From: "Kurt Manucredo" <fuzzybritches0@gmail.com>
To: yhs@fb.com, alexei.starovoitov@gmail.com
Cc: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com,
andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org,
daniel@iogearbox.net, davem@davemloft.net, hawk@kernel.org,
john.fastabend@gmail.com, kafai@fb.com, kpsingh@kernel.org,
kuba@kernel.org, linux-kernel@vger.kernel.org,
netdev@vger.kernel.org, songliubraving@fb.com,
syzkaller-bugs@googlegroups.com, nathan@kernel.org,
ndesaulniers@google.com, clang-built-linux@googlegroups.com,
linux-kernel-mentees@lists.linuxfoundation.org,
skhan@linuxfoundation.org, gregkh@linuxfoundation.org
Subject: Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run
Date: Sun, 06 Jun 2021 21:44:32 +0200 [thread overview]
Message-ID: <10175-15986-curtm@phaethon> (raw)
In-Reply-To: <f045d171-15ff-8755-bcb7-4e20ca79b28a@fb.com>
On Sat, 5 Jun 2021 14:39:57 -0700, Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 6/5/21 12:10 PM, Alexei Starovoitov 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.
> >>
> >> This is not enough. We need more information on why this happens
> >> so we can judge whether the patch indeed fixed the issue.
> >>
> >>>
> >>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid
> >>> missing them and return with error when detected.
> >>>
> >>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com
> >>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com>
> >>> ---
> >>>
> >>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231
> >>>
> >>> Changelog:
> >>> ----------
> >>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals.
> >>> Fix commit message.
> >>> v3 - Make it clearer what the fix is for.
> >>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
> >>> check in check_alu_op() in verifier.c.
> >>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
> >>> check in ___bpf_prog_run().
> >>>
> >>> thanks
> >>>
> >>> kind regards
> >>>
> >>> Kurt
> >>>
> >>> kernel/bpf/verifier.c | 30 +++++++++---------------------
> >>> 1 file changed, 9 insertions(+), 21 deletions(-)
> >>>
> >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >>> index 94ba5163d4c5..ed0eecf20de5 100644
> >>> --- a/kernel/bpf/verifier.c
> >>> +++ b/kernel/bpf/verifier.c
> >>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> >>> u32_min_val = src_reg.u32_min_value;
> >>> u32_max_val = src_reg.u32_max_value;
> >>>
> >>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) &&
> >>> + umax_val >= insn_bitness) {
> >>> + /* Shifts greater than 31 or 63 are undefined.
> >>> + * This includes shifts by a negative number.
> >>> + */
> >>> + verbose(env, "invalid shift %lldn", umax_val);
> >>> + return -EINVAL;
> >>> + }
> >>
> >> I think your fix is good. I would like to move after
> >
> > I suspect such change will break valid programs that do shift by register.
>
> Oh yes, you are correct. We should guard it with src_known.
> But this should be extremely rare with explicit shifting amount being
> greater than 31/64 and if it is the case, the compiler will has a
> warning.
>
> >
> >> the following code though:
> >>
> >> if (!src_known &&
> >> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
> >> __mark_reg_unknown(env, dst_reg);
> >> return 0;
> >> }
> >>
> >>> +
> >>> if (alu32) {
> >>> src_known = tnum_subreg_is_const(src_reg.var_off);
> >>> if ((src_known &&
> >>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> >>> scalar_min_max_xor(dst_reg, &src_reg);
> >>> break;
> >>> case BPF_LSH:
> >>> - if (umax_val >= insn_bitness) {
> >>> - /* Shifts greater than 31 or 63 are undefined.
> >>> - * This includes shifts by a negative number.
> >>> - */
> >>> - mark_reg_unknown(env, regs, insn->dst_reg);
> >>> - break;
> >>> - }
> >>
> >> 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.
>
> Agree. This makes sense.
Thanks for your input. If you find I should look closer into this bug
just let me know. I'd love to help. If not it's fine, too. :-)
kind regards,
Kurt Manucredo
next prev parent reply other threads:[~2021-06-06 19:49 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-10 16:05 [syzbot] UBSAN: shift-out-of-bounds in ___bpf_prog_run syzbot
2021-03-28 3:38 ` syzbot
2021-06-02 21:27 ` [PATCH v3] bpf: core: fix " Kurt Manucredo
2021-06-03 4:43 ` Greg KH
2021-06-05 15:01 ` [PATCH v4] " Kurt Manucredo
2021-06-05 17:55 ` Yonghong Song
2021-06-05 19:10 ` Alexei Starovoitov
2021-06-05 21:39 ` Yonghong Song
2021-06-06 19:44 ` Kurt Manucredo [this message]
2021-06-07 7:38 ` Dmitry Vyukov
2021-06-09 18:20 ` Kees Cook
2021-06-09 23:40 ` Yonghong Song
2021-06-10 5:32 ` Dmitry Vyukov
2021-06-10 6:06 ` Yonghong Song
2021-06-10 17:06 ` Kees Cook
2021-06-10 17:52 ` Alexei Starovoitov
2021-06-10 20:00 ` Eric Biggers
2021-06-15 16:42 ` [PATCH v5] " Kurt Manucredo
2021-06-15 18:51 ` Edward Cree
2021-06-15 19:33 ` Eric Biggers
2021-06-15 21:08 ` Daniel Borkmann
2021-06-15 21:32 ` Eric Biggers
2021-06-15 21:38 ` Eric Biggers
2021-06-15 21:54 ` Daniel Borkmann
2021-06-15 22:07 ` Eric Biggers
2021-06-15 22:31 ` Kurt Manucredo
2021-06-17 10:09 ` Daniel Borkmann
2021-06-06 19:15 ` [PATCH v4] " Kurt Manucredo
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