Date: Thu, 10 Jun 2021 10:06:42 -0700 From: Kees Cook <keescook@...omium.org> To: Yonghong Song <yhs@...com> Cc: Dmitry Vyukov <dvyukov@...gle.com>, Alexei Starovoitov <alexei.starovoitov@...il.com>, Kurt Manucredo <fuzzybritches0@...il.com>, syzbot+bed360704c521841c85d@...kaller.appspotmail.com, Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net>, "David S. Miller" <davem@...emloft.net>, Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, Martin KaFai Lau <kafai@...com>, KP Singh <kpsingh@...nel.org>, Jakub Kicinski <kuba@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, Song Liu <songliubraving@...com>, syzkaller-bugs <syzkaller-bugs@...glegroups.com>, nathan@...nel.org, Nick Desaulniers <ndesaulniers@...gle.com>, Clang-Built-Linux ML <clang-built-linux@...glegroups.com>, linux-kernel-mentees@...ts.linuxfoundation.org, Shuah Khan <skhan@...uxfoundation.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, kasan-dev <kasan-dev@...glegroups.com> Subject: Re: [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@...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@...il.com> wrote: > > > > > > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@...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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