linux/kernel/bpf/trampoline.c
KP Singh 9e4e01dfd3 bpf: lsm: Implement attach, detach and execution
JITed BPF programs are dynamically attached to the LSM hooks
using BPF trampolines. The trampoline prologue generates code to handle
conversion of the signature of the hook to the appropriate BPF context.

The allocated trampoline programs are attached to the nop functions
initialized as LSM hooks.

BPF_PROG_TYPE_LSM programs must have a GPL compatible license and
and need CAP_SYS_ADMIN (required for loading eBPF programs).

Upon attachment:

* A BPF fexit trampoline is used for LSM hooks with a void return type.
* A BPF fmod_ret trampoline is used for LSM hooks which return an
  int. The attached programs can override the return value of the
  bpf LSM hook to indicate a MAC Policy decision.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Florent Revest <revest@google.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: James Morris <jamorris@linux.microsoft.com>
Link: https://lore.kernel.org/bpf/20200329004356.27286-5-kpsingh@chromium.org
2020-03-30 01:34:00 +02:00

415 lines
10 KiB
C

// SPDX-License-Identifier: GPL-2.0-only
/* Copyright (c) 2019 Facebook */
#include <linux/hash.h>
#include <linux/bpf.h>
#include <linux/filter.h>
#include <linux/ftrace.h>
#include <linux/rbtree_latch.h>
#include <linux/perf_event.h>
#include <linux/btf.h>
/* dummy _ops. The verifier will operate on target program's ops. */
const struct bpf_verifier_ops bpf_extension_verifier_ops = {
};
const struct bpf_prog_ops bpf_extension_prog_ops = {
};
/* btf_vmlinux has ~22k attachable functions. 1k htab is enough. */
#define TRAMPOLINE_HASH_BITS 10
#define TRAMPOLINE_TABLE_SIZE (1 << TRAMPOLINE_HASH_BITS)
static struct hlist_head trampoline_table[TRAMPOLINE_TABLE_SIZE];
/* serializes access to trampoline_table */
static DEFINE_MUTEX(trampoline_mutex);
void *bpf_jit_alloc_exec_page(void)
{
void *image;
image = bpf_jit_alloc_exec(PAGE_SIZE);
if (!image)
return NULL;
set_vm_flush_reset_perms(image);
/* Keep image as writeable. The alternative is to keep flipping ro/rw
* everytime new program is attached or detached.
*/
set_memory_x((long)image, 1);
return image;
}
void bpf_image_ksym_add(void *data, struct bpf_ksym *ksym)
{
ksym->start = (unsigned long) data;
ksym->end = ksym->start + PAGE_SIZE;
bpf_ksym_add(ksym);
perf_event_ksymbol(PERF_RECORD_KSYMBOL_TYPE_BPF, ksym->start,
PAGE_SIZE, false, ksym->name);
}
void bpf_image_ksym_del(struct bpf_ksym *ksym)
{
bpf_ksym_del(ksym);
perf_event_ksymbol(PERF_RECORD_KSYMBOL_TYPE_BPF, ksym->start,
PAGE_SIZE, true, ksym->name);
}
static void bpf_trampoline_ksym_add(struct bpf_trampoline *tr)
{
struct bpf_ksym *ksym = &tr->ksym;
snprintf(ksym->name, KSYM_NAME_LEN, "bpf_trampoline_%llu", tr->key);
bpf_image_ksym_add(tr->image, ksym);
}
struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
{
struct bpf_trampoline *tr;
struct hlist_head *head;
void *image;
int i;
mutex_lock(&trampoline_mutex);
head = &trampoline_table[hash_64(key, TRAMPOLINE_HASH_BITS)];
hlist_for_each_entry(tr, head, hlist) {
if (tr->key == key) {
refcount_inc(&tr->refcnt);
goto out;
}
}
tr = kzalloc(sizeof(*tr), GFP_KERNEL);
if (!tr)
goto out;
/* is_root was checked earlier. No need for bpf_jit_charge_modmem() */
image = bpf_jit_alloc_exec_page();
if (!image) {
kfree(tr);
tr = NULL;
goto out;
}
tr->key = key;
INIT_HLIST_NODE(&tr->hlist);
hlist_add_head(&tr->hlist, head);
refcount_set(&tr->refcnt, 1);
mutex_init(&tr->mutex);
for (i = 0; i < BPF_TRAMP_MAX; i++)
INIT_HLIST_HEAD(&tr->progs_hlist[i]);
tr->image = image;
INIT_LIST_HEAD_RCU(&tr->ksym.lnode);
bpf_trampoline_ksym_add(tr);
out:
mutex_unlock(&trampoline_mutex);
return tr;
}
static int is_ftrace_location(void *ip)
{
long addr;
addr = ftrace_location((long)ip);
if (!addr)
return 0;
if (WARN_ON_ONCE(addr != (long)ip))
return -EFAULT;
return 1;
}
static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
{
void *ip = tr->func.addr;
int ret;
if (tr->func.ftrace_managed)
ret = unregister_ftrace_direct((long)ip, (long)old_addr);
else
ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, NULL);
return ret;
}
static int modify_fentry(struct bpf_trampoline *tr, void *old_addr, void *new_addr)
{
void *ip = tr->func.addr;
int ret;
if (tr->func.ftrace_managed)
ret = modify_ftrace_direct((long)ip, (long)old_addr, (long)new_addr);
else
ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, new_addr);
return ret;
}
/* first time registering */
static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
{
void *ip = tr->func.addr;
int ret;
ret = is_ftrace_location(ip);
if (ret < 0)
return ret;
tr->func.ftrace_managed = ret;
if (tr->func.ftrace_managed)
ret = register_ftrace_direct((long)ip, (long)new_addr);
else
ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, NULL, new_addr);
return ret;
}
static struct bpf_tramp_progs *
bpf_trampoline_get_progs(const struct bpf_trampoline *tr, int *total)
{
const struct bpf_prog_aux *aux;
struct bpf_tramp_progs *tprogs;
struct bpf_prog **progs;
int kind;
*total = 0;
tprogs = kcalloc(BPF_TRAMP_MAX, sizeof(*tprogs), GFP_KERNEL);
if (!tprogs)
return ERR_PTR(-ENOMEM);
for (kind = 0; kind < BPF_TRAMP_MAX; kind++) {
tprogs[kind].nr_progs = tr->progs_cnt[kind];
*total += tr->progs_cnt[kind];
progs = tprogs[kind].progs;
hlist_for_each_entry(aux, &tr->progs_hlist[kind], tramp_hlist)
*progs++ = aux->prog;
}
return tprogs;
}
static int bpf_trampoline_update(struct bpf_trampoline *tr)
{
void *old_image = tr->image + ((tr->selector + 1) & 1) * PAGE_SIZE/2;
void *new_image = tr->image + (tr->selector & 1) * PAGE_SIZE/2;
struct bpf_tramp_progs *tprogs;
u32 flags = BPF_TRAMP_F_RESTORE_REGS;
int err, total;
tprogs = bpf_trampoline_get_progs(tr, &total);
if (IS_ERR(tprogs))
return PTR_ERR(tprogs);
if (total == 0) {
err = unregister_fentry(tr, old_image);
tr->selector = 0;
goto out;
}
if (tprogs[BPF_TRAMP_FEXIT].nr_progs ||
tprogs[BPF_TRAMP_MODIFY_RETURN].nr_progs)
flags = BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_SKIP_FRAME;
/* Though the second half of trampoline page is unused a task could be
* preempted in the middle of the first half of trampoline and two
* updates to trampoline would change the code from underneath the
* preempted task. Hence wait for tasks to voluntarily schedule or go
* to userspace.
*/
synchronize_rcu_tasks();
err = arch_prepare_bpf_trampoline(new_image, new_image + PAGE_SIZE / 2,
&tr->func.model, flags, tprogs,
tr->func.addr);
if (err < 0)
goto out;
if (tr->selector)
/* progs already running at this address */
err = modify_fentry(tr, old_image, new_image);
else
/* first time registering */
err = register_fentry(tr, new_image);
if (err)
goto out;
tr->selector++;
out:
kfree(tprogs);
return err;
}
static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(struct bpf_prog *prog)
{
switch (prog->expected_attach_type) {
case BPF_TRACE_FENTRY:
return BPF_TRAMP_FENTRY;
case BPF_MODIFY_RETURN:
return BPF_TRAMP_MODIFY_RETURN;
case BPF_TRACE_FEXIT:
return BPF_TRAMP_FEXIT;
case BPF_LSM_MAC:
if (!prog->aux->attach_func_proto->type)
/* The function returns void, we cannot modify its
* return value.
*/
return BPF_TRAMP_FEXIT;
else
return BPF_TRAMP_MODIFY_RETURN;
default:
return BPF_TRAMP_REPLACE;
}
}
int bpf_trampoline_link_prog(struct bpf_prog *prog)
{
enum bpf_tramp_prog_type kind;
struct bpf_trampoline *tr;
int err = 0;
int cnt;
tr = prog->aux->trampoline;
kind = bpf_attach_type_to_tramp(prog);
mutex_lock(&tr->mutex);
if (tr->extension_prog) {
/* cannot attach fentry/fexit if extension prog is attached.
* cannot overwrite extension prog either.
*/
err = -EBUSY;
goto out;
}
cnt = tr->progs_cnt[BPF_TRAMP_FENTRY] + tr->progs_cnt[BPF_TRAMP_FEXIT];
if (kind == BPF_TRAMP_REPLACE) {
/* Cannot attach extension if fentry/fexit are in use. */
if (cnt) {
err = -EBUSY;
goto out;
}
tr->extension_prog = prog;
err = bpf_arch_text_poke(tr->func.addr, BPF_MOD_JUMP, NULL,
prog->bpf_func);
goto out;
}
if (cnt >= BPF_MAX_TRAMP_PROGS) {
err = -E2BIG;
goto out;
}
if (!hlist_unhashed(&prog->aux->tramp_hlist)) {
/* prog already linked */
err = -EBUSY;
goto out;
}
hlist_add_head(&prog->aux->tramp_hlist, &tr->progs_hlist[kind]);
tr->progs_cnt[kind]++;
err = bpf_trampoline_update(prog->aux->trampoline);
if (err) {
hlist_del(&prog->aux->tramp_hlist);
tr->progs_cnt[kind]--;
}
out:
mutex_unlock(&tr->mutex);
return err;
}
/* bpf_trampoline_unlink_prog() should never fail. */
int bpf_trampoline_unlink_prog(struct bpf_prog *prog)
{
enum bpf_tramp_prog_type kind;
struct bpf_trampoline *tr;
int err;
tr = prog->aux->trampoline;
kind = bpf_attach_type_to_tramp(prog);
mutex_lock(&tr->mutex);
if (kind == BPF_TRAMP_REPLACE) {
WARN_ON_ONCE(!tr->extension_prog);
err = bpf_arch_text_poke(tr->func.addr, BPF_MOD_JUMP,
tr->extension_prog->bpf_func, NULL);
tr->extension_prog = NULL;
goto out;
}
hlist_del(&prog->aux->tramp_hlist);
tr->progs_cnt[kind]--;
err = bpf_trampoline_update(prog->aux->trampoline);
out:
mutex_unlock(&tr->mutex);
return err;
}
void bpf_trampoline_put(struct bpf_trampoline *tr)
{
if (!tr)
return;
mutex_lock(&trampoline_mutex);
if (!refcount_dec_and_test(&tr->refcnt))
goto out;
WARN_ON_ONCE(mutex_is_locked(&tr->mutex));
if (WARN_ON_ONCE(!hlist_empty(&tr->progs_hlist[BPF_TRAMP_FENTRY])))
goto out;
if (WARN_ON_ONCE(!hlist_empty(&tr->progs_hlist[BPF_TRAMP_FEXIT])))
goto out;
bpf_image_ksym_del(&tr->ksym);
/* wait for tasks to get out of trampoline before freeing it */
synchronize_rcu_tasks();
bpf_jit_free_exec(tr->image);
hlist_del(&tr->hlist);
kfree(tr);
out:
mutex_unlock(&trampoline_mutex);
}
/* The logic is similar to BPF_PROG_RUN, but with an explicit
* rcu_read_lock() and migrate_disable() which are required
* for the trampoline. The macro is split into
* call _bpf_prog_enter
* call prog->bpf_func
* call __bpf_prog_exit
*/
u64 notrace __bpf_prog_enter(void)
__acquires(RCU)
{
u64 start = 0;
rcu_read_lock();
migrate_disable();
if (static_branch_unlikely(&bpf_stats_enabled_key))
start = sched_clock();
return start;
}
void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start)
__releases(RCU)
{
struct bpf_prog_stats *stats;
if (static_branch_unlikely(&bpf_stats_enabled_key) &&
/* static_key could be enabled in __bpf_prog_enter
* and disabled in __bpf_prog_exit.
* And vice versa.
* Hence check that 'start' is not zero.
*/
start) {
stats = this_cpu_ptr(prog->aux->stats);
u64_stats_update_begin(&stats->syncp);
stats->cnt++;
stats->nsecs += sched_clock() - start;
u64_stats_update_end(&stats->syncp);
}
migrate_enable();
rcu_read_unlock();
}
int __weak
arch_prepare_bpf_trampoline(void *image, void *image_end,
const struct btf_func_model *m, u32 flags,
struct bpf_tramp_progs *tprogs,
void *orig_call)
{
return -ENOTSUPP;
}
static int __init init_trampolines(void)
{
int i;
for (i = 0; i < TRAMPOLINE_TABLE_SIZE; i++)
INIT_HLIST_HEAD(&trampoline_table[i]);
return 0;
}
late_initcall(init_trampolines);