On Thu, Jan 05, 2023 at 04:02:11AM IST, Andrii Nakryiko wrote:
> On Sun, Jan 1, 2023 at 12:34 AM Kumar Kartikeya Dwivedi
> <memxor@xxxxxxxxx> wrote:
> >
> > Currently, the dynptr function is not checking the variable offset part
> > of PTR_TO_STACK that it needs to check. The fixed offset is considered
> > when computing the stack pointer index, but if the variable offset was
> > not a constant (such that it could not be accumulated in reg->off), we
> > will end up a discrepency where runtime pointer does not point to the
> > actual stack slot we mark as STACK_DYNPTR.
> >
> > It is impossible to precisely track dynptr state when variable offset is
> > not constant, hence, just like bpf_timer, kptr, bpf_spin_lock, etc.
> > simply reject the case where reg->var_off is not constant. Then,
> > consider both reg->off and reg->var_off.value when computing the stack
> > pointer index.
> >
> > A new helper dynptr_get_spi is introduced to hide over these details
> > since the dynptr needs to be located in multiple places outside the
> > process_dynptr_func checks, hence once we know it's a PTR_TO_STACK, we
> > need to enforce these checks in all places.
> >
> > Note that it is disallowed for unprivileged users to have a non-constant
> > var_off, so this problem should only be possible to trigger from
> > programs having CAP_PERFMON. However, its effects can vary.
> >
> > Without the fix, it is possible to replace the contents of the dynptr
> > arbitrarily by making verifier mark different stack slots than actual
> > location and then doing writes to the actual stack address of dynptr at
> > runtime.
> >
> > Fixes: 97e03f521050 ("bpf: Add verifier support for dynptrs")
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> > ---
> > kernel/bpf/verifier.c | 83 ++++++++++++++-----
> > .../bpf/prog_tests/kfunc_dynptr_param.c | 2 +-
> > .../testing/selftests/bpf/progs/dynptr_fail.c | 6 +-
> > 3 files changed, 66 insertions(+), 25 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index f7248235e119..ca970f80e395 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -638,11 +638,34 @@ static void print_liveness(struct bpf_verifier_env *env,
> > verbose(env, "D");
> > }
> >
> > -static int get_spi(s32 off)
> > +static int __get_spi(s32 off)
> > {
> > return (-off - 1) / BPF_REG_SIZE;
> > }
> >
> > +static int dynptr_get_spi(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
> > +{
> > + int off, spi;
> > +
> > + if (!tnum_is_const(reg->var_off)) {
> > + verbose(env, "dynptr has to be at the constant offset\n");
> > + return -EINVAL;
> > + }
> > +
> > + off = reg->off + reg->var_off.value;
> > + if (off % BPF_REG_SIZE) {
> > + verbose(env, "cannot pass in dynptr at an offset=%d\n", reg->off);
>
> s/reg->off/off/ ?
>
Yep, thanks for catching.
> > + return -EINVAL;
> > + }
> > +
> > + spi = __get_spi(off);
> > + if (spi < 1) {
> > + verbose(env, "cannot pass in dynptr at an offset=%d\n", (int)(off + reg->var_off.value));
>
> s/(int)(off + reg->var_off.value)/off/?
>
Same, yes.
> > + return -EINVAL;
> > + }
> > + return spi;
> > +}
> > +
>
> [...]
>
> > @@ -2422,7 +2456,9 @@ static int mark_dynptr_read(struct bpf_verifier_env *env, struct bpf_reg_state *
> > */
> > if (reg->type == CONST_PTR_TO_DYNPTR)
> > return 0;
> > - spi = get_spi(reg->off);
> > + spi = dynptr_get_spi(env, reg);
> > + if (WARN_ON_ONCE(spi < 0))
> > + return spi;
> > /* Caller ensures dynptr is valid and initialized, which means spi is in
> > * bounds and spi is the first dynptr slot. Simply mark stack slot as
> > * read.
> > @@ -5946,6 +5982,11 @@ static int process_kptr_func(struct bpf_verifier_env *env, int regno,
> > return 0;
> > }
> >
> > +static bool arg_type_is_release(enum bpf_arg_type type)
> > +{
> > + return type & OBJ_RELEASE;
> > +}
> > +
>
> no need to move it?
>
Yeah, will fix.
> > /* There are two register types representing a bpf_dynptr, one is PTR_TO_STACK
> > * which points to a stack slot, and the other is CONST_PTR_TO_DYNPTR.
> > *
> > @@ -5986,12 +6027,14 @@ int process_dynptr_func(struct bpf_verifier_env *env, int regno,
> > }
> > /* CONST_PTR_TO_DYNPTR already has fixed and var_off as 0 due to
> > * check_func_arg_reg_off's logic. We only need to check offset
> > - * alignment for PTR_TO_STACK.
> > + * and its alignment for PTR_TO_STACK.
> > */
> > - if (reg->type == PTR_TO_STACK && (reg->off % BPF_REG_SIZE)) {
> > - verbose(env, "cannot pass in dynptr at an offset=%d\n", reg->off);
> > - return -EINVAL;
> > + if (reg->type == PTR_TO_STACK) {
> > + err = dynptr_get_spi(env, reg);
> > + if (err < 0)
> > + return err;
> > }
> > +
> > /* MEM_UNINIT - Points to memory that is an appropriate candidate for
> > * constructing a mutable bpf_dynptr object.
> > *
> > @@ -6070,11 +6113,6 @@ static bool arg_type_is_mem_size(enum bpf_arg_type type)
> > type == ARG_CONST_SIZE_OR_ZERO;
> > }
> >
> > -static bool arg_type_is_release(enum bpf_arg_type type)
> > -{
> > - return type & OBJ_RELEASE;
> > -}
> > -
> > static bool arg_type_is_dynptr(enum bpf_arg_type type)
> > {
> > return base_type(type) == ARG_PTR_TO_DYNPTR;
> > @@ -6404,8 +6442,9 @@ static u32 dynptr_ref_obj_id(struct bpf_verifier_env *env, struct bpf_reg_state
>
> why not make dynptr_ref_obj_id return int and <0 on error? There seems
> to be just one place where we call dynptr_ref_obj_id and we can check
> and report error there
>
Good suggestion, I'll make that change.
> >
> > if (reg->type == CONST_PTR_TO_DYNPTR)
> > return reg->ref_obj_id;
> > -
> > - spi = get_spi(reg->off);
> > + spi = dynptr_get_spi(env, reg);
> > + if (WARN_ON_ONCE(spi < 0))
> > + return U32_MAX;
> > return state->stack[spi].spilled_ptr.ref_obj_id;
> > }
> >
>
> [...]
