[PATCH bpf-next v2 17/25] bpf: Support constant scalar arguments for kfuncs

Kumar Kartikeya Dwivedi
2022-10-13T06:24:27Z

Allow passing known constant scalars as arguments to kfuncs that do not
represent a size parameter. This makes the search pruning optimization
of verifier more conservative for such kfunc calls, and each
non-distinct argument is considered unequivalent.

We will use this support to then expose a global bpf_kptr_alloc function
where it takes the local type ID in program BTF, and returns a
PTR_TO_BTF_ID to the local type. These will be called local kptrs, and
allows programs to allocate their own objects.

However, this is still not completely safe, as mark_chain_precision
logic is buggy without more work when the constant argument is not a
size, but still needs precise marker propagation for pruning checks.
Next patch will fix this problem.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
Documentation/bpf/kfuncs.rst | 30 ++++++++++++++++++
kernel/bpf/verifier.c | 59 +++++++++++++++++++++++++++---------
2 files changed, 75 insertions(+), 14 deletions(-)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 0f858156371d..08f9a968d06d 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -72,6 +72,36 @@ argument as its size. By default, without __sz annotation, the size of the type
of the pointer is used. Without __sz annotation, a kfunc cannot accept a void
pointer.

+2.2.1 __k Annotation
+--------------------
+
+This annotation is only understood for scalar arguments, where it indicates that
+the verifier must check the scalar argument to be a known constant, which does
+not indicate a size parameter. This distinction is important, as when the scalar
+argument does not represent a size parameter, verifier is more conservative in
+state search pruning and does not consider two arguments equivalent for safety
+purposes if the already verified value was within range of the new one.
+
+This assumption holds well for sizes (as memory accessed within smaller bounds
+in old verified state will also work for bigger bounds in current to be explored
+state), but not for other constant arguments where each carries a distinct
+semantic effect.
+
+An example is given below::
+
+ void *bpf_mem_alloc(u32 local_type_id__k)
+ {
+ ...
+ }
+
+Here, bpf_mem_alloc uses local_type_id argument to find out the size of that
+type ID in program's BTF and return a sized pointer to it. Each type ID will
+have a distinct size, hence it is crucial to treat each such call as distinct
+when values don't match.
+
+Hence, whenever a constant scalar argument is accepted by a kfunc which is not a
+size parameter, __k suffix must be used.
+
.. _BPF_kfunc_nodef:

2.3 Using an existing kernel function
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 0ff021ab3064..7c3d7d07773f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7676,6 +7676,10 @@ struct bpf_kfunc_call_arg_meta {
u8 release_regno;
bool r0_rdonly;
u64 r0_size;
+ struct {
+ u64 value;
+ bool found;
+ } arg_constant;
};

static bool is_kfunc_acquire(struct bpf_kfunc_call_arg_meta *meta)
@@ -7713,30 +7717,40 @@ static bool is_kfunc_arg_kptr_get(struct bpf_kfunc_call_arg_meta *meta, int arg)
return arg == 0 && (meta->kfunc_flags & KF_KPTR_GET);
}

-static bool is_kfunc_arg_mem_size(const struct btf *btf,
- const struct btf_param *arg,
- const struct bpf_reg_state *reg)
+static bool __kfunc_param_match_suffix(const struct btf *btf,
+ const struct btf_param *arg,
+ const char *suffix)
{
- int len, sfx_len = sizeof("__sz") - 1;
- const struct btf_type *t;
+ int suffix_len = strlen(suffix), len;
const char *param_name;

- t = btf_type_skip_modifiers(btf, arg->type, NULL);
- if (!btf_type_is_scalar(t) || reg->type != SCALAR_VALUE)
- return false;
-
/* In the future, this can be ported to use BTF tagging */
param_name = btf_name_by_offset(btf, arg->name_off);
if (str_is_empty(param_name))
return false;
len = strlen(param_name);
- if (len < sfx_len)
+ if (len < suffix_len)
return false;
- param_name += len - sfx_len;
- if (strncmp(param_name, "__sz", sfx_len))
+ param_name += len - suffix_len;
+ return !strncmp(param_name, suffix, suffix_len);
+}
+
+static bool is_kfunc_arg_mem_size(const struct btf *btf,
+ const struct btf_param *arg,
+ const struct bpf_reg_state *reg)
+{
+ const struct btf_type *t;
+
+ t = btf_type_skip_modifiers(btf, arg->type, NULL);
+ if (!btf_type_is_scalar(t) || reg->type != SCALAR_VALUE)
return false;

- return true;
+ return __kfunc_param_match_suffix(btf, arg, "__sz");
+}
+
+static bool is_kfunc_arg_sfx_constant(const struct btf *btf, const struct btf_param *arg)
+{
+ return __kfunc_param_match_suffix(btf, arg, "__k");
}

static bool is_kfunc_arg_ret_buf_size(const struct btf *btf,
@@ -8013,7 +8027,24 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
verbose(env, "R%d is not a scalar\n", regno);
return -EINVAL;
}
- if (is_kfunc_arg_ret_buf_size(btf, &args[i], reg, "rdonly_buf_size")) {
+ if (is_kfunc_arg_sfx_constant(meta->btf, &args[i])) {
+ /* kfunc is already bpf_capable() only, no need
+ * to check it here.
+ */
+ if (meta->arg_constant.found) {
+ verbose(env, "verifier internal error: only one constant argument permitted\n");
+ return -EFAULT;
+ }
+ if (!tnum_is_const(reg->var_off)) {
+ verbose(env, "R%d must be a known constant\n", regno);
+ return -EINVAL;
+ }
+ ret = mark_chain_precision(env, regno);
+ if (ret < 0)
+ return ret;
+ meta->arg_constant.found = true;
+ meta->arg_constant.value = reg->var_off.value;
+ } else if (is_kfunc_arg_ret_buf_size(btf, &args[i], reg, "rdonly_buf_size")) {
meta->r0_rdonly = true;
is_ret_buf_sz = true;
} else if (is_kfunc_arg_ret_buf_size(btf, &args[i], reg, "rdwr_buf_size")) {
--
2.38.0