> On Dec 9, 2020, at 5:33 PM, Yonghong Song <yhs@xxxxxx> wrote:
>
> Currently, when checking stack memory accessed by helper calls,
> for spills, only PTR_TO_BTF_ID and SCALAR_VALUE are
> allowed.
>
> Song discovered an issue where the below bpf program
> int dump_task(struct bpf_iter__task *ctx)
> {
> struct seq_file *seq = ctx->meta->seq;
> static char[] info = "abc";
> BPF_SEQ_PRINTF(seq, "%s\n", info);
> return 0;
> }
> may cause a verifier failure.
>
> The verifier output looks like:
> ; struct seq_file *seq = ctx->meta->seq;
> 1: (79) r1 = *(u64 *)(r1 +0)
> ; BPF_SEQ_PRINTF(seq, "%s\n", info);
> 2: (18) r2 = 0xffff9054400f6000
> 4: (7b) *(u64 *)(r10 -8) = r2
> 5: (bf) r4 = r10
> ;
> 6: (07) r4 += -8
> ; BPF_SEQ_PRINTF(seq, "%s\n", info);
> 7: (18) r2 = 0xffff9054400fe000
> 9: (b4) w3 = 4
> 10: (b4) w5 = 8
> 11: (85) call bpf_seq_printf#126
> R1_w=ptr_seq_file(id=0,off=0,imm=0) R2_w=map_value(id=0,off=0,ks=4,vs=4,imm=0)
> R3_w=inv4 R4_w=fp-8 R5_w=inv8 R10=fp0 fp-8_w=map_value
> last_idx 11 first_idx 0
> regs=8 stack=0 before 10: (b4) w5 = 8
> regs=8 stack=0 before 9: (b4) w3 = 4
> invalid indirect read from stack off -8+0 size 8
>
> Basically, the verifier complains the map_value pointer at "fp-8" location.
> To fix the issue, if env->allow_ptr_leaks is true, let us also permit
> pointers on the stack to be accessible by the helper.
>
> Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> Reported-by: Song Liu <songliubraving@xxxxxx>
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
Acked-by: Song Liu <songliubraving@xxxxxx>
Thanks for the fix!
> ---
> kernel/bpf/verifier.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 93def76cf32b..9159c9822ede 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3769,7 +3769,8 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
> goto mark;
>
> if (state->stack[spi].slot_type[0] == STACK_SPILL &&
> - state->stack[spi].spilled_ptr.type == SCALAR_VALUE) {
> + (state->stack[spi].spilled_ptr.type == SCALAR_VALUE ||
> + env->allow_ptr_leaks)) {
> __mark_reg_unknown(env, &state->stack[spi].spilled_ptr);
> for (j = 0; j < BPF_REG_SIZE; j++)
> state->stack[spi].slot_type[j] = STACK_MISC;
> --
> 2.24.1
>
