bpf: add per-insn complexity limit

malicious bpf program may try to force the verifier to remember
a lot of distinct verifier states.
Put a limit to number of per-insn 'struct bpf_verifier_state'.
Note that hitting the limit doesn't reject the program.
It potentially makes the verifier do more steps to analyze the program.
It means that malicious programs will hit BPF_COMPLEXITY_LIMIT_INSNS sooner
instead of spending cpu time walking long link list.

The limit of BPF_COMPLEXITY_LIMIT_STATES==64 affects cilium progs
with slight increase in number of "steps" it takes to successfully verify
the programs:
                       before    after
bpf_lb-DLB_L3.o         1940      1940
bpf_lb-DLB_L4.o         3089      3089
bpf_lb-DUNKNOWN.o       1065      1065
bpf_lxc-DDROP_ALL.o     28052  |  28162
bpf_lxc-DUNKNOWN.o      35487  |  35541
bpf_netdev.o            10864     10864
bpf_overlay.o           6643      6643
bpf_lcx_jit.o           38437     38437

But it also makes malicious program to be rejected in 0.4 seconds vs 6.5
Hence apply this limit to unprivileged programs only.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 55a49703f423e28689827575b7182bdcd06ba651..fc760d00a38c497502c28b56aad1a8d426565560 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -175,6 +175,7 @@ struct bpf_verifier_stack_elem {
 
 #define BPF_COMPLEXITY_LIMIT_INSNS     131072
 #define BPF_COMPLEXITY_LIMIT_STACK     1024
+#define BPF_COMPLEXITY_LIMIT_STATES    64
 
 #define BPF_MAP_PTR_UNPRIV     1UL
 #define BPF_MAP_PTR_POISON     ((void *)((0xeB9FUL << 1) +     \
@@ -5047,7 +5048,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
        struct bpf_verifier_state_list *new_sl;
        struct bpf_verifier_state_list *sl;
        struct bpf_verifier_state *cur = env->cur_state, *new;
-       int i, j, err;
+       int i, j, err, states_cnt = 0;
 
        sl = env->explored_states[insn_idx];
        if (!sl)
@@ -5074,8 +5075,12 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
                        return 1;
                }
                sl = sl->next;
+               states_cnt++;
        }
 
+       if (!env->allow_ptr_leaks && states_cnt > BPF_COMPLEXITY_LIMIT_STATES)
+               return 0;
+
        /* there were no equivalent states, remember current one.
         * technically the current state is not proven to be safe yet,
         * but it will either reach outer most bpf_exit (which means it's safe)