On Wed, Apr 28, 2021 at 9:19 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> The recursion check in __bpf_prog_enter and __bpf_prog_exit
> leaves some (not inlined) functions unprotected:
>
> In __bpf_prog_enter:
> - migrate_disable is called before prog->active is checked
>
> In __bpf_prog_exit:
> - migrate_enable,rcu_read_unlock_strict are called after
> prog->active is decreased
>
> When attaching trampoline to them we get panic like:
>
> traps: PANIC: double fault, error_code: 0x0
> double fault: 0000 [#1] SMP PTI
> RIP: 0010:__bpf_prog_enter+0x4/0x50
> ...
> Call Trace:
> <IRQ>
> bpf_trampoline_6442466513_0+0x18/0x1000
> migrate_disable+0x5/0x50
> __bpf_prog_enter+0x9/0x50
> bpf_trampoline_6442466513_0+0x18/0x1000
> migrate_disable+0x5/0x50
> __bpf_prog_enter+0x9/0x50
> bpf_trampoline_6442466513_0+0x18/0x1000
> migrate_disable+0x5/0x50
> __bpf_prog_enter+0x9/0x50
> bpf_trampoline_6442466513_0+0x18/0x1000
> migrate_disable+0x5/0x50
> ...
>
> Fixing this by adding deny list of btf ids for tracing
> and ext programs and checking btf id during program
> verification. Adding above functions to this list.
>
> Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
> kernel/bpf/verifier.c | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 2579f6fbb5c3..4ffd64eaffda 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -13112,6 +13112,17 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
> return 0;
> }
>
> +BTF_SET_START(btf_id_deny)
> +BTF_ID_UNUSED
> +#ifdef CONFIG_SMP
> +BTF_ID(func, migrate_disable)
> +BTF_ID(func, migrate_enable)
> +#endif
> +#if !defined CONFIG_PREEMPT_RCU && !defined CONFIG_TINY_RCU
> +BTF_ID(func, rcu_read_unlock_strict)
> +#endif
> +BTF_SET_END(btf_id_deny)
> +
> static int check_attach_btf_id(struct bpf_verifier_env *env)
> {
> struct bpf_prog *prog = env->prog;
> @@ -13171,6 +13182,10 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> ret = bpf_lsm_verify_prog(&env->log, prog);
> if (ret < 0)
> return ret;
> + } else if ((prog->type == BPF_PROG_TYPE_TRACING ||
> + prog->type == BPF_PROG_TYPE_EXT) &&
BPF_PROG_TYP_EXT can only replace other BPF programs/subprograms, it
can't replace kernel functions, so the deny list shouldn't be checked
for them.
> + btf_id_set_contains(&btf_id_deny, btf_id)) {
> + return -EINVAL;
> }
>
> key = bpf_trampoline_compute_key(tgt_prog, prog->aux->attach_btf, btf_id);
> --
> 2.30.2
>
