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    SubjectRe: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run
    On Wed, Jun 09, 2021 at 11:06:31PM -0700, Yonghong Song wrote:
    >
    >
    > On 6/9/21 10:32 PM, Dmitry Vyukov wrote:
    > > On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs@fb.com> wrote:
    > > > On 6/9/21 11:20 AM, Kees Cook wrote:
    > > > > On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote:
    > > > > > On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov
    > > > > > <alexei.starovoitov@gmail.com> wrote:
    > > > > > > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote:
    > > > > > > > On 6/5/21 8:01 AM, Kurt Manucredo wrote:
    > > > > > > > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run()
    > > > > > > > > kernel/bpf/core.c:1414:2.
    > > > > > > > [...]
    > > > > > > >
    > > > > > > > I think this is what happens. For the above case, we simply
    > > > > > > > marks the dst reg as unknown and didn't fail verification.
    > > > > > > > So later on at runtime, the shift optimization will have wrong
    > > > > > > > shift value (> 31/64). Please correct me if this is not right
    > > > > > > > analysis. As I mentioned in the early please write detailed
    > > > > > > > analysis in commit log.
    > > > > > >
    > > > > > > The large shift is not wrong. It's just undefined.
    > > > > > > syzbot has to ignore such cases.
    > > > > >
    > > > > > Hi Alexei,
    > > > > >
    > > > > > The report is produced by KUBSAN. I thought there was an agreement on
    > > > > > cleaning up KUBSAN reports from the kernel (the subset enabled on
    > > > > > syzbot at least).
    > > > > > What exactly cases should KUBSAN ignore?
    > > > > > +linux-hardening/kasan-dev for KUBSAN false positive
    > > > >
    > > > > Can check_shl_overflow() be used at all? Best to just make things
    > > > > readable and compiler-happy, whatever the implementation. :)
    > > >
    > > > This is not a compile issue. If the shift amount is a constant,
    > > > compiler should have warned and user should fix the warning.
    > > >
    > > > This is because user code has
    > > > something like
    > > > a << s;
    > > > where s is a unknown variable and
    > > > verifier just marked the result of a << s as unknown value.
    > > > Verifier may not reject the code depending on how a << s result
    > > > is used.

    Ah, gotcha: it's the BPF code itself that needs to catch it.

    > > > If bpf program writer uses check_shl_overflow() or some kind
    > > > of checking for shift value and won't do shifting if the
    > > > shifting may cause an undefined result, there should not
    > > > be any kubsan warning.

    Right.

    > > I guess the main question: what should happen if a bpf program writer
    > > does _not_ use compiler nor check_shl_overflow()?

    I think the BPF runtime needs to make such actions defined, instead of
    doing a blind shift. It needs to check the size of the shift explicitly
    when handling the shift instruction.

    > If kubsan is not enabled, everything should work as expected even with
    > shl overflow may cause undefined result.
    >
    > if kubsan is enabled, the reported shift-out-of-bounds warning
    > should be ignored. You could disasm the insn to ensure that
    > there indeed exists a potential shl overflow.

    Sure, but the point of UBSAN is to find and alert about undefined
    behavior, so we still need to fix this.


    --
    Kees Cook

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