Tao Lyu wrote:
> The verifier previously aimed to reject partial overwrite on an 8-byte stack slot
> that contains a spilled pointer.
> However, it rejects all partial stack overwrites
> as long as the targeted stack slot is a spilled register,
> because it does not check if the stack slot is a spilled pointer.
>
> Finally, incomplete checks will result in the rejection of valid programs,
> which spill narrower scalar values onto scalar slots, as shown below.
>
> 0: R1=ctx() R10=fp0
> ; asm volatile ( @ repro.bpf.c:679
> 0: (7a) *(u64 *)(r10 -8) = 1 ; R10=fp0 fp-8_w=1
> 1: (62) *(u32 *)(r10 -8) = 1
> attempt to corrupt spilled pointer on stack
> processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0.
>
> The issue is fixed by adding a check on the spilled register type of targeted slots.
>
> Fixes: ab125ed3ec1c ("bpf: fix check for attempt to corrupt spilled pointer")
> Signed-off-by: Tao Lyu <tao.lyu@xxxxxxx>
> ---
A test case would be nice.
> kernel/bpf/verifier.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 17c06f1505e4..3064ba7c140f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4493,6 +4493,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> */
> if (!env->allow_ptr_leaks &&
> is_spilled_reg(&state->stack[spi]) &&
> + state->stack[spi].spilled_ptr.type != SCALAR_VALUE &&
> size != BPF_REG_SIZE) {
Might be worth a helper we have is_spilled_reg and is_spilled_scalar_reg
maybe a is_spilled_not_scalar_reg() is nice. Perhaps Andrii has a style
opinion. Or,
is_spilled_reg(...) && !is_spilled_scalar_reg(...)
Not sure I like that any better than whats there in patch though.
> verbose(env, "attempt to corrupt spilled pointer on stack\n");
> return -EACCES;
> --
> 2.25.1
>
>
