linux-kernel - [PATCH 4.19 087/103] bpf: add per-insn complexity limit

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Date:   Tue, 29 Jan 2019 12:36:04 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Edward Cree <ecree@...arflare.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.19 087/103] bpf: add per-insn complexity limit

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

[ commit ceefbc96fa5c5b975d87bf8e89ba8416f6b764d9 upstream ]

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@...nel.org>
Acked-by: Daniel Borkmann <daniel@...earbox.net>
Acked-by: Edward Cree <ecree@...arflare.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/bpf/verifier.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3d093003c723..2bbb98535b70 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -156,6 +156,7 @@ 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) +	\
@@ -4735,7 +4736,7 @@ 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;
-	int i, j, err;
+	int i, j, err, states_cnt = 0;
 
 	sl = env->explored_states[insn_idx];
 	if (!sl)
@@ -4762,8 +4763,12 @@ 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)
-- 
2.19.1