[PATCH v5 1/3] bpf: Allow trusted pointers to be passed to KF_TRUSTED_ARGS kfuncs

David Vernet
2022-10-14T21:21:43Z

Kfuncs currently support specifying the KF_TRUSTED_ARGS flag to signal
to the verifier that it should enforce that a BPF program passes it a
"safe", trusted pointer. Currently, "safe" means that the pointer is
either PTR_TO_CTX, or is refcounted. There may be cases, however, where
the kernel passes a BPF program a safe / trusted pointer to an object
that the BPF program wishes to use as a kptr, but because the object
does not yet have a ref_obj_id from the perspective of the verifier, the
program would be unable to pass it to a KF_ACQUIRE | KF_TRUSTED_ARGS
kfunc.

The solution is to expand the set of pointers that are considered
trusted according to KF_TRUSTED_ARGS, so that programs can invoke kfuncs
with these pointers without getting rejected by the verifier.

There is already a PTR_UNTRUSTED flag that is set in some scenarios,
such as when a BPF program reads a kptr directly from a map
without performing a bpf_kptr_xchg() call. These pointers of course can
and should be rejected by the verifier. Unfortunately, however,
PTR_UNTRUSTED does not cover all the cases for safety that need to
be addressed to adequately protect kfuncs. Specifically, pointers
obtained by a BPF program "walking" a struct are _not_ considered
PTR_UNTRUSTED according to BPF. For example, say that we were to add a
kfunc called bpf_task_acquire(), with KF_ACQUIRE | KF_TRUSTED_ARGS, to
acquire a struct task_struct *. If we only used PTR_UNTRUSTED to signal
that a task was unsafe to pass to a kfunc, the verifier would mistakenly
allow the following unsafe BPF program to be loaded:

SEC("tp_btf/task_newtask")
int BPF_PROG(unsafe_acquire_task,
struct task_struct *task,
u64 clone_flags)
{
struct task_struct *acquired, *nested;

nested = task->last_wakee;

/* Would not be rejected by the verifier. */
acquired = bpf_task_acquire(nested);
if (!acquired)
return 0;

bpf_task_release(acquired);
return 0;
}

To address this, this patch defines a new type flag called PTR_NESTED
which tracks whether a PTR_TO_BTF_ID pointer was retrieved from walking
a struct. A pointer passed directly from the kernel begins with
(PTR_NESTED & type) == 0, meaning of course that it is not nested. Any
pointer received from walking that object, however, would inherit that
flag and become a nested pointer.

With that flag, this patch also updates btf_check_func_arg_match() to
only flag a PTR_TO_BTF_ID object as requiring a refcount if it has any
type modifiers (which of course includes both PTR_UNTRUSTED and
PTR_NESTED). Otherwise, the pointer passes this check and continues
onto the others in btf_check_func_arg_match().

A subsequent patch will add kfuncs for storing a task kfunc as a kptr,
and then another patch will validate this feature by ensuring that the
verifier rejects a kfunc invocation with a nested pointer.

Signed-off-by: David Vernet <void@manifault.com>
---
include/linux/bpf.h | 6 ++++++
kernel/bpf/btf.c | 11 ++++++++++-
kernel/bpf/verifier.c | 12 +++++++++++-
tools/testing/selftests/bpf/verifier/calls.c | 4 ++--
4 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 9e7d46d16032..b624024edb4e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -457,6 +457,12 @@ enum bpf_type_flag {
/* Size is known at compile time. */
MEM_FIXED_SIZE = BIT(10 + BPF_BASE_TYPE_BITS),

+ /* PTR was obtained from walking a struct. This is used with
+ * PTR_TO_BTF_ID to determine whether the pointer is safe to pass to a
+ * kfunc with KF_TRUSTED_ARGS.
+ */
+ PTR_NESTED = BIT(11 + BPF_BASE_TYPE_BITS),
+
__BPF_TYPE_FLAG_MAX,
__BPF_TYPE_LAST_FLAG = __BPF_TYPE_FLAG_MAX - 1,
};
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index eba603cec2c5..3d7bad11b10b 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6333,8 +6333,17 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
/* Check if argument must be a referenced pointer, args + i has
* been verified to be a pointer (after skipping modifiers).
* PTR_TO_CTX is ok without having non-zero ref_obj_id.
+ *
+ * All object pointers must be refcounted, other than:
+ * - PTR_TO_CTX
+ * - Trusted pointers (i.e. pointers with no type modifiers)
*/
- if (is_kfunc && trusted_args && (obj_ptr && reg->type != PTR_TO_CTX) && !reg->ref_obj_id) {
+ if (is_kfunc &&
+ trusted_args &&
+ obj_ptr &&
+ base_type(reg->type) != PTR_TO_CTX &&
+ type_flag(reg->type) &&
+ !reg->ref_obj_id) {
bpf_log(log, "R%d must be referenced\n", regno);
return -EINVAL;
}
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6f6d2d511c06..d16a08ca507b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -581,6 +581,8 @@ static const char *reg_type_str(struct bpf_verifier_env *env,
strncpy(prefix, "user_", 32);
if (type & MEM_PERCPU)
strncpy(prefix, "percpu_", 32);
+ if (type & PTR_NESTED)
+ strncpy(prefix, "nested_", 32);
if (type & PTR_UNTRUSTED)
strncpy(prefix, "untrusted_", 32);

@@ -4558,6 +4560,9 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
if (type_flag(reg->type) & PTR_UNTRUSTED)
flag |= PTR_UNTRUSTED;

+ /* All pointers obtained by walking a struct are nested. */
+ flag |= PTR_NESTED;
+
if (atype == BPF_READ && value_regno >= 0)
mark_btf_ld_reg(env, regs, value_regno, ret, reg->btf, btf_id, flag);

@@ -5694,7 +5699,12 @@ static const struct bpf_reg_types scalar_types = { .types = { SCALAR_VALUE } };
static const struct bpf_reg_types context_types = { .types = { PTR_TO_CTX } };
static const struct bpf_reg_types alloc_mem_types = { .types = { PTR_TO_MEM | MEM_ALLOC } };
static const struct bpf_reg_types const_map_ptr_types = { .types = { CONST_PTR_TO_MAP } };
-static const struct bpf_reg_types btf_ptr_types = { .types = { PTR_TO_BTF_ID } };
+static const struct bpf_reg_types btf_ptr_types = {
+ .types = {
+ PTR_TO_BTF_ID,
+ PTR_TO_BTF_ID | PTR_NESTED
+ },
+};
static const struct bpf_reg_types spin_lock_types = { .types = { PTR_TO_MAP_VALUE } };
static const struct bpf_reg_types percpu_btf_ptr_types = { .types = { PTR_TO_BTF_ID | MEM_PERCPU } };
static const struct bpf_reg_types func_ptr_types = { .types = { PTR_TO_FUNC } };
diff --git a/tools/testing/selftests/bpf/verifier/calls.c b/tools/testing/selftests/bpf/verifier/calls.c
index e1a937277b54..496c29b1a298 100644
--- a/tools/testing/selftests/bpf/verifier/calls.c
+++ b/tools/testing/selftests/bpf/verifier/calls.c
@@ -181,7 +181,7 @@
},
.result_unpriv = REJECT,
.result = REJECT,
- .errstr = "negative offset ptr_ ptr R1 off=-4 disallowed",
+ .errstr = "negative offset nested_ptr_ ptr R1 off=-4 disallowed",
},
{
"calls: invalid kfunc call: PTR_TO_BTF_ID with variable offset",
@@ -243,7 +243,7 @@
},
.result_unpriv = REJECT,
.result = REJECT,
- .errstr = "R1 must be referenced",
+ .errstr = "arg#0 pointer type STRUCT prog_test_ref_kfunc must point to scalar",
},
{
"calls: valid kfunc call: referenced arg needs refcounted PTR_TO_BTF_ID",
--
2.38.0