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bpf: Allow trusted pointers to be passed to KF_TRUSTED_ARGS kfuncs 

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_TRUSTED
which tracks whether a PTR_TO_BTF_ID pointer is safe to pass to a
KF_TRUSTED_ARGS kfunc or a BPF helper function. PTR_TRUSTED pointers are
passed directly from the kernel as a tracepoint or struct_ops callback
argument. Any nested pointer that is obtained from walking a PTR_TRUSTED
pointer is no longer PTR_TRUSTED. From the example above, the struct
task_struct *task argument is PTR_TRUSTED, but the 'nested' pointer
obtained from 'task->last_wakee' is not PTR_TRUSTED.

A subsequent patch will add kfuncs for storing a task kfunc as a kptr,
and then another patch will add selftests to validate.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221120051004.3605026-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
David Vernet 2022-11-19 23:10:02 -06:00 committed by Alexei Starovoitov
parent ef66c5475d
commit 3f00c52393
10 changed files with 161 additions and 54 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
Documentation/bpf/kfuncs.rst
View File

@ -161,22 +161,20 @@ KF_ACQUIRE and KF_RET_NULL flags.
--------------------------
The KF_TRUSTED_ARGS flag is used for kfuncs taking pointer arguments. It
indicates that the all pointer arguments will always have a guaranteed lifetime,
and pointers to kernel objects are always passed to helpers in their unmodified
form (as obtained from acquire kfuncs).
indicates that the all pointer arguments are valid, and that all pointers to
BTF objects have been passed in their unmodified form (that is, at a zero
offset, and without having been obtained from walking another pointer).
It can be used to enforce that a pointer to a refcounted object acquired from a
kfunc or BPF helper is passed as an argument to this kfunc without any
modifications (e.g. pointer arithmetic) such that it is trusted and points to
the original object.
There are two types of pointers to kernel objects which are considered "valid":
Meanwhile, it is also allowed pass pointers to normal memory to such kfuncs,
but those can have a non-zero offset.
1. Pointers which are passed as tracepoint or struct_ops callback arguments.
2. Pointers which were returned from a KF_ACQUIRE or KF_KPTR_GET kfunc.
This flag is often used for kfuncs that operate (change some property, perform
some operation) on an object that was obtained using an acquire kfunc. Such
kfuncs need an unchanged pointer to ensure the integrity of the operation being
performed on the expected object.
Pointers to non-BTF objects (e.g. scalar pointers) may also be passed to
KF_TRUSTED_ARGS kfuncs, and may have a non-zero offset.
The definition of "valid" pointers is subject to change at any time, and has
absolutely no ABI stability guarantees.
2.4.6 KF_SLEEPABLE flag
-----------------------

30
include/linux/bpf.h
View File

@ -543,6 +543,35 @@ enum bpf_type_flag {
*/
MEM_ALLOC = BIT(11 + BPF_BASE_TYPE_BITS),
/* PTR was passed from the kernel in a trusted context, and may be
* passed to KF_TRUSTED_ARGS kfuncs or BPF helper functions.
* Confusingly, this is _not_ the opposite of PTR_UNTRUSTED above.
* PTR_UNTRUSTED refers to a kptr that was read directly from a map
* without invoking bpf_kptr_xchg(). What we really need to know is
* whether a pointer is safe to pass to a kfunc or BPF helper function.
* While PTR_UNTRUSTED pointers are unsafe to pass to kfuncs and BPF
* helpers, they do not cover all possible instances of unsafe
* pointers. For example, a pointer that was obtained from walking a
* struct will _not_ get the PTR_UNTRUSTED type modifier, despite the
* fact that it may be NULL, invalid, etc. This is due to backwards
* compatibility requirements, as this was the behavior that was first
* introduced when kptrs were added. The behavior is now considered
* deprecated, and PTR_UNTRUSTED will eventually be removed.
*
* PTR_TRUSTED, on the other hand, is a pointer that the kernel
* guarantees to be valid and safe to pass to kfuncs and BPF helpers.
* For example, pointers passed to tracepoint arguments are considered
* PTR_TRUSTED, as are pointers that are passed to struct_ops
* callbacks. As alluded to above, pointers that are obtained from
* walking PTR_TRUSTED pointers are _not_ trusted. For example, if a
* struct task_struct *task is PTR_TRUSTED, then accessing
* task->last_wakee will lose the PTR_TRUSTED modifier when it's stored
* in a BPF register. Similarly, pointers passed to certain programs
* types such as kretprobes are not guaranteed to be valid, as they may
* for example contain an object that was recently freed.
*/
PTR_TRUSTED = BIT(12 + BPF_BASE_TYPE_BITS),
__BPF_TYPE_FLAG_MAX,
__BPF_TYPE_LAST_FLAG = __BPF_TYPE_FLAG_MAX - 1,
};
@ -636,6 +665,7 @@ enum bpf_return_type {
RET_PTR_TO_RINGBUF_MEM_OR_NULL = PTR_MAYBE_NULL | MEM_RINGBUF | RET_PTR_TO_MEM,
RET_PTR_TO_DYNPTR_MEM_OR_NULL = PTR_MAYBE_NULL | RET_PTR_TO_MEM,
RET_PTR_TO_BTF_ID_OR_NULL = PTR_MAYBE_NULL | RET_PTR_TO_BTF_ID,
RET_PTR_TO_BTF_ID_TRUSTED = PTR_TRUSTED | RET_PTR_TO_BTF_ID,
/* This must be the last entry. Its purpose is to ensure the enum is
* wide enough to hold the higher bits reserved for bpf_type_flag.

7
include/linux/bpf_verifier.h
View File

@ -680,4 +680,11 @@ static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
}
}
#define BPF_REG_TRUSTED_MODIFIERS (MEM_ALLOC | PTR_TRUSTED)
static inline bool bpf_type_has_unsafe_modifiers(u32 type)
{
return type_flag(type) & ~BPF_REG_TRUSTED_MODIFIERS;
}
#endif /* _LINUX_BPF_VERIFIER_H */

65
include/linux/btf.h
View File

@ -19,36 +19,53 @@
#define KF_RELEASE (1 << 1) /* kfunc is a release function */
#define KF_RET_NULL (1 << 2) /* kfunc returns a pointer that may be NULL */
#define KF_KPTR_GET (1 << 3) /* kfunc returns reference to a kptr */
/* Trusted arguments are those which are meant to be referenced arguments with
* unchanged offset. It is used to enforce that pointers obtained from acquire
* kfuncs remain unmodified when being passed to helpers taking trusted args.
/* Trusted arguments are those which are guaranteed to be valid when passed to
* the kfunc. It is used to enforce that pointers obtained from either acquire
* kfuncs, or from the main kernel on a tracepoint or struct_ops callback
* invocation, remain unmodified when being passed to helpers taking trusted
* args.
*
* Consider
* struct foo {
* int data;
* struct foo *next;
* };
* Consider, for example, the following new task tracepoint:
*
* struct bar {
* int data;
* struct foo f;
* };
* SEC("tp_btf/task_newtask")
* int BPF_PROG(new_task_tp, struct task_struct *task, u64 clone_flags)
* {
* ...
* }
*
* struct foo *f = alloc_foo(); // Acquire kfunc
* struct bar *b = alloc_bar(); // Acquire kfunc
* And the following kfunc:
*
* If a kfunc set_foo_data() wants to operate only on the allocated object, it
* will set the KF_TRUSTED_ARGS flag, which will prevent unsafe usage like:
* BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
*
* set_foo_data(f, 42); // Allowed
* set_foo_data(f->next, 42); // Rejected, non-referenced pointer
* set_foo_data(&f->next, 42);// Rejected, referenced, but wrong type
* set_foo_data(&b->f, 42); // Rejected, referenced, but bad offset
* All invocations to the kfunc must pass the unmodified, unwalked task:
*
* In the final case, usually for the purposes of type matching, it is deduced
* by looking at the type of the member at the offset, but due to the
* requirement of trusted argument, this deduction will be strict and not done
* for this case.
* bpf_task_acquire(task); // Allowed
* bpf_task_acquire(task->last_wakee); // Rejected, walked task
*
* Programs may also pass referenced tasks directly to the kfunc:
*
* struct task_struct *acquired;
*
* acquired = bpf_task_acquire(task); // Allowed, same as above
* bpf_task_acquire(acquired); // Allowed
* bpf_task_acquire(task); // Allowed
* bpf_task_acquire(acquired->last_wakee); // Rejected, walked task
*
* Programs may _not_, however, pass a task from an arbitrary fentry/fexit, or
* kprobe/kretprobe to the kfunc, as BPF cannot guarantee that all of these
* pointers are guaranteed to be safe. For example, the following BPF program
* would be rejected:
*
* SEC("kretprobe/free_task")
* int BPF_PROG(free_task_probe, struct task_struct *tsk)
* {
* struct task_struct *acquired;
*
* acquired = bpf_task_acquire(acquired); // Rejected, not a trusted pointer
* bpf_task_release(acquired);
*
* return 0;
* }
*/
#define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer arguments */
#define KF_SLEEPABLE (1 << 5) /* kfunc may sleep */

8
kernel/bpf/btf.c
View File

@ -5799,6 +5799,11 @@ static u32 get_ctx_arg_idx(struct btf *btf, const struct btf_type *func_proto,
return nr_args + 1;
}
static bool prog_type_args_trusted(enum bpf_prog_type prog_type)
{
return prog_type == BPF_PROG_TYPE_TRACING || prog_type == BPF_PROG_TYPE_STRUCT_OPS;
}
bool btf_ctx_access(int off, int size, enum bpf_access_type type,
const struct bpf_prog *prog,
struct bpf_insn_access_aux *info)
@ -5942,6 +5947,9 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
}
info->reg_type = PTR_TO_BTF_ID;
if (prog_type_args_trusted(prog->type))
info->reg_type |= PTR_TRUSTED;
if (tgt_prog) {
enum bpf_prog_type tgt_type;

69
kernel/bpf/verifier.c
View File

@ -589,12 +589,13 @@ static const char *reg_type_str(struct bpf_verifier_env *env,
strncpy(postfix, "_or_null", 16);
}
snprintf(prefix, sizeof(prefix), "%s%s%s%s%s",
snprintf(prefix, sizeof(prefix), "%s%s%s%s%s%s",
type & MEM_RDONLY ? "rdonly_" : "",
type & MEM_RINGBUF ? "ringbuf_" : "",
type & MEM_USER ? "user_" : "",
type & MEM_PERCPU ? "percpu_" : "",
type & PTR_UNTRUSTED ? "untrusted_" : ""
type & PTR_UNTRUSTED ? "untrusted_" : "",
type & PTR_TRUSTED ? "trusted_" : ""
);
snprintf(env->type_str_buf, TYPE_STR_BUF_LEN, "%s%s%s",
@ -3856,7 +3857,7 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
struct bpf_reg_state *reg, u32 regno)
{
const char *targ_name = kernel_type_name(kptr_field->kptr.btf, kptr_field->kptr.btf_id);
int perm_flags = PTR_MAYBE_NULL;
int perm_flags = PTR_MAYBE_NULL | PTR_TRUSTED;
const char *reg_name = "";
/* Only unreferenced case accepts untrusted pointers */
@ -4732,6 +4733,9 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
if (type_flag(reg->type) & PTR_UNTRUSTED)
flag |= PTR_UNTRUSTED;
/* Any pointer obtained from walking a trusted pointer is no longer trusted. */
flag &= ~PTR_TRUSTED;
if (atype == BPF_READ && value_regno >= 0)
mark_btf_ld_reg(env, regs, value_regno, ret, reg->btf, btf_id, flag);
@ -5844,6 +5848,7 @@ static const struct bpf_reg_types btf_id_sock_common_types = {
PTR_TO_TCP_SOCK,
PTR_TO_XDP_SOCK,
PTR_TO_BTF_ID,
PTR_TO_BTF_ID | PTR_TRUSTED,
},
.btf_id = &btf_sock_ids[BTF_SOCK_TYPE_SOCK_COMMON],
};
@ -5884,8 +5889,18 @@ 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 ringbuf_mem_types = { .types = { PTR_TO_MEM | MEM_RINGBUF } };
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 percpu_btf_ptr_types = { .types = { PTR_TO_BTF_ID | MEM_PERCPU } };
static const struct bpf_reg_types btf_ptr_types = {
.types = {
PTR_TO_BTF_ID,
PTR_TO_BTF_ID | PTR_TRUSTED,
},
};
static const struct bpf_reg_types percpu_btf_ptr_types = {
.types = {
PTR_TO_BTF_ID | MEM_PERCPU,
PTR_TO_BTF_ID | MEM_PERCPU | PTR_TRUSTED,
}
};
static const struct bpf_reg_types func_ptr_types = { .types = { PTR_TO_FUNC } };
static const struct bpf_reg_types stack_ptr_types = { .types = { PTR_TO_STACK } };
static const struct bpf_reg_types const_str_ptr_types = { .types = { PTR_TO_MAP_VALUE } };
@ -5973,7 +5988,7 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
return -EACCES;
found:
if (reg->type == PTR_TO_BTF_ID) {
if (reg->type == PTR_TO_BTF_ID || reg->type & PTR_TRUSTED) {
/* For bpf_sk_release, it needs to match against first member
* 'struct sock_common', hence make an exception for it. This
* allows bpf_sk_release to work for multiple socket types.
@ -6055,6 +6070,8 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
*/
case PTR_TO_BTF_ID:
case PTR_TO_BTF_ID | MEM_ALLOC:
case PTR_TO_BTF_ID | PTR_TRUSTED:
case PTR_TO_BTF_ID | MEM_ALLOC | PTR_TRUSTED:
/* When referenced PTR_TO_BTF_ID is passed to release function,
* it's fixed offset must be 0. In the other cases, fixed offset
* can be non-zero.
@ -7939,6 +7956,25 @@ 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_trusted_reg(const struct bpf_reg_state *reg)
{
/* A referenced register is always trusted. */
if (reg->ref_obj_id)
return true;
/* If a register is not referenced, it is trusted if it has either the
* MEM_ALLOC or PTR_TRUSTED type modifiers, and no others. Some of the
* other type modifiers may be safe, but we elect to take an opt-in
* approach here as some (e.g. PTR_UNTRUSTED and PTR_MAYBE_NULL) are
* not.
*
* Eventually, we should make PTR_TRUSTED the single source of truth
* for whether a register is trusted.
*/
return type_flag(reg->type) & BPF_REG_TRUSTED_MODIFIERS &&
!bpf_type_has_unsafe_modifiers(reg->type);
}
static bool __kfunc_param_match_suffix(const struct btf *btf,
const struct btf_param *arg,
const char *suffix)
@ -8220,7 +8256,7 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
const char *reg_ref_tname;
u32 reg_ref_id;
if (reg->type == PTR_TO_BTF_ID) {
if (base_type(reg->type) == PTR_TO_BTF_ID) {
reg_btf = reg->btf;
reg_ref_id = reg->btf_id;
} else {
@ -8366,6 +8402,7 @@ static int check_reg_allocation_locked(struct bpf_verifier_env *env, struct bpf_
ptr = reg->map_ptr;
break;
case PTR_TO_BTF_ID | MEM_ALLOC:
case PTR_TO_BTF_ID | MEM_ALLOC | PTR_TRUSTED:
ptr = reg->btf;
break;
default:
@ -8596,8 +8633,9 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
case KF_ARG_PTR_TO_BTF_ID:
if (!is_kfunc_trusted_args(meta))
break;
if (!reg->ref_obj_id) {
verbose(env, "R%d must be referenced\n", regno);
if (!is_trusted_reg(reg)) {
verbose(env, "R%d must be referenced or trusted\n", regno);
return -EINVAL;
}
fallthrough;
@ -8702,9 +8740,13 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
break;
case KF_ARG_PTR_TO_BTF_ID:
/* Only base_type is checked, further checks are done here */
if (reg->type != PTR_TO_BTF_ID &&
(!reg2btf_ids[base_type(reg->type)] || type_flag(reg->type))) {
verbose(env, "arg#%d expected pointer to btf or socket\n", i);
if ((base_type(reg->type) != PTR_TO_BTF_ID ||
bpf_type_has_unsafe_modifiers(reg->type)) &&
!reg2btf_ids[base_type(reg->type)]) {
verbose(env, "arg#%d is %s ", i, reg_type_str(env, reg->type));
verbose(env, "expected %s or socket\n",
reg_type_str(env, base_type(reg->type) |
(type_flag(reg->type) & BPF_REG_TRUSTED_MODIFIERS)));
return -EINVAL;
}
ret = process_kf_arg_ptr_to_btf_id(env, reg, ref_t, ref_tname, ref_id, meta, i);
@ -14713,6 +14755,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
break;
case PTR_TO_BTF_ID:
case PTR_TO_BTF_ID | PTR_UNTRUSTED:
case PTR_TO_BTF_ID | PTR_TRUSTED:
/* PTR_TO_BTF_ID | MEM_ALLOC always has a valid lifetime, unlike
* PTR_TO_BTF_ID, and an active ref_obj_id, but the same cannot
* be said once it is marked PTR_UNTRUSTED, hence we must handle
@ -14720,6 +14763,8 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
* for this case.
*/
case PTR_TO_BTF_ID | MEM_ALLOC | PTR_UNTRUSTED:
case PTR_TO_BTF_ID | PTR_UNTRUSTED | PTR_TRUSTED:
case PTR_TO_BTF_ID | PTR_UNTRUSTED | MEM_ALLOC | PTR_TRUSTED:
if (type == BPF_READ) {
insn->code = BPF_LDX | BPF_PROBE_MEM |
BPF_SIZE((insn)->code);

2
kernel/trace/bpf_trace.c
View File

@ -774,7 +774,7 @@ BPF_CALL_0(bpf_get_current_task_btf)
const struct bpf_func_proto bpf_get_current_task_btf_proto = {
.func = bpf_get_current_task_btf,
.gpl_only = true,
.ret_type = RET_PTR_TO_BTF_ID,
.ret_type = RET_PTR_TO_BTF_ID_TRUSTED,
.ret_btf_id = &btf_tracing_ids[BTF_TRACING_TYPE_TASK],
};

4
net/ipv4/bpf_tcp_ca.c
View File

@ -61,7 +61,9 @@ static bool bpf_tcp_ca_is_valid_access(int off, int size,
if (!bpf_tracing_btf_ctx_access(off, size, type, prog, info))
return false;
if (info->reg_type == PTR_TO_BTF_ID && info->btf_id == sock_id)
if (base_type(info->reg_type) == PTR_TO_BTF_ID &&
!bpf_type_has_unsafe_modifiers(info->reg_type) &&
info->btf_id == sock_id)
/* promote it to tcp_sock */
info->btf_id = tcp_sock_id;

2
tools/testing/selftests/bpf/verifier/calls.c
View File

@ -109,7 +109,7 @@
},
.prog_type = BPF_PROG_TYPE_SCHED_CLS,
.result = REJECT,
.errstr = "arg#0 expected pointer to btf or socket",
.errstr = "arg#0 is ptr_or_null_ expected ptr_ or socket",
.fixup_kfunc_btf_id = {
{ "bpf_kfunc_call_test_acquire", 3 },
{ "bpf_kfunc_call_test_release", 5 },

4
tools/testing/selftests/bpf/verifier/ref_tracking.c
View File

@ -142,7 +142,7 @@
.kfunc = "bpf",
.expected_attach_type = BPF_LSM_MAC,
.flags = BPF_F_SLEEPABLE,
.errstr = "arg#0 expected pointer to btf or socket",
.errstr = "arg#0 is ptr_or_null_ expected ptr_ or socket",
.fixup_kfunc_btf_id = {
{ "bpf_lookup_user_key", 2 },
{ "bpf_key_put", 4 },
@ -163,7 +163,7 @@
.kfunc = "bpf",
.expected_attach_type = BPF_LSM_MAC,
.flags = BPF_F_SLEEPABLE,
.errstr = "arg#0 expected pointer to btf or socket",
.errstr = "arg#0 is ptr_or_null_ expected ptr_ or socket",
.fixup_kfunc_btf_id = {
{ "bpf_lookup_system_key", 1 },
{ "bpf_key_put", 3 },