Re: [syzbot] [bpf?] [net?] BUG: unable to handle kernel NULL pointer dereference in __build_skb_around

[Alexander Lobakin]

From: Syzbot <syzbot+e1d1b65f7c32f2a86a9f@xxxxxxxxxxxxxxxxxxxxxxxxx>
Date: Wed, 15 Mar 2023 05:03:47 -0700

> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    3c2611bac08a selftests/bpf: Fix trace_virtqueue_add_sgs te..
> git tree:       bpf-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=1026d472c80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cab35c936731a347
> dashboard link: https://syzkaller.appspot.com/bug?extid=e1d1b65f7c32f2a86a9f
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15826bc6c80000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15cd12e2c80000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/36a32f4d222a/disk-3c2611ba.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/f5c0da04f143/vmlinux-3c2611ba.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/ae2ca9bce51a/bzImage-3c2611ba.xz
> 
> The issue was bisected to:
> 
> commit 9c94bbf9a87b264294f42e6cc0f76d87854733ec
> Author: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
> Date:   Mon Mar 13 21:55:52 2023 +0000
> 
>     xdp: recycle Page Pool backed skbs built from XDP frames
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11deec2ac80000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=13deec2ac80000
> console output: https://syzkaller.appspot.com/x/log.txt?x=15deec2ac80000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+e1d1b65f7c32f2a86a9f@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 9c94bbf9a87b ("xdp: recycle Page Pool backed skbs built from XDP frames")
> 
> BUG: kernel NULL pointer dereference, address: 0000000000000d28
> #PF: supervisor write access in kernel mode
> #PF: error_code(0x0002) - not-present page
> PGD 7b741067 P4D 7b741067 PUD 7c1ca067 PMD 0 
> Oops: 0002 [#1] PREEMPT SMP KASAN
> CPU: 1 PID: 5080 Comm: syz-executor371 Not tainted 6.2.0-syzkaller-13030-g3c2611bac08a #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/02/2023
> RIP: 0010:memset_erms+0xd/0x20 arch/x86/lib/memset_64.S:66
> Code: 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 66 0f 1f 00 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 66 0f 1f
> RSP: 0018:ffffc90003baf730 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff888028b94000 RCX: 0000000000000020
> RDX: 0000000000000020 RSI: 0000000000000000 RDI: 0000000000000d28
> RBP: 0000000000000001 R08: 0000000000000001 R09: 0000000000000d28
> R10: ffffed100517281c R11: 0000000000094001 R12: 0000000000000d48
> R13: 0000000000000d28 R14: 0000000000000f68 R15: 0000000000000100
> FS:  0000555555979300(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000d28 CR3: 0000000028e2d000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  __finalize_skb_around net/core/skbuff.c:321 [inline]
>  __build_skb_around+0x232/0x3a0 net/core/skbuff.c:379
>  build_skb_around+0x32/0x290 net/core/skbuff.c:444
>  __xdp_build_skb_from_frame+0x121/0x760 net/core/xdp.c:622
>  xdp_recv_frames net/bpf/test_run.c:248 [inline]
>  xdp_test_run_batch net/bpf/test_run.c:334 [inline]
>  bpf_test_run_xdp_live+0x1289/0x1930 net/bpf/test_run.c:362
>  bpf_prog_test_run_xdp+0xa05/0x14e0 net/bpf/test_run.c:1418
>  bpf_prog_test_run kernel/bpf/syscall.c:3675 [inline]
>  __sys_bpf+0x1598/0x5100 kernel/bpf/syscall.c:5028
>  __do_sys_bpf kernel/bpf/syscall.c:5114 [inline]
>  __se_sys_bpf kernel/bpf/syscall.c:5112 [inline]
>  __x64_sys_bpf+0x79/0xc0 kernel/bpf/syscall.c:5112
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f320b4efca9
> Code: 28 c3 e8 2a 14 00 00 66 2e 0f 1f 84 00 00 00 00 00 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 c0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffd2c9924d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f320b4efca9
> RDX: 0000000000000048 RSI: 0000000020000080 RDI: 000000000000000a
> RBP: 00007f320b4b3e50 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f320b4b3ee0
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>  </TASK>
> Modules linked in:
> CR2: 0000000000000d28
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:memset_erms+0xd/0x20 arch/x86/lib/memset_64.S:66
> Code: 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 66 0f 1f 00 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 66 0f 1f
> RSP: 0018:ffffc90003baf730 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff888028b94000 RCX: 0000000000000020
> RDX: 0000000000000020 RSI: 0000000000000000 RDI: 0000000000000d28
> RBP: 0000000000000001 R08: 0000000000000001 R09: 0000000000000d28
> R10: ffffed100517281c R11: 0000000000094001 R12: 0000000000000d48
> R13: 0000000000000d28 R14: 0000000000000f68 R15: 0000000000000100
> FS:  0000555555979300(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000d28 CR3: 0000000028e2d000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> ----------------
> Code disassembly (best guess), 1 bytes skipped:
>    0:	48 0f af c6          	imul   %rsi,%rax
>    4:	f3 48 ab             	rep stos %rax,%es:(%rdi)
>    7:	89 d1                	mov    %edx,%ecx
>    9:	f3 aa                	rep stos %al,%es:(%rdi)
>    b:	4c 89 c8             	mov    %r9,%rax
>    e:	c3                   	retq
>    f:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>   16:	00 00 00 00
>   1a:	66 90                	xchg   %ax,%ax
>   1c:	66 0f 1f 00          	nopw   (%rax)
>   20:	49 89 f9             	mov    %rdi,%r9
>   23:	40 88 f0             	mov    %sil,%al
>   26:	48 89 d1             	mov    %rdx,%rcx
> * 29:	f3 aa                	rep stos %al,%es:(%rdi) <-- trapping instruction

Looks like skb_shinfo() returns %NULL inside __finalize_skb_around(). My
code didn't touch this at all, but I'm digging this already anyway :s

+ Toke, test_run author :p

>   2b:	4c 89 c8             	mov    %r9,%rax
>   2e:	c3                   	retq
>   2f:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>   36:	00 00 00 00
>   3a:	66 90                	xchg   %ax,%ax
>   3c:	66                   	data16
>   3d:	0f                   	.byte 0xf
>   3e:	1f                   	(bad)
> 
> 
> ---
> 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.
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> syzbot can test patches for this issue, for details see:
> https://goo.gl/tpsmEJ#testing-patches

Thanks,
Olek