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Message-ID: <522C998F-A1C7-4948-BBF3-F474FBDEFFB2@fb.com>
Date: Fri, 28 Jun 2019 19:36:49 +0000
From: Lawrence Brakmo <brakmo@...com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>,
Alexei Starovoitov <ast@...nel.org>
CC: "David S. Miller" <davem@...emloft.net>,
Daniel Borkmann <daniel@...earbox.net>,
Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next] bpf: fix precision tracking
On 6/28/19, 12:33 PM, "netdev-owner@...r.kernel.org on behalf of Andrii Nakryiko" <netdev-owner@...r.kernel.org on behalf of andrii.nakryiko@...il.com> wrote:
On Fri, Jun 28, 2019 at 9:25 AM Alexei Starovoitov <ast@...nel.org> wrote:
>
> When equivalent state is found the current state needs to propagate precision marks.
> Otherwise the verifier will prune the search incorrectly.
>
> There is a price for correctness:
> before before broken fixed
> cnst spill precise precise
> bpf_lb-DLB_L3.o 1923 8128 1863 1898
> bpf_lb-DLB_L4.o 3077 6707 2468 2666
> bpf_lb-DUNKNOWN.o 1062 1062 544 544
> bpf_lxc-DDROP_ALL.o 166729 380712 22629 36823
> bpf_lxc-DUNKNOWN.o 174607 440652 28805 45325
> bpf_netdev.o 8407 31904 6801 7002
> bpf_overlay.o 5420 23569 4754 4858
> bpf_lxc_jit.o 39389 359445 50925 69631
> Overall precision tracking is still very effective.
>
> Fixes: b5dc0163d8fd ("bpf: precise scalar_value tracking")
> Reported-by: Lawrence Brakmo <brakmo@...com>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
Acked-by: Andrii Nakryiko <andriin@...com>
Tested-by: Lawrence Brakmo <brakmo@...com>
> Sending the fix early w/o tests, since I'm traveling.
> Will add proper tests when I'm back.
> ---
> kernel/bpf/verifier.c | 121 +++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 107 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6b5623d320f9..62afc4058ced 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1659,16 +1659,18 @@ static void mark_all_scalars_precise(struct bpf_verifier_env *env,
> }
> }
>
> -static int mark_chain_precision(struct bpf_verifier_env *env, int regno)
> +static int __mark_chain_precision(struct bpf_verifier_env *env, int regno,
> + int spi)
> {
> struct bpf_verifier_state *st = env->cur_state;
> int first_idx = st->first_insn_idx;
> int last_idx = env->insn_idx;
> struct bpf_func_state *func;
> struct bpf_reg_state *reg;
> - u32 reg_mask = 1u << regno;
> - u64 stack_mask = 0;
> + u32 reg_mask = regno >= 0 ? 1u << regno : 0;
> + u64 stack_mask = spi >= 0 ? 1ull << spi : 0;
> bool skip_first = true;
> + bool new_marks = false;
> int i, err;
>
> if (!env->allow_ptr_leaks)
> @@ -1676,18 +1678,43 @@ static int mark_chain_precision(struct bpf_verifier_env *env, int regno)
> return 0;
>
> func = st->frame[st->curframe];
> - reg = &func->regs[regno];
> - if (reg->type != SCALAR_VALUE) {
> - WARN_ONCE(1, "backtracing misuse");
> - return -EFAULT;
> + if (regno >= 0) {
> + reg = &func->regs[regno];
> + if (reg->type != SCALAR_VALUE) {
> + WARN_ONCE(1, "backtracing misuse");
> + return -EFAULT;
> + }
> + if (!reg->precise)
> + new_marks = true;
> + else
> + reg_mask = 0;
> + reg->precise = true;
> }
> - if (reg->precise)
> - return 0;
> - func->regs[regno].precise = true;
>
> + while (spi >= 0) {
> + if (func->stack[spi].slot_type[0] != STACK_SPILL) {
> + stack_mask = 0;
> + break;
> + }
> + reg = &func->stack[spi].spilled_ptr;
> + if (reg->type != SCALAR_VALUE) {
> + stack_mask = 0;
> + break;
> + }
> + if (!reg->precise)
> + new_marks = true;
> + else
> + stack_mask = 0;
> + reg->precise = true;
> + break;
> + }
> +
> + if (!new_marks)
> + return 0;
> + if (!reg_mask && !stack_mask)
> + return 0;
> for (;;) {
> DECLARE_BITMAP(mask, 64);
> - bool new_marks = false;
> u32 history = st->jmp_history_cnt;
>
> if (env->log.level & BPF_LOG_LEVEL)
> @@ -1730,12 +1757,15 @@ static int mark_chain_precision(struct bpf_verifier_env *env, int regno)
> if (!st)
> break;
>
> + new_marks = false;
> func = st->frame[st->curframe];
> bitmap_from_u64(mask, reg_mask);
> for_each_set_bit(i, mask, 32) {
> reg = &func->regs[i];
> - if (reg->type != SCALAR_VALUE)
> + if (reg->type != SCALAR_VALUE) {
> + reg_mask &= ~(1u << i);
> continue;
> + }
> if (!reg->precise)
> new_marks = true;
> reg->precise = true;
> @@ -1756,11 +1786,15 @@ static int mark_chain_precision(struct bpf_verifier_env *env, int regno)
> return -EFAULT;
> }
>
> - if (func->stack[i].slot_type[0] != STACK_SPILL)
> + if (func->stack[i].slot_type[0] != STACK_SPILL) {
> + stack_mask &= ~(1ull << i);
> continue;
> + }
> reg = &func->stack[i].spilled_ptr;
> - if (reg->type != SCALAR_VALUE)
> + if (reg->type != SCALAR_VALUE) {
> + stack_mask &= ~(1ull << i);
> continue;
> + }
> if (!reg->precise)
> new_marks = true;
> reg->precise = true;
> @@ -1772,6 +1806,8 @@ static int mark_chain_precision(struct bpf_verifier_env *env, int regno)
> reg_mask, stack_mask);
> }
>
> + if (!reg_mask && !stack_mask)
> + break;
> if (!new_marks)
> break;
>
> @@ -1781,6 +1817,15 @@ static int mark_chain_precision(struct bpf_verifier_env *env, int regno)
> return 0;
> }
>
> +static int mark_chain_precision(struct bpf_verifier_env *env, int regno)
> +{
> + return __mark_chain_precision(env, regno, -1);
> +}
> +
> +static int mark_chain_precision_stack(struct bpf_verifier_env *env, int spi)
> +{
> + return __mark_chain_precision(env, -1, spi);
> +}
>
> static bool is_spillable_regtype(enum bpf_reg_type type)
> {
> @@ -7114,6 +7159,46 @@ static int propagate_liveness(struct bpf_verifier_env *env,
> return 0;
> }
>
> +/* find precise scalars in the previous equivalent state and
> + * propagate them into the current state
> + */
> +static int propagate_precision(struct bpf_verifier_env *env,
> + const struct bpf_verifier_state *old)
> +{
> + struct bpf_reg_state *state_reg;
> + struct bpf_func_state *state;
> + int i, err = 0;
> +
> + state = old->frame[old->curframe];
> + state_reg = state->regs;
> + for (i = 0; i < BPF_REG_FP; i++, state_reg++) {
> + if (state_reg->type != SCALAR_VALUE ||
> + !state_reg->precise)
> + continue;
> + if (env->log.level & BPF_LOG_LEVEL2)
> + verbose(env, "propagating r%d\n", i);
> + err = mark_chain_precision(env, i);
> + if (err < 0)
> + return err;
> + }
> +
> + for (i = 0; i < state->allocated_stack / BPF_REG_SIZE; i++) {
> + if (state->stack[i].slot_type[0] != STACK_SPILL)
> + continue;
> + state_reg = &state->stack[i].spilled_ptr;
> + if (state_reg->type != SCALAR_VALUE ||
> + !state_reg->precise)
> + continue;
> + if (env->log.level & BPF_LOG_LEVEL2)
> + verbose(env, "propagating fp%d\n",
> + (-i - 1) * BPF_REG_SIZE);
> + err = mark_chain_precision_stack(env, i);
> + if (err < 0)
> + return err;
> + }
> + return 0;
> +}
> +
> static bool states_maybe_looping(struct bpf_verifier_state *old,
> struct bpf_verifier_state *cur)
> {
> @@ -7206,6 +7291,14 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> * this state and will pop a new one.
> */
> err = propagate_liveness(env, &sl->state, cur);
> +
> + /* if previous state reached the exit with precision and
> + * current state is equivalent to it (except precsion marks)
> + * the precision needs to be propagated back in
> + * the current state.
> + */
> + err = err ? : push_jmp_history(env, cur);
> + err = err ? : propagate_precision(env, &sl->state);
> if (err)
> return err;
> return 1;
> --
> 2.20.0
>
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