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

[Andrei Matei]

+ Edward

On Thu, Mar 21, 2024 at 3:33 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> 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().

check_stack_access_within_bounds() tries to catch positive offsets;
It does: [1]

err = check_stack_slot_within_bounds(env, min_off, state, type);
if (!err && max_off > 0)
  err = -EINVAL; /* out of stack access into non-negative offsets */

Notice the max_off > 0 in there.
And we have various tests that seem to check that positive offsets are
rejected. Do you know what the bug is?
I'm thinking maybe there's some overflow going on, except that UBSAN
reported an index of -1 as being the problem.

Edward, I see that you've been tickling the robot trying to narrow the issue;
perhaps you've figured it out?

If the bug is not immediately apparent to anyone, I would really appreciate a
bit of tutoring around how to reproduce and get verifier logs. I have tried a
bunch of cases of constant- and variable-offset accesses, and couldn't repro. I
can run syzkaller's repro on its own vm image, and indeed it crashes. But I'm
not sure how to get verifier logs out of the C reproducer.  Alternatively, I'm
not sure how to figure out the actual BPF program corresponding to the "syz
repro" in [2] and turn it into a test_progs test. How do you guys do it?

Thanks a lot!

[1] https://github.com/torvalds/linux/blob/70293240c5ce675a67bfc48f419b093023b862b3/kernel/bpf/verifier.c#L6695
[2] https://syzkaller.appspot.com/x/repro.syz?x=1763a479180000


> 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.
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> >
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