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bpf: Support bpf_trampoline on functions with IPMODIFY (e.g. livepatch)
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When tracing a function with IPMODIFY ftrace_ops (livepatch), the bpf
trampoline must follow the instruction pointer saved on stack. This needs
extra handling for bpf trampolines with BPF_TRAMP_F_CALL_ORIG flag.

Implement bpf_tramp_ftrace_ops_func and use it for the ftrace_ops used
by BPF trampoline. This enables tracing functions with livepatch.

This also requires moving bpf trampoline to *_ftrace_direct_mult APIs.

Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/all/20220602193706.2607681-2-song@kernel.org/
Link: https://lore.kernel.org/bpf/20220720002126.803253-5-song@kernel.org
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@liu-song-6 @borkmann
liu-song-6 authored and borkmann committed Jul 22, 2022
1 parent 316cba6 commit 00963a2
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Showing 2 changed files with 149 additions and 17 deletions.
8 changes: 8 additions & 0 deletions include/linux/bpf.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -47,6 +47,7 @@ struct kobject;
struct mem_cgroup;
struct module;
struct bpf_func_state;
struct ftrace_ops;

extern struct idr btf_idr;
extern spinlock_t btf_idr_lock;
Expand Down Expand Up @@ -756,6 +757,11 @@ struct btf_func_model {
*/
#define BPF_TRAMP_F_ORIG_STACK BIT(5)

/* This trampoline is on a function with another ftrace_ops with IPMODIFY,
* e.g., a live patch. This flag is set and cleared by ftrace call backs,
*/
#define BPF_TRAMP_F_SHARE_IPMODIFY BIT(6)

/* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
* bytes on x86.
*/
Expand Down Expand Up @@ -838,9 +844,11 @@ struct bpf_tramp_image {
struct bpf_trampoline {
/* hlist for trampoline_table */
struct hlist_node hlist;
struct ftrace_ops *fops;
/* serializes access to fields of this trampoline */
struct mutex mutex;
refcount_t refcnt;
u32 flags;
u64 key;
struct {
struct btf_func_model model;
Expand Down
158 changes: 141 additions & 17 deletions kernel/bpf/trampoline.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@
#include <linux/static_call.h>
#include <linux/bpf_verifier.h>
#include <linux/bpf_lsm.h>
#include <linux/delay.h>

/* dummy _ops. The verifier will operate on target program's ops. */
const struct bpf_verifier_ops bpf_extension_verifier_ops = {
Expand All @@ -29,6 +30,81 @@ static struct hlist_head trampoline_table[TRAMPOLINE_TABLE_SIZE];
/* serializes access to trampoline_table */
static DEFINE_MUTEX(trampoline_mutex);

#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mutex);

static int bpf_tramp_ftrace_ops_func(struct ftrace_ops *ops, enum ftrace_ops_cmd cmd)
{
struct bpf_trampoline *tr = ops->private;
int ret = 0;

if (cmd == FTRACE_OPS_CMD_ENABLE_SHARE_IPMODIFY_SELF) {
/* This is called inside register_ftrace_direct_multi(), so
* tr->mutex is already locked.
*/
lockdep_assert_held_once(&tr->mutex);

/* Instead of updating the trampoline here, we propagate
* -EAGAIN to register_ftrace_direct_multi(). Then we can
* retry register_ftrace_direct_multi() after updating the
* trampoline.
*/
if ((tr->flags & BPF_TRAMP_F_CALL_ORIG) &&
!(tr->flags & BPF_TRAMP_F_ORIG_STACK)) {
if (WARN_ON_ONCE(tr->flags & BPF_TRAMP_F_SHARE_IPMODIFY))
return -EBUSY;

tr->flags |= BPF_TRAMP_F_SHARE_IPMODIFY;
return -EAGAIN;
}

return 0;
}

/* The normal locking order is
* tr->mutex => direct_mutex (ftrace.c) => ftrace_lock (ftrace.c)
*
* The following two commands are called from
*
* prepare_direct_functions_for_ipmodify
* cleanup_direct_functions_after_ipmodify
*
* In both cases, direct_mutex is already locked. Use
* mutex_trylock(&tr->mutex) to avoid deadlock in race condition
* (something else is making changes to this same trampoline).
*/
if (!mutex_trylock(&tr->mutex)) {
/* sleep 1 ms to make sure whatever holding tr->mutex makes
* some progress.
*/
msleep(1);
return -EAGAIN;
}

switch (cmd) {
case FTRACE_OPS_CMD_ENABLE_SHARE_IPMODIFY_PEER:
tr->flags |= BPF_TRAMP_F_SHARE_IPMODIFY;

if ((tr->flags & BPF_TRAMP_F_CALL_ORIG) &&
!(tr->flags & BPF_TRAMP_F_ORIG_STACK))
ret = bpf_trampoline_update(tr, false /* lock_direct_mutex */);
break;
case FTRACE_OPS_CMD_DISABLE_SHARE_IPMODIFY_PEER:
tr->flags &= ~BPF_TRAMP_F_SHARE_IPMODIFY;

if (tr->flags & BPF_TRAMP_F_ORIG_STACK)
ret = bpf_trampoline_update(tr, false /* lock_direct_mutex */);
break;
default:
ret = -EINVAL;
break;
};

mutex_unlock(&tr->mutex);
return ret;
}
#endif

bool bpf_prog_has_trampoline(const struct bpf_prog *prog)
{
enum bpf_attach_type eatype = prog->expected_attach_type;
Expand Down Expand Up @@ -89,6 +165,16 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
tr = kzalloc(sizeof(*tr), GFP_KERNEL);
if (!tr)
goto out;
#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
tr->fops = kzalloc(sizeof(struct ftrace_ops), GFP_KERNEL);
if (!tr->fops) {
kfree(tr);
tr = NULL;
goto out;
}
tr->fops->private = tr;
tr->fops->ops_func = bpf_tramp_ftrace_ops_func;
#endif

tr->key = key;
INIT_HLIST_NODE(&tr->hlist);
Expand Down Expand Up @@ -128,7 +214,7 @@ static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
int ret;

if (tr->func.ftrace_managed)
ret = unregister_ftrace_direct((long)ip, (long)old_addr);
ret = unregister_ftrace_direct_multi(tr->fops, (long)old_addr);
else
ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, NULL);

Expand All @@ -137,15 +223,20 @@ static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
return ret;
}

static int modify_fentry(struct bpf_trampoline *tr, void *old_addr, void *new_addr)
static int modify_fentry(struct bpf_trampoline *tr, void *old_addr, void *new_addr,
bool lock_direct_mutex)
{
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
if (tr->func.ftrace_managed) {
if (lock_direct_mutex)
ret = modify_ftrace_direct_multi(tr->fops, (long)new_addr);
else
ret = modify_ftrace_direct_multi_nolock(tr->fops, (long)new_addr);
} else {
ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, new_addr);
}
return ret;
}

Expand All @@ -163,10 +254,12 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
if (bpf_trampoline_module_get(tr))
return -ENOENT;

if (tr->func.ftrace_managed)
ret = register_ftrace_direct((long)ip, (long)new_addr);
else
if (tr->func.ftrace_managed) {
ftrace_set_filter_ip(tr->fops, (unsigned long)ip, 0, 0);
ret = register_ftrace_direct_multi(tr->fops, (long)new_addr);
} else {
ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, NULL, new_addr);
}

if (ret)
bpf_trampoline_module_put(tr);
Expand Down Expand Up @@ -332,11 +425,11 @@ static struct bpf_tramp_image *bpf_tramp_image_alloc(u64 key, u32 idx)
return ERR_PTR(err);
}

static int bpf_trampoline_update(struct bpf_trampoline *tr)
static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mutex)
{
struct bpf_tramp_image *im;
struct bpf_tramp_links *tlinks;
u32 flags = BPF_TRAMP_F_RESTORE_REGS;
u32 orig_flags = tr->flags;
bool ip_arg = false;
int err, total;

Expand All @@ -358,18 +451,31 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr)
goto out;
}

/* clear all bits except SHARE_IPMODIFY */
tr->flags &= BPF_TRAMP_F_SHARE_IPMODIFY;

if (tlinks[BPF_TRAMP_FEXIT].nr_links ||
tlinks[BPF_TRAMP_MODIFY_RETURN].nr_links)
tlinks[BPF_TRAMP_MODIFY_RETURN].nr_links) {
/* NOTE: BPF_TRAMP_F_RESTORE_REGS and BPF_TRAMP_F_SKIP_FRAME
* should not be set together.
*/
flags = BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_SKIP_FRAME;
tr->flags |= BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_SKIP_FRAME;
} else {
tr->flags |= BPF_TRAMP_F_RESTORE_REGS;
}

if (ip_arg)
flags |= BPF_TRAMP_F_IP_ARG;
tr->flags |= BPF_TRAMP_F_IP_ARG;

#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
again:
if ((tr->flags & BPF_TRAMP_F_SHARE_IPMODIFY) &&
(tr->flags & BPF_TRAMP_F_CALL_ORIG))
tr->flags |= BPF_TRAMP_F_ORIG_STACK;
#endif

err = arch_prepare_bpf_trampoline(im, im->image, im->image + PAGE_SIZE,
&tr->func.model, flags, tlinks,
&tr->func.model, tr->flags, tlinks,
tr->func.addr);
if (err < 0)
goto out;
Expand All @@ -378,17 +484,34 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr)
WARN_ON(!tr->cur_image && tr->selector);
if (tr->cur_image)
/* progs already running at this address */
err = modify_fentry(tr, tr->cur_image->image, im->image);
err = modify_fentry(tr, tr->cur_image->image, im->image, lock_direct_mutex);
else
/* first time registering */
err = register_fentry(tr, im->image);

#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
if (err == -EAGAIN) {
/* -EAGAIN from bpf_tramp_ftrace_ops_func. Now
* BPF_TRAMP_F_SHARE_IPMODIFY is set, we can generate the
* trampoline again, and retry register.
*/
/* reset fops->func and fops->trampoline for re-register */
tr->fops->func = NULL;
tr->fops->trampoline = 0;
goto again;
}
#endif
if (err)
goto out;

if (tr->cur_image)
bpf_tramp_image_put(tr->cur_image);
tr->cur_image = im;
tr->selector++;
out:
/* If any error happens, restore previous flags */
if (err)
tr->flags = orig_flags;
kfree(tlinks);
return err;
}
Expand Down Expand Up @@ -454,7 +577,7 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_tr

hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
tr->progs_cnt[kind]++;
err = bpf_trampoline_update(tr);
err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
if (err) {
hlist_del_init(&link->tramp_hlist);
tr->progs_cnt[kind]--;
Expand Down Expand Up @@ -487,7 +610,7 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_
}
hlist_del_init(&link->tramp_hlist);
tr->progs_cnt[kind]--;
return bpf_trampoline_update(tr);
return bpf_trampoline_update(tr, true /* lock_direct_mutex */);
}

/* bpf_trampoline_unlink_prog() should never fail. */
Expand Down Expand Up @@ -715,6 +838,7 @@ void bpf_trampoline_put(struct bpf_trampoline *tr)
* multiple rcu callbacks.
*/
hlist_del(&tr->hlist);
kfree(tr->fops);
kfree(tr);
out:
mutex_unlock(&trampoline_mutex);
Expand Down

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