On Tue, Nov 10, 2020 at 08:47:13PM -0800, Andrii Nakryiko wrote:
> On Tue, Nov 10, 2020 at 1:03 PM Dmitrii Banshchikov <[email protected]>
> 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 <[email protected]>
> > 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.
>
> > 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.
>
> >
> > 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.
>
>
> > --
> > 2.24.1
> >
