx
  1c:	48 b8 00 00 00 00 00 	movabs $0xdffffc0000000000,%rax
  23:	fc ff df
  26:	48 c1 ea 03          	shr    $0x3,%rdx
* 2a:	80 3c 02 00          	cmpb   $0x0,(%rdx,%rax,1) <-- trapping instruction
  2e:	0f 85 da 02 00 00    	jne    0x30e
  34:	49 8d 7e 3a          	lea    0x3a(%r14),%rdi
  38:	4d 8b 26             	mov    (%r14),%r12
  3b:	48                   	rex.W
  3c:	b8                   	.byte 0xb8
  3d:	00 00                	add    %al,(%rax)