On Sun, Jan 1, 2023 at 12:34 AM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> Currently, while reads are disallowed for dynptr stack slots, writes are
> not. Reads don't work from both direct access and helpers, while writes
> do work in both cases, but have the effect of overwriting the slot_type.
>
> While this is fine, handling for a few edge cases is missing. Firstly,
> a user can overwrite the stack slots of dynptr partially.
>
> Consider the following layout:
> spi: [d][d][?]
> 2 1 0
>
> First slot is at spi 2, second at spi 1.
> Now, do a write of 1 to 8 bytes for spi 1.
>
> This will essentially either write STACK_MISC for all slot_types or
> STACK_MISC and STACK_ZERO (in case of size < BPF_REG_SIZE partial write
> of zeroes). The end result is that slot is scrubbed.
>
> Now, the layout is:
> spi: [d][m][?]
> 2 1 0
>
> Suppose if user initializes spi = 1 as dynptr.
> We get:
> spi: [d][d][d]
> 2 1 0
>
> But this time, both spi 2 and spi 1 have first_slot = true.
>
> Now, when passing spi 2 to dynptr helper, it will consider it as
> initialized as it does not check whether second slot has first_slot ==
> false. And spi 1 should already work as normal.
>
> This effectively replaced size + offset of first dynptr, hence allowing
> invalid OOB reads and writes.
>
> Make a few changes to protect against this:
> When writing to PTR_TO_STACK using BPF insns, when we touch spi of a
> STACK_DYNPTR type, mark both first and second slot (regardless of which
> slot we touch) as STACK_INVALID. Reads are already prevented.
>
> Second, prevent writing to stack memory from helpers if the range may
> contain any STACK_DYNPTR slots. Reads are already prevented.
>
> For helpers, we cannot allow it to destroy dynptrs from the writes as
> depending on arguments, helper may take uninit_mem and dynptr both at
> the same time. This would mean that helper may write to uninit_mem
> before it reads the dynptr, which would be bad.
>
> PTR_TO_MEM: [?????dd]
>
> Depending on the code inside the helper, it may end up overwriting the
> dynptr contents first and then read those as the dynptr argument.
>
> Verifier would only simulate destruction when it does byte by byte
> access simulation in check_helper_call for meta.access_size, and
> fail to catch this case, as it happens after argument checks.
>
> The same would need to be done for any other non-trivial objects created
> on the stack in the future, such as bpf_list_head on stack, or
> bpf_rb_root on stack.
>
> A common misunderstanding in the current code is that MEM_UNINIT means
> writes, but note that writes may also be performed even without
> MEM_UNINIT in case of helpers, in that case the code after handling meta
> && meta->raw_mode will complain when it sees STACK_DYNPTR. So that
> invalid read case also covers writes to potential STACK_DYNPTR slots.
> The only loophole was in case of meta->raw_mode which simulated writes
> through instructions which could overwrite them.
>
> A future series sequenced after this will focus on the clean up of
> helper access checks and bugs around that.
>
> Fixes: 97e03f521050 ("bpf: Add verifier support for dynptrs")
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> ---
> kernel/bpf/verifier.c | 73 +++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 73 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index ca970f80e395..b985d90505cc 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -769,6 +769,8 @@ static void mark_dynptr_cb_reg(struct bpf_reg_state *reg,
> __mark_dynptr_reg(reg, type, true);
> }
>
> +static void destroy_stack_slots_dynptr(struct bpf_verifier_env *env,
> + struct bpf_func_state *state, int spi);
>
> static int mark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
> enum bpf_arg_type arg_type, int insn_idx)
> @@ -858,6 +860,44 @@ static int unmark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_re
> return 0;
> }
>
> +static void destroy_stack_slots_dynptr(struct bpf_verifier_env *env,
> + struct bpf_func_state *state, int spi)
> +{
> + int i;
> +
> + /* We always ensure that STACK_DYNPTR is never set partially,
> + * hence just checking for slot_type[0] is enough. This is
> + * different for STACK_SPILL, where it may be only set for
> + * 1 byte, so code has to use is_spilled_reg.
> + */
> + if (state->stack[spi].slot_type[0] != STACK_DYNPTR)
> + return;
nit: an empty line here helps readability
> + /* Reposition spi to first slot */
> + if (!state->stack[spi].spilled_ptr.dynptr.first_slot)
> + spi = spi + 1;
> +
> + mark_stack_slot_scratched(env, spi);
> + mark_stack_slot_scratched(env, spi - 1);
> +
> + /* Writing partially to one dynptr stack slot destroys both. */
> + for (i = 0; i < BPF_REG_SIZE; i++) {
> + state->stack[spi].slot_type[i] = STACK_INVALID;
> + state->stack[spi - 1].slot_type[i] = STACK_INVALID;
> + }
> +
> + /* Do not release reference state, we are destroying dynptr on stack,
> + * not using some helper to release it. Just reset register.
> + */
I agree with Andrii's point - I think it'd be more helpful if we error
out here if the dynptr is refcounted. It'd be easy to check too, we
already have dynptr_type_refcounted().
> + __mark_reg_not_init(env, &state->stack[spi].spilled_ptr);
> + __mark_reg_not_init(env, &state->stack[spi - 1].spilled_ptr);
> +
> + /* Same reason as unmark_stack_slots_dynptr above */
> + state->stack[spi].spilled_ptr.live |= REG_LIVE_WRITTEN;
> + state->stack[spi - 1].spilled_ptr.live |= REG_LIVE_WRITTEN;
> +
> + return;
I think we should also invalidate any data slices associated with the
dynptrs? It seems natural that once a dynptr is invalidated, none of
its data slices should be usable.
> +}
> +
> static bool is_dynptr_reg_valid_uninit(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
> {
> struct bpf_func_state *state = func(env, reg);
> @@ -3384,6 +3424,8 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> env->insn_aux_data[insn_idx].sanitize_stack_spill = true;
> }
>
> + destroy_stack_slots_dynptr(env, state, spi);
> +
> mark_stack_slot_scratched(env, spi);
> if (reg && !(off % BPF_REG_SIZE) && register_is_bounded(reg) &&
> !register_is_null(reg) && env->bpf_capable) {
> @@ -3497,6 +3539,13 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env,
> if (err)
> return err;
>
> + for (i = min_off; i < max_off; i++) {
> + int slot, spi;
> +
> + slot = -i - 1;
> + spi = slot / BPF_REG_SIZE;
I think you can just use __get_spi() here
> + destroy_stack_slots_dynptr(env, state, spi);
I think here too,
if (state->stack[spi].slot_type[0] == STACK_DYNPTR)
destroy_stack_slots_dynptr(env, state, spi)
makes it more readable.
And if it is a STACK_DYNPTR, we can also fast-forward i.
> + }
>
> /* Variable offset writes destroy any spilled pointers in range. */
> for (i = min_off; i < max_off; i++) {
> @@ -5524,6 +5573,30 @@ static int check_stack_range_initialized(
> }
>
> if (meta && meta->raw_mode) {
> + /* Ensure we won't be overwriting dynptrs when simulating byte
> + * by byte access in check_helper_call using meta.access_size.
> + * This would be a problem if we have a helper in the future
> + * which takes:
> + *
> + * helper(uninit_mem, len, dynptr)
> + *
> + * Now, uninint_mem may overlap with dynptr pointer. Hence, it
> + * may end up writing to dynptr itself when touching memory from
> + * arg 1. This can be relaxed on a case by case basis for known
> + * safe cases, but reject due to the possibilitiy of aliasing by
> + * default.
> + */
> + for (i = min_off; i < max_off + access_size; i++) {
> + slot = -i - 1;
> + spi = slot / BPF_REG_SIZE;
nit: here too, we can use __get_spi()
> + /* raw_mode may write past allocated_stack */
> + if (state->allocated_stack <= slot)
> + continue;
> + if (state->stack[spi].slot_type[slot % BPF_REG_SIZE] == STACK_DYNPTR) {
> + verbose(env, "potential write to dynptr at off=%d disallowed\n", i);
> + return -EACCES;
> + }
> + }
> meta->access_size = access_size;
> meta->regno = regno;
> return 0;
> --
> 2.39.0
>
