stack access issue. Re: [syzbot] [bpf?] UBSAN: array-index-out-of-bounds in check_stack_range_initialized

[Alexei Starovoitov]

Hi Andrei,

looks like the refactoring of stack access introduced a bug.
See the reproducer below.
positive offsets are not caught by check_stack_access_within_bounds().
So both slot and spi become negative and access
stack[spi].slot_type[slot % BPF_REG_SIZE]
returns garbage.

On Tue, Mar 19, 2024 at 11:12 AM syzbot
<syzbot+33f4297b5f927648741a@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    0740b6427e90 Merge branch 'bpf-arena-followups'
> git tree:       bpf
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=12fed769180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=6fb1be60a193d440
> dashboard link: https://syzkaller.appspot.com/bug?extid=33f4297b5f927648741a
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1763a479180000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15c38711180000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/c9e6e9f97566/disk-0740b642.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/78476a588b62/vmlinux-0740b642.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/50cd6fab9ead/bzImage-0740b642.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+33f4297b5f927648741a@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ------------[ cut here ]------------
> UBSAN: array-index-out-of-bounds in kernel/bpf/verifier.c:7190:12
> index -1 is out of range for type 'u8[8]' (aka 'unsigned char[8]')
> CPU: 0 PID: 5071 Comm: syz-executor474 Not tainted 6.8.0-syzkaller-05226-g0740b6427e90 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0x1e7/0x2e0 lib/dump_stack.c:106
>  ubsan_epilogue lib/ubsan.c:217 [inline]
>  __ubsan_handle_out_of_bounds+0x121/0x150 lib/ubsan.c:415
>  check_stack_range_initialized+0x1668/0x19a0 kernel/bpf/verifier.c:7190
>  check_helper_mem_access+0x2eb/0xfa0 kernel/bpf/verifier.c:7294
>  check_helper_call+0x263c/0x7220 kernel/bpf/verifier.c:10252
>  do_check+0x9e29/0x10530 kernel/bpf/verifier.c:17801
>  do_check_common+0x14bd/0x1dd0 kernel/bpf/verifier.c:20500
>  do_check_main kernel/bpf/verifier.c:20591 [inline]
>  bpf_check+0x136ab/0x19010 kernel/bpf/verifier.c:21261
>  bpf_prog_load+0x1667/0x20f0 kernel/bpf/syscall.c:2895
>  __sys_bpf+0x4ee/0x810 kernel/bpf/syscall.c:5631
>  __do_sys_bpf kernel/bpf/syscall.c:5738 [inline]
>  __se_sys_bpf kernel/bpf/syscall.c:5736 [inline]
>  __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5736
>  do_syscall_64+0xfb/0x240
>  entry_SYSCALL_64_after_hwframe+0x6d/0x75
> RIP: 0033:0x7f8416194629
> Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 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 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffdc6f0fdb8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> RAX: ffffffffffffffda RBX: 00007ffdc6f0ff88 RCX: 00007f8416194629
> RDX: 0000000000000090 RSI: 00000000200000c0 RDI: 0000000000000005
> RBP: 00007f8416207610 R08: 0000000000000000 R09: 00007ffdc6f0ff88
> R10: 00000000fffffff8 R11: 0000000000000246 R12: 0000000000000001
> R13: 00007ffdc6f0ff78 R14: 0000000000000001 R15: 0000000000000001
>  </TASK>
> ---[ end trace ]---
>
>
> ---
> 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 report is already addressed, 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 report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
>
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report
>
> If you want to undo deduplication, reply with:
> #syz undup