From aa30eb3260b2dea3a68d3c42a39f9a09c5e99cee Mon Sep 17 00:00:00 2001 From: Eduard Zingerman Date: Tue, 29 Oct 2024 10:26:40 -0700 Subject: [PATCH] bpf: Force checkpoint when jmp history is too long A specifically crafted program might trick verifier into growing very long jump history within a single bpf_verifier_state instance. Very long jump history makes mark_chain_precision() unreasonably slow, especially in case if verifier processes a loop. Mitigate this by forcing new state in is_state_visited() in case if current state's jump history is too long. Use same constant as in `skip_inf_loop_check`, but multiply it by arbitrarily chosen value 2 to account for jump history containing not only information about jumps, but also information about stack access. For an example of problematic program consider the code below, w/o this patch the example is processed by verifier for ~15 minutes, before failing to allocate big-enough chunk for jmp_history. 0: r7 = *(u16 *)(r1 +0);" 1: r7 += 0x1ab064b9;" 2: if r7 & 0x702000 goto 1b; 3: r7 &= 0x1ee60e;" 4: r7 += r1;" 5: if r7 s> 0x37d2 goto +0;" 6: r0 = 0;" 7: exit;" Perf profiling shows that most of the time is spent in mark_chain_precision() ~95%. The easiest way to explain why this program causes problems is to apply the following patch: diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 0c216e71cec7..4b4823961abe 100644 \--- a/include/linux/bpf.h \+++ b/include/linux/bpf.h \@@ -1926,7 +1926,7 @@ struct bpf_array { }; }; -#define BPF_COMPLEXITY_LIMIT_INSNS 1000000 /* yes. 1M insns */ +#define BPF_COMPLEXITY_LIMIT_INSNS 256 /* yes. 1M insns */ #define MAX_TAIL_CALL_CNT 33 /* Maximum number of loops for bpf_loop and bpf_iter_num. diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index f514247ba8ba..75e88be3bb3e 100644 \--- a/kernel/bpf/verifier.c \+++ b/kernel/bpf/verifier.c \@@ -18024,8 +18024,13 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) skip_inf_loop_check: if (!force_new_state && env->jmps_processed - env->prev_jmps_processed < 20 && - env->insn_processed - env->prev_insn_processed < 100) + env->insn_processed - env->prev_insn_processed < 100) { + verbose(env, "is_state_visited: suppressing checkpoint at %d, %d jmps processed, cur->jmp_history_cnt is %d\n", + env->insn_idx, + env->jmps_processed - env->prev_jmps_processed, + cur->jmp_history_cnt); add_new_state = false; + } goto miss; } /* If sl->state is a part of a loop and this loop's entry is a part of \@@ -18142,6 +18147,9 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) if (!add_new_state) return 0; + verbose(env, "is_state_visited: new checkpoint at %d, resetting env->jmps_processed\n", + env->insn_idx); + /* There were no equivalent states, remember the 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) And observe verification log: ... is_state_visited: new checkpoint at 5, resetting env->jmps_processed 5: R1=ctx() R7=ctx(...) 5: (65) if r7 s> 0x37d2 goto pc+0 ; R7=ctx(...) 6: (b7) r0 = 0 ; R0_w=0 7: (95) exit from 5 to 6: R1=ctx() R7=ctx(...) R10=fp0 6: R1=ctx() R7=ctx(...) R10=fp0 6: (b7) r0 = 0 ; R0_w=0 7: (95) exit is_state_visited: suppressing checkpoint at 1, 3 jmps processed, cur->jmp_history_cnt is 74 from 2 to 1: R1=ctx() R7_w=scalar(...) R10=fp0 1: R1=ctx() R7_w=scalar(...) R10=fp0 1: (07) r7 += 447767737 is_state_visited: suppressing checkpoint at 2, 3 jmps processed, cur->jmp_history_cnt is 75 2: R7_w=scalar(...) 2: (45) if r7 & 0x702000 goto pc-2 ... mark_precise 152 steps for r7 ... 2: R7_w=scalar(...) is_state_visited: suppressing checkpoint at 1, 4 jmps processed, cur->jmp_history_cnt is 75 1: (07) r7 += 447767737 is_state_visited: suppressing checkpoint at 2, 4 jmps processed, cur->jmp_history_cnt is 76 2: R7_w=scalar(...) 2: (45) if r7 & 0x702000 goto pc-2 ... BPF program is too large. Processed 257 insn The log output shows that checkpoint at label (1) is never created, because it is suppressed by `skip_inf_loop_check` logic: a. When 'if' at (2) is processed it pushes a state with insn_idx (1) onto stack and proceeds to (3); b. At (5) checkpoint is created, and this resets env->{jmps,insns}_processed. c. Verification proceeds and reaches `exit`; d. State saved at step (a) is popped from stack and is_state_visited() considers if checkpoint needs to be added, but because env->{jmps,insns}_processed had been just reset at step (b) the `skip_inf_loop_check` logic forces `add_new_state` to false. e. Verifier proceeds with current state, which slowly accumulates more and more entries in the jump history. The accumulation of entries in the jump history is a problem because of two factors: - it eventually exhausts memory available for kmalloc() allocation; - mark_chain_precision() traverses the jump history of a state, meaning that if `r7` is marked precise, verifier would iterate ever growing jump history until parent state boundary is reached. (note: the log also shows a REG INVARIANTS VIOLATION warning upon jset processing, but that's another bug to fix). With this patch applied, the example above is rejected by verifier under 1s of time, reaching 1M instructions limit. The program is a simplified reproducer from syzbot report. Previous discussion could be found at [1]. The patch does not cause any changes in verification performance, when tested on selftests from veristat.cfg and cilium programs taken from [2]. [1] https://lore.kernel.org/bpf/20241009021254.2805446-1-eddyz87@gmail.com/ [2] https://github.com/anakryiko/cilium Changelog: - v1 -> v2: - moved patch to bpf tree; - moved force_new_state variable initialization after declaration and shortened the comment. v1: https://lore.kernel.org/bpf/20241018020307.1766906-1-eddyz87@gmail.com/ Fixes: 2589726d12a1 ("bpf: introduce bounded loops") Reported-by: syzbot+7e46cdef14bf496a3ab4@syzkaller.appspotmail.com Signed-off-by: Eduard Zingerman Signed-off-by: Andrii Nakryiko Acked-by: Daniel Borkmann Link: https://lore.kernel.org/bpf/20241029172641.1042523-1-eddyz87@gmail.com Closes: https://lore.kernel.org/bpf/670429f6.050a0220.49194.0517.GAE@google.com/ --- kernel/bpf/verifier.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 587a6c76e564..3254c27085b8 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -17886,9 +17886,11 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) struct bpf_verifier_state_list *sl, **pprev; struct bpf_verifier_state *cur = env->cur_state, *new, *loop_entry; int i, j, n, err, states_cnt = 0; - bool force_new_state = env->test_state_freq || is_force_checkpoint(env, insn_idx); - bool add_new_state = force_new_state; - bool force_exact; + bool force_new_state, add_new_state, force_exact; + + force_new_state = env->test_state_freq || is_force_checkpoint(env, insn_idx) || + /* Avoid accumulating infinitely long jmp history */ + cur->jmp_history_cnt > 40; /* bpf progs typically have pruning point every 4 instructions * http://vger.kernel.org/bpfconf2019.html#session-1 @@ -17898,6 +17900,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) * In tests that amounts to up to 50% reduction into total verifier * memory consumption and 20% verifier time speedup. */ + add_new_state = force_new_state; if (env->jmps_processed - env->prev_jmps_processed >= 2 && env->insn_processed - env->prev_insn_processed >= 8) add_new_state = true;