Date:   Wed, 06 Sep 2023 15:40:26 +0300
From:   Eduard Zingerman <eddyz87@...il.com>
To:     syzbot <syzbot+291100dcb32190ec02a8@...kaller.appspotmail.com>,
        andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
        daniel@...earbox.net, davem@...emloft.net, haoluo@...gle.com,
        hawk@...nel.org, john.fastabend@...il.com, jolsa@...nel.org,
        kpsingh@...nel.org, kuba@...nel.org, linux-kernel@...r.kernel.org,
        martin.lau@...ux.dev, netdev@...r.kernel.org, sdf@...gle.com,
        song@...nel.org, syzkaller-bugs@...glegroups.com,
        yonghong.song@...ux.dev
Subject: Re: [syzbot] [bpf?] general protection fault in
 bpf_prog_offload_verifier_prep

On Sun, 2023-09-03 at 12:55 -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    fa09bc40b21a igb: disable virtualization features on 82580
> git tree:       net
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=13382fa8680000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=634e05b4025da9da
> dashboard link: https://syzkaller.appspot.com/bug?extid=291100dcb32190ec02a8
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1529c448680000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15db0248680000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/7ab461d84992/disk-fa09bc40.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/3ac6d43ab2db/vmlinux-fa09bc40.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/778d096a134e/bzImage-fa09bc40.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+291100dcb32190ec02a8@...kaller.appspotmail.com
> 
> general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> CPU: 1 PID: 5055 Comm: syz-executor625 Not tainted 6.5.0-syzkaller-04012-gfa09bc40b21a #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/26/2023
> RIP: 0010:bpf_prog_offload_verifier_prep+0xaa/0x170 kernel/bpf/offload.c:295
> Code: 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 a1 00 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b 65 10 4c 89 e2 48 c1 ea 03 <80> 3c 02 00 0f 85 93 00 00 00 48 b8 00 00 00 00 00 fc ff df 4d 8b
> RSP: 0018:ffffc900039ff7f8 EFLAGS: 00010246
> RAX: dffffc0000000000 RBX: ffffc9000156e000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: ffffffff81a8cf76 RDI: ffff888021b25f10
> RBP: ffff888021b25f00 R08: 0000000000000001 R09: fffffbfff195203d
> R10: ffffffff8ca901ef R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000005 R14: 0000000000000003 R15: ffffc9000156e060
> FS:  0000555556071380(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020000100 CR3: 0000000022f6b000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  bpf_check+0x52f3/0xabd0 kernel/bpf/verifier.c:19762
>  bpf_prog_load+0x153a/0x2270 kernel/bpf/syscall.c:2708
>  __sys_bpf+0xbb6/0x4e90 kernel/bpf/syscall.c:5335
>  __do_sys_bpf kernel/bpf/syscall.c:5439 [inline]
>  __se_sys_bpf kernel/bpf/syscall.c:5437 [inline]
>  __x64_sys_bpf+0x78/0xc0 kernel/bpf/syscall.c:5437
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f7c0df78ea9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 d1 19 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:00007ffde3592128 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f7c0df78ea9
> RDX: 0000000000000090 RSI: 0000000020000940 RDI: 0000000000000005
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000100000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>  </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:bpf_prog_offload_verifier_prep+0xaa/0x170 kernel/bpf/offload.c:295
> Code: 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 a1 00 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b 65 10 4c 89 e2 48 c1 ea 03 <80> 3c 02 00 0f 85 93 00 00 00 48 b8 00 00 00 00 00 fc ff df 4d 8b
> RSP: 0018:ffffc900039ff7f8 EFLAGS: 00010246
> RAX: dffffc0000000000 RBX: ffffc9000156e000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: ffffffff81a8cf76 RDI: ffff888021b25f10
> RBP: ffff888021b25f00 R08: 0000000000000001 R09: fffffbfff195203d
> R10: ffffffff8ca901ef R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000005 R14: 0000000000000003 R15: ffffc9000156e060
> FS:  0000555556071380(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020000100 CR3: 0000000022f6b000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> ----------------
> Code disassembly (best guess), 3 bytes skipped:
>    0:	df 48 89             	fisttps -0x77(%rax)
>    3:	fa                   	cli
>    4:	48 c1 ea 03          	shr    $0x3,%rdx
>    8:	80 3c 02 00          	cmpb   $0x0,(%rdx,%rax,1)
>    c:	0f 85 a1 00 00 00    	jne    0xb3
>   12:	48 b8 00 00 00 00 00 	movabs $0xdffffc0000000000,%rax
>   19:	fc ff df
>   1c:	4c 8b 65 10          	mov    0x10(%rbp),%r12
>   20:	4c 89 e2             	mov    %r12,%rdx
>   23:	48 c1 ea 03          	shr    $0x3,%rdx
> * 27:	80 3c 02 00          	cmpb   $0x0,(%rdx,%rax,1) <-- trapping instruction
>   2b:	0f 85 93 00 00 00    	jne    0xc4
>   31:	48 b8 00 00 00 00 00 	movabs $0xdffffc0000000000,%rax
>   38:	fc ff df
>   3b:	4d                   	rex.WRB
>   3c:	8b                   	.byte 0x8b
> 
> 
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
> 
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> 
> If the bug is already fixed, let syzbot know by replying with:
> #syz fix: exact-commit-title
> 
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.
> 
> If you want to overwrite bug's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
> 
> If the bug is a duplicate of another bug, reply with:
> #syz dup: exact-subject-of-another-report
> 
> If you want to undo deduplication, reply with:
> #syz undup
> 

I have an explanation of why this error occurs, but I need an advice
on how to fix it.

Then NULL pointer deference occurs in the following function from offload.c:

    int bpf_prog_offload_verifier_prep(struct bpf_prog *prog)
    {
        struct bpf_prog_offload *offload;
        int ret = -ENODEV;
    
        down_read(&bpf_devs_lock);
        offload = prog->aux->offload;
        if (offload) {
            ret = offload->offdev->ops->prepare(prog);
                           ^^^^^^
                           this pointer is NULL
            offload->dev_state = !ret;
        }
        up_read(&bpf_devs_lock);
    
        return ret;
    }

# Short explanation

(a) call chain bpf_prog_load -> bpf_prog_dev_bound_init -> __bpf_prog_dev_bound_init
               -> __bpf_offload_dev_netdev_register
    might insert an instance of struct bpf_offload_netdev with {.offdev == NULL}
    into hash table offload.c:offdevs;
(b) call chain bpf_prog_load -> bpf_check -> bpf_prog_offload_verifier_prep
    assumes that from (prog->aux->offload != NULL)
              follows (prog->aux->offload->offdev != NULL)
    which is not the case because of (a).

# Long explanation

The reproducer generated by testbot has the following structure:
- in a loop call function execute_one(), which does the following
  system calls in sequence:
  - socket(AF_INET6, SOCK_RAW, IPPROTO_IGMP) = <some fd>
  - ioctl(3, SIOCGIFINDEX, {ifr_name="batadv_slave_1"}) = 0
  - bpf(BPF_PROG_LOAD,
        {prog_type=BPF_PROG_TYPE_XDP, ... prog_flags=0x40, prog_ifindex=29, ...}) = -1 EINVAL
    (referred to as program #1 below)
  - socket(AF_INET6, SOCK_RAW, IPPROTO_IGMP) = <some fd>
  - ioctl(4, SIOCGIFINDEX, {ifr_name="batadv_slave_1"}) = 0
  - bpf(BPF_PROG_LOAD,
        {prog_type=BPF_PROG_TYPE_XDP, ... prog_flags=0, ... prog_ifindex=29}) = -1 EINVAL
    (referred to as program #2 below)

The error occurs when second bpf call is processed.
Interestingly, if sleep(1) is inserted somewhere between first and
second bpf calls error does not occur:

    @@ -1246,6 +1246,7 @@ void execute_one(void)
       *(uint32_t*)0x200009cc = 4;
       syscall(__NR_bpf, /*cmd=*/5ul, /*arg=*/0x20000940ul, /*size=*/0x90ul);
       res = syscall(__NR_socket, /*domain=*/0xaul, /*type=*/3ul, /*proto=*/2);
    +  // sleep(1); /* uncomment to hide the error */
       if (res != -1)
         r[2] = res;
       memcpy((void*)0x20000100, "batadv_slave_1\000\000", 16);

## Control flow when error occurs

For program #1:
- bpf_prog_load():
  - bpf_prog_is_dev_bound(prog->aux) is true
    - bpf_prog_dev_bound_init
      - prog->aux->offload_requested is 0 (because of 0x40 prog_flags)
      - __bpf_prog_dev_bound_init
        - netdev is "batadv_slave_1"
        - bpf_offload_find_netdev(offload->netdev) == NULL,
          (this is a lookup in hash table offload.c:offdevs)
          which triggers a call to __bpf_offload_dev_netdev_register
          - __bpf_offload_dev_netdev_register(NULL, offload->netdev)
            registers struct bpf_offload_netdev with {.offdev = NULL}
            for netdev "batadv_slave_1" in offload.c:offdevs hash table.

For program #2:
- bpf_prog_load():
  - bpf_prog_is_dev_bound(prog->aux) is true
    - bpf_prog_dev_bound_init
      - prog->aux->offload_requested is 1 (because of 0x0 prog_flags)
      - __bpf_prog_dev_bound_init
        - netdev is "batadv_slave_1"
        - bpf_offload_find_netdev(offload->netdev) != NULL,
          this is struct bpf_offload_netdev with {.offdev = NULL}
          created for program #1
        - prog->aux->offload = struct bpf_prog_offload {.offload -> {.offdev = NULL}},
          The bpf_prog_offload remembered for prog points to bpf_offload_netdev
          with .offdev == NULL.
  - ...
  - bpf_check
    - bpf_prog_offload_verifier_prep
      - prog->aux->offload != NULL, but prog->aux->offload->offdev == NULL
        => null pointer deference.

## Control flow when error does not occur

For program #1:
- ... all as in the previous case ...

Some worker thread:
- kernel/bpf/core.c:bpf_prog_free_deferred, registered for program #1:
  - bpf_prog_is_dev_bound(aux) is true
  - bpf_prog_dev_bound_destroy
    - netdev is "batadv_slave_1"
    - (!ondev->offdev && list_empty(&ondev->progs)) is true
      - __bpf_offload_dev_netdev_unregister
        this removes struct bpf_offload_netdev with {.offdev = NULL}
        from offload.c:offdevs hash table.

For program #2:
- bpf_prog_load():
  - bpf_prog_is_dev_bound(prog->aux) is true
    - bpf_prog_dev_bound_init
      - prog->aux->offload_requested is 1 (because of 0x0 prog_flags)
      - __bpf_prog_dev_bound_init
        - netdev is "batadv_slave_1"
        - bpf_offload_find_netdev(offload->netdev) == NULL
        - bpf_prog_is_offloaded(prog->aux) is true
        - -EINVAL is returned.

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