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SubjectRe: [PATCH bpf-next 4/7] bpf: Attachment verification for BPF_MODIFY_RETURN
On 03-Mär 14:44, Andrii Nakryiko wrote:
> On Tue, Mar 3, 2020 at 6:12 AM KP Singh <kpsingh@chromium.org> wrote:
> >
> > From: KP Singh <kpsingh@google.com>
> >
> > - Functions that are whitlisted by for error injection i.e.
> > within_error_injection_list.
> > - Security hooks, this is expected to be cleaned up after the KRSI
> > patches introduce the LSM_HOOK macro:
> >
> > https://lore.kernel.org/bpf/20200220175250.10795-1-kpsingh@chromium.org/
>
> Commit message can use a bit more work for sure. Why (and even what)
> of the changes is not really explained well.

Added some more details.

>
> >
> > - The attachment is currently limited to functions that return an int.
> > This can be extended later other types (e.g. PTR).
> >
> > Signed-off-by: KP Singh <kpsingh@google.com>
> > ---
> > kernel/bpf/btf.c | 28 ++++++++++++++++++++--------
> > kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
> > 2 files changed, 51 insertions(+), 8 deletions(-)
> >

[...]

> > + t = btf_type_skip_modifiers(btf, t->type, NULL);
> > + if (!btf_type_is_int(t)) {
>
> Should the size of int be verified here? E.g., if some function
> returns u8, is that ok for BPF program to return, say, (1<<30) ?

Would this work?

if (size != t->size) {
bpf_log(log,
"size accessed = %d should be %d\n",
size, t->size);
return false;
}

- KP

>
> > + bpf_log(log,
> > + "ret type %s not allowed for fmod_ret\n",
> > + btf_kind_str[BTF_INFO_KIND(t->info)]);
> > + return false;
> > + }
> > + }
> > } else if (arg >= nr_args) {
> > bpf_log(log, "func '%s' doesn't have %d-th argument\n",
> > tname, arg + 1);
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 2460c8e6b5be..ae32517d4ccd 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -19,6 +19,7 @@
> > #include <linux/sort.h>
> > #include <linux/perf_event.h>
> > #include <linux/ctype.h>
> > +#include <linux/error-injection.h>
> >
> > #include "disasm.h"
> >
> > @@ -9800,6 +9801,33 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
> >
> > return 0;
> > }
> > +#define SECURITY_PREFIX "security_"
> > +
> > +static int check_attach_modify_return(struct bpf_verifier_env *env)
> > +{
> > + struct bpf_prog *prog = env->prog;
> > + unsigned long addr = (unsigned long) prog->aux->trampoline->func.addr;
> > +
> > + if (within_error_injection_list(addr))
> > + return 0;
> > +
> > + /* This is expected to be cleaned up in the future with the KRSI effort
> > + * introducing the LSM_HOOK macro for cleaning up lsm_hooks.h.
> > + */
> > + if (!strncmp(SECURITY_PREFIX, prog->aux->attach_func_name,
> > + sizeof(SECURITY_PREFIX) - 1)) {
> > +
> > + if (!capable(CAP_MAC_ADMIN))
> > + return -EPERM;
> > +
> > + return 0;
> > + }
> > +
> > + verbose(env, "fmod_ret attach_btf_id %u (%s) is not modifiable\n",
> > + prog->aux->attach_btf_id, prog->aux->attach_func_name);
> > +
> > + return -EINVAL;
> > +}
> >
> > static int check_attach_btf_id(struct bpf_verifier_env *env)
> > {
> > @@ -10000,6 +10028,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> > }
> > tr->func.addr = (void *)addr;
> > prog->aux->trampoline = tr;
> > +
> > + if (prog->expected_attach_type == BPF_MODIFY_RETURN)
> > + ret = check_attach_modify_return(env);
> > out:
> > mutex_unlock(&tr->mutex);
> > if (ret)
> > --
> > 2.20.1
> >

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