Re: bpf: incorrect passing infinate loop causing rcu detected stall during bpf_prog_run()

[Hao Sun]

On Sun, Oct 29, 2023 at 2:35 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Fri, Oct 27, 2023 at 2:09 AM Hao Sun <sunhao.th@xxxxxxxxx> wrote:
> >
> > Hi,
> >
> > The following C repro contains a bpf program that can cause rcu
> > stall/soft lockup during running in bpf_prog_run(). Seems the verifier
> > incorrectly passed the program with an infinite loop.
> >
> > C repro: https://pastebin.com/raw/ymzAxjeU
>
> Thanks for the report.
> Did you debug what exactly caused this bug?
> Are you planning to work on the fix?

This bug is really hard to debug. Here is a simplified view of
the original program:

loop:
0: r4 = r8
1: r1 = 0x1f
2: r8 -= -8
3: if r1 > r7 goto pc+1
4: r7 <<= r1         ; LSH r7 by 31
5: r5 = r0
6: r5 *= 2
7: if r5 < r0 goto pc+1
8: r8 s>>= 6
9: w7 &= w7       ; r7 = 0 after the first iter
10: r8 -= r7
11: r8 -= -1
12: if r4 >= 0x9 goto loop
13: exit

At runtime, r7 is updated to 0 through #4 and #9 at the first iteration,
so the following iteration will not take #3 to #4, so #3 can be ignored
after the first iteration. r0 is init by get_current_task, and r5 is always
smaller than r0 at runtime, so #7 to #8 will never run. So, the update
to r8 is only #2 and #11, which together add 9 to r8. Since r4 is set
to r8 at the start of each iteration, so it's an infinite loop at runtime.

Based on the log, the verifier keeps tracking #7 to #8 and to #9, and
at some point, the verifier prunes states and path from #7 to #9, so
it stops checking. The log is huge and hard to follow, the issue is likely
in pruning logic, but I don't have much knowledge about that part.

>
> > Verifier's log: https://pastebin.com/raw/thZDTFJc
>
> log is trimmed.

Full log: https://pastebin.com/raw/cTC8wmDH