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Date: Wed, 11 Nov 2020 14:33:11 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Dmitrii Banshchikov <me@...que.spb.ru>
Cc: bpf <bpf@...r.kernel.org>, Kernel Team <kernel-team@...com>,
Andrey Ignatov <rdna@...com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
Yonghong Song <yhs@...com>,
Andrii Nakryiko <andrii@...nel.org>,
john fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...omium.org>,
Toke Høiland-Jørgensen <toke@...hat.com>,
Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH] bpf: relax return code check for subprograms
On Wed, Nov 11, 2020 at 2:38 AM Dmitrii Banshchikov <me@...que.spb.ru> wrote:
>
> On Tue, Nov 10, 2020 at 08:47:13PM -0800, Andrii Nakryiko wrote:
> > On Tue, Nov 10, 2020 at 1:03 PM Dmitrii Banshchikov <me@...que.spb.ru> wrote:
> > >
> > > Currently verifier enforces return code checks for subprograms in the
> > > same manner as it does for program entry points. This prevents returning
> > > arbitrary scalar values from subprograms. Scalar type of returned values
> > > is checked by btf_prepare_func_args() and hence it should be safe to
> > > allow only scalars for now. Relax return code checks for subprograms and
> > > allow any correct scalar values.
> > >
> > > Signed-off-by: Dmitrii Banshchikov <me@...que.spb.ru>
> > > Fixes: 51c39bb1d5d10 (bpf: Introduce function-by-function verification)
> > > ---
> >
> > Please make sure that your subject has [PATCH bpf-next], if it's
> > targeted against bpf-next tree.
> >
> > > kernel/bpf/verifier.c | 26 ++++++++++++++-----
> > > .../bpf/prog_tests/test_global_funcs.c | 1 +
> > > .../selftests/bpf/progs/test_global_func8.c | 25 ++++++++++++++++++
> > > 3 files changed, 45 insertions(+), 7 deletions(-)
> > > create mode 100644 tools/testing/selftests/bpf/progs/test_global_func8.c
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 10da26e55130..c108b19e1fad 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -7791,7 +7791,7 @@ static int check_ld_abs(struct bpf_verifier_env *env, struct bpf_insn *insn)
> > > return 0;
> > > }
> > >
> > > -static int check_return_code(struct bpf_verifier_env *env)
> > > +static int check_return_code(struct bpf_verifier_env *env, bool is_subprog)
> > > {
> > > struct tnum enforce_attach_type_range = tnum_unknown;
> > > const struct bpf_prog *prog = env->prog;
> > > @@ -7801,10 +7801,12 @@ static int check_return_code(struct bpf_verifier_env *env)
> > > int err;
> > >
> > > /* LSM and struct_ops func-ptr's return type could be "void" */
> > > - if ((prog_type == BPF_PROG_TYPE_STRUCT_OPS ||
> > > - prog_type == BPF_PROG_TYPE_LSM) &&
> > > - !prog->aux->attach_func_proto->type)
> > > - return 0;
> > > + if (!is_subprog) {
> >
> > I think just adding `!is_subprog` && to existing if is cleaner and
> > more succinct.
> >
> > > + if ((prog_type == BPF_PROG_TYPE_STRUCT_OPS ||
> > > + prog_type == BPF_PROG_TYPE_LSM) &&
> > > + !prog->aux->attach_func_proto->type)
> > > + return 0;
> > > + }
> > >
> > > /* eBPF calling convetion is such that R0 is used
> > > * to return the value from eBPF program.
> > > @@ -7821,6 +7823,16 @@ static int check_return_code(struct bpf_verifier_env *env)
> > > return -EACCES;
> > > }
> > >
> > > + reg = cur_regs(env) + BPF_REG_0;
> > > + if (is_subprog) {
> > > + if (reg->type != SCALAR_VALUE) {
> > > + verbose(env, "At subprogram exit the register R0 is not a scalar value (%s)\n",
> > > + reg_type_str[reg->type]);
> > > + return -EINVAL;
> > > + }
> > > + return 0;
> > > + }
> > > +
> >
> > It's not clear why reg->type != SCALAR_VALUE check is done after
> > prog_type-specific check. Is there any valid case where we'd allow
> > non-scalar return? Maybe Alexei can chime in here.
> >
> > If not, then I'd just move the existing SCALAR_VALUE check below up
> > here, unconditionally for subprog and non-subprog. And then just exit
> > after that, if we are processing a subprog.
>
> As comment says BPF_PROG_TYPE_STRUCT_OPS and BPF_PROG_TYPE_LSM
> progs may return void. Hence we want allow this only for
> entry points and not for subprograms as btf_prepare_func_args()
> guarantees that subprogram return value has SCALAR type.
>
> Beside that there are other cases when SCALAR type is not
> enforced for return value: e.g. BPF_PROG_TYPE_TRACING with
> BPF_MODIFY_RETURN expected attach type.
I'm surprised we allow returning FP or map_value pointers or something
like that, regardless of program type. Which is why I raised this
question. But that's a separate topic, let's punt it.
>
> >
> > > switch (prog_type) {
> > > case BPF_PROG_TYPE_CGROUP_SOCK_ADDR:
> > > if (env->prog->expected_attach_type == BPF_CGROUP_UDP4_RECVMSG ||
> > > @@ -7874,7 +7886,6 @@ static int check_return_code(struct bpf_verifier_env *env)
> > > return 0;
> > > }
> > >
> > > - reg = cur_regs(env) + BPF_REG_0;
> > > if (reg->type != SCALAR_VALUE) {
> > > verbose(env, "At program exit the register R0 is not a known value (%s)\n",
> > > reg_type_str[reg->type]);
> > > @@ -9266,6 +9277,7 @@ static int do_check(struct bpf_verifier_env *env)
> > > int insn_cnt = env->prog->len;
> > > bool do_print_state = false;
> > > int prev_insn_idx = -1;
> > > + const bool is_subprog = env->cur_state->frame[0]->subprogno;
> >
> > this can probably be done inside check_return_code(), no?
>
> No.
> Frame stack may be empty when check_return_code() is called.
How can that happen? check_reg_arg() in check_return_code() relies on
having a frame available. So does cur_regs() function, also used
there. What am I missing?
>
>
> >
> > >
> > > for (;;) {
> > > struct bpf_insn *insn;
> > > @@ -9530,7 +9542,7 @@ static int do_check(struct bpf_verifier_env *env)
> > > if (err)
> > > return err;
> > >
> > > - err = check_return_code(env);
> > > + err = check_return_code(env, is_subprog);
> > > if (err)
> > > return err;
> > > process_bpf_exit:
> > > diff --git a/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c b/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c
> > > index 193002b14d7f..32e4348b714b 100644
> > > --- a/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c
> > > +++ b/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c
> > > @@ -60,6 +60,7 @@ void test_test_global_funcs(void)
> > > { "test_global_func5.o" , "expected pointer to ctx, but got PTR" },
> > > { "test_global_func6.o" , "modified ctx ptr R2" },
> > > { "test_global_func7.o" , "foo() doesn't return scalar" },
> > > + { "test_global_func8.o" },
> > > };
> > > libbpf_print_fn_t old_print_fn = NULL;
> > > int err, i, duration = 0;
> > > diff --git a/tools/testing/selftests/bpf/progs/test_global_func8.c b/tools/testing/selftests/bpf/progs/test_global_func8.c
> > > new file mode 100644
> > > index 000000000000..1e9a87f30b7c
> > > --- /dev/null
> > > +++ b/tools/testing/selftests/bpf/progs/test_global_func8.c
> > > @@ -0,0 +1,25 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +/* Copyright (c) 2020 Facebook */
> > > +#include <stddef.h>
> > > +#include <linux/bpf.h>
> > > +#include <bpf/bpf_helpers.h>
> > > +
> > > +__attribute__ ((noinline))
> >
> > nit: use __noinline, it's defined in bpf_helpers.h
> >
> > > +int bar(struct __sk_buff *skb)
> > > +{
> > > + return bpf_get_prandom_u32();
> > > +}
> > > +
> > > +static __always_inline int foo(struct __sk_buff *skb)
> >
> > foo is not essential, just inline it in test_cls below
> >
> > > +{
> > > + if (!bar(skb))
> > > + return 0;
> > > +
> > > + return 1;
> > > +}
> > > +
> > > +SEC("cgroup_skb/ingress")
> > > +int test_cls(struct __sk_buff *skb)
> > > +{
> > > + return foo(skb);
> > > +}
> >
> > I also wonder what happens if __noinline function has return type
> > void? Do you mind adding another BPF program that uses non-inline
> > global void function? We might need to handle that case in the
> > verifier explicitly.
>
> btf_prepare_func_args() guarantees that a subprogram may have only
> SCALAR return type.
Right, I didn't know about this, thanks. We might want to lift that
restriction eventually.
>
> >
> >
> > > --
> > > 2.24.1
> > >
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