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bpf: keep track of max number of bpf_loop callback iterations
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commit bb124da upstream.

In some cases verifier can't infer convergence of the bpf_loop()
iteration. E.g. for the following program:

    static int cb(__u32 idx, struct num_context* ctx)
    {
        ctx->i++;
        return 0;
    }

    SEC("?raw_tp")
    int prog(void *_)
    {
        struct num_context ctx = { .i = 0 };
        __u8 choice_arr[2] = { 0, 1 };

        bpf_loop(2, cb, &ctx, 0);
        return choice_arr[ctx.i];
    }

Each 'cb' simulation would eventually return to 'prog' and reach
'return choice_arr[ctx.i]' statement. At which point ctx.i would be
marked precise, thus forcing verifier to track multitude of separate
states with {.i=0}, {.i=1}, ... at bpf_loop() callback entry.

This commit allows "brute force" handling for such cases by limiting
number of callback body simulations using 'umax' value of the first
bpf_loop() parameter.

For this, extend bpf_func_state with 'callback_depth' field.
Increment this field when callback visiting state is pushed to states
traversal stack. For frame #N it's 'callback_depth' field counts how
many times callback with frame depth N+1 had been executed.
Use bpf_func_state specifically to allow independent tracking of
callback depths when multiple nested bpf_loop() calls are present.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231121020701.26440-11-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@eddyz87 @gregkh
eddyz87 authored and gregkh committed Feb 1, 2024
1 parent 5cac3cb commit bfc5c19
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Showing 3 changed files with 53 additions and 12 deletions.
11 changes: 11 additions & 0 deletions include/linux/bpf_verifier.h
View file
Expand Up @@ -300,6 +300,17 @@ struct bpf_func_state {
bool in_callback_fn;
struct tnum callback_ret_range;
bool in_async_callback_fn;
/* For callback calling functions that limit number of possible
* callback executions (e.g. bpf_loop) keeps track of current
* simulated iteration number.
* Value in frame N refers to number of times callback with frame
* N+1 was simulated, e.g. for the following call:
*
* bpf_loop(..., fn, ...); | suppose current frame is N
* | fn would be simulated in frame N+1
* | number of simulations is tracked in frame N
*/
u32 callback_depth;

/* The following fields should be last. See copy_func_state() */
int acquired_refs;
Expand Down
19 changes: 17 additions & 2 deletions kernel/bpf/verifier.c
View file
Expand Up @@ -9301,6 +9301,8 @@ static int push_callback_call(struct bpf_verifier_env *env, struct bpf_insn *ins
return err;

callback_state->callback_unroll_depth++;
callback_state->frame[callback_state->curframe - 1]->callback_depth++;
caller->callback_depth = 0;
return 0;
}

Expand Down Expand Up @@ -10090,8 +10092,21 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
break;
case BPF_FUNC_loop:
update_loop_inline_state(env, meta.subprogno);
err = push_callback_call(env, insn, insn_idx, meta.subprogno,
set_loop_callback_state);
/* Verifier relies on R1 value to determine if bpf_loop() iteration
* is finished, thus mark it precise.
*/
err = mark_chain_precision(env, BPF_REG_1);
if (err)
return err;
if (cur_func(env)->callback_depth < regs[BPF_REG_1].umax_value) {
err = push_callback_call(env, insn, insn_idx, meta.subprogno,
set_loop_callback_state);
} else {
cur_func(env)->callback_depth = 0;
if (env->log.level & BPF_LOG_LEVEL2)
verbose(env, "frame%d bpf_loop iteration limit reached\n",
env->cur_state->curframe);
}
break;
case BPF_FUNC_dynptr_from_mem:
if (regs[BPF_REG_1].type != PTR_TO_MAP_VALUE) {
Expand Down
35 changes: 25 additions & 10 deletions tools/testing/selftests/bpf/progs/verifier_subprog_precision.c
View file
Expand Up @@ -119,7 +119,23 @@ __naked int global_subprog_result_precise(void)

SEC("?raw_tp")
__success __log_level(2)
/* First simulated path does not include callback body */
/* First simulated path does not include callback body,
* r1 and r4 are always precise for bpf_loop() calls.
*/
__msg("9: (85) call bpf_loop#181")
__msg("mark_precise: frame0: last_idx 9 first_idx 9 subseq_idx -1")
__msg("mark_precise: frame0: parent state regs=r4 stack=:")
__msg("mark_precise: frame0: last_idx 8 first_idx 0 subseq_idx 9")
__msg("mark_precise: frame0: regs=r4 stack= before 8: (b7) r4 = 0")
__msg("mark_precise: frame0: last_idx 9 first_idx 9 subseq_idx -1")
__msg("mark_precise: frame0: parent state regs=r1 stack=:")
__msg("mark_precise: frame0: last_idx 8 first_idx 0 subseq_idx 9")
__msg("mark_precise: frame0: regs=r1 stack= before 8: (b7) r4 = 0")
__msg("mark_precise: frame0: regs=r1 stack= before 7: (b7) r3 = 0")
__msg("mark_precise: frame0: regs=r1 stack= before 6: (bf) r2 = r8")
__msg("mark_precise: frame0: regs=r1 stack= before 5: (bf) r1 = r6")
__msg("mark_precise: frame0: regs=r6 stack= before 4: (b7) r6 = 3")
/* r6 precision propagation */
__msg("14: (0f) r1 += r6")
__msg("mark_precise: frame0: last_idx 14 first_idx 9")
__msg("mark_precise: frame0: regs=r6 stack= before 13: (bf) r1 = r7")
Expand All @@ -134,10 +150,9 @@ __msg("17: (b7) r0 = 0")
__msg("18: (95) exit")
__msg("returning from callee:")
__msg("to caller at 9:")
/* r4 (flags) is always precise for bpf_loop() */
__msg("frame 0: propagating r4")
__msg("frame 0: propagating r1,r4")
__msg("mark_precise: frame0: last_idx 9 first_idx 9 subseq_idx -1")
__msg("mark_precise: frame0: regs=r4 stack= before 18: (95) exit")
__msg("mark_precise: frame0: regs=r1,r4 stack= before 18: (95) exit")
__msg("from 18 to 9: safe")
__naked int callback_result_precise(void)
{
Expand Down Expand Up @@ -264,12 +279,12 @@ __msg("15: (b7) r0 = 0")
__msg("16: (95) exit")
__msg("returning from callee:")
__msg("to caller at 9:")
/* r4 (flags) is always precise for bpf_loop(),
/* r1, r4 are always precise for bpf_loop(),
* r6 was marked before backtracking to callback body.
*/
__msg("frame 0: propagating r4,r6")
__msg("frame 0: propagating r1,r4,r6")
__msg("mark_precise: frame0: last_idx 9 first_idx 9 subseq_idx -1")
__msg("mark_precise: frame0: regs=r4,r6 stack= before 16: (95) exit")
__msg("mark_precise: frame0: regs=r1,r4,r6 stack= before 16: (95) exit")
__msg("mark_precise: frame1: regs= stack= before 15: (b7) r0 = 0")
__msg("mark_precise: frame1: regs= stack= before 9: (85) call bpf_loop")
__msg("mark_precise: frame0: parent state regs= stack=:")
Expand Down Expand Up @@ -422,12 +437,12 @@ __msg("17: (b7) r0 = 0")
__msg("18: (95) exit")
__msg("returning from callee:")
__msg("to caller at 10:")
/* r4 (flags) is always precise for bpf_loop(),
/* r1, r4 are always precise for bpf_loop(),
* fp-8 was marked before backtracking to callback body.
*/
__msg("frame 0: propagating r4,fp-8")
__msg("frame 0: propagating r1,r4,fp-8")
__msg("mark_precise: frame0: last_idx 10 first_idx 10 subseq_idx -1")
__msg("mark_precise: frame0: regs=r4 stack=-8 before 18: (95) exit")
__msg("mark_precise: frame0: regs=r1,r4 stack=-8 before 18: (95) exit")
__msg("mark_precise: frame1: regs= stack= before 17: (b7) r0 = 0")
__msg("mark_precise: frame1: regs= stack= before 10: (85) call bpf_loop#181")
__msg("mark_precise: frame0: parent state regs= stack=:")
Expand Down

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