Re: [syzbot] [bpf?] general protection fault in bpf_prog_offload_verifier_prep

From: Eduard Zingerman
Date: Wed Sep 06 2023 - 08:40:36 EST

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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@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> 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@xxxxxxxxxxxxxxxx.
>
> 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:
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> If you attach or paste a git patch, syzbot will apply it before testing.
>
> If you want to overwrite bug's subsystems, reply with:
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> (See the list of subsystem names on the web dashboard)
>
> If the bug is a duplicate of another bug, reply with:
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>
> 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.

Next message: Matti Vaittinen: "[PATCH 3/3] MAINTAINERS: Add ROHM BM1390"
Previous message: Andrew Lunn: "Re: [PATCH 1/2] net: ethernet: davinci_emac: Use MAC Address from Device Tree"
In reply to: syzbot: "[syzbot] [bpf?] general protection fault in bpf_prog_offload_verifier_prep"
Next in thread: Eduard Zingerman: "Re: [syzbot] [bpf?] general protection fault in bpf_prog_offload_verifier_prep"
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