[PATCH AUTOSEL 5.4 326/350] bpf: Switch bpf_map ref counter to atomic64_t so bpf_map_inc() never fails

Sasha Levin
2019-12-10T21:13:42Z

From: Andrii Nakryiko <andriin@fb.com>

[ Upstream commit 1e0bd5a091e5d9e0f1d5b0e6329b87bb1792f784 ]

92117d8443bc ("bpf: fix refcnt overflow") turned refcounting of bpf_map into
potentially failing operation, when refcount reaches BPF_MAX_REFCNT limit
(32k). Due to using 32-bit counter, it's possible in practice to overflow
refcounter and make it wrap around to 0, causing erroneous map free, while
there are still references to it, causing use-after-free problems.

But having a failing refcounting operations are problematic in some cases. One
example is mmap() interface. After establishing initial memory-mapping, user
is allowed to arbitrarily map/remap/unmap parts of mapped memory, arbitrarily
splitting it into multiple non-contiguous regions. All this happening without
any control from the users of mmap subsystem. Rather mmap subsystem sends
notifications to original creator of memory mapping through open/close
callbacks, which are optionally specified during initial memory mapping
creation. These callbacks are used to maintain accurate refcount for bpf_map
(see next patch in this series). The problem is that open() callback is not
supposed to fail, because memory-mapped resource is set up and properly
referenced. This is posing a problem for using memory-mapping with BPF maps.

One solution to this is to maintain separate refcount for just memory-mappings
and do single bpf_map_inc/bpf_map_put when it goes from/to zero, respectively.
There are similar use cases in current work on tcp-bpf, necessitating extra
counter as well. This seems like a rather unfortunate and ugly solution that
doesn't scale well to various new use cases.

Another approach to solve this is to use non-failing refcount_t type, which
uses 32-bit counter internally, but, once reaching overflow state at UINT_MAX,
stays there. This utlimately causes memory leak, but prevents use after free.

But given refcounting is not the most performance-critical operation with BPF
maps (it's not used from running BPF program code), we can also just switch to
64-bit counter that can't overflow in practice, potentially disadvantaging
32-bit platforms a tiny bit. This simplifies semantics and allows above
described scenarios to not worry about failing refcount increment operation.

In terms of struct bpf_map size, we are still good and use the same amount of
space:

BEFORE (3 cache lines, 8 bytes of padding at the end):
struct bpf_map {
const struct bpf_map_ops * ops __attribute__((__aligned__(64))); /* 0 8 */
struct bpf_map * inner_map_meta; /* 8 8 */
void * security; /* 16 8 */
enum bpf_map_type map_type; /* 24 4 */
u32 key_size; /* 28 4 */
u32 value_size; /* 32 4 */
u32 max_entries; /* 36 4 */
u32 map_flags; /* 40 4 */
int spin_lock_off; /* 44 4 */
u32 id; /* 48 4 */
int numa_node; /* 52 4 */
u32 btf_key_type_id; /* 56 4 */
u32 btf_value_type_id; /* 60 4 */
/* --- cacheline 1 boundary (64 bytes) --- */
struct btf * btf; /* 64 8 */
struct bpf_map_memory memory; /* 72 16 */
bool unpriv_array; /* 88 1 */
bool frozen; /* 89 1 */

/* XXX 38 bytes hole, try to pack */

/* --- cacheline 2 boundary (128 bytes) --- */
atomic_t refcnt __attribute__((__aligned__(64))); /* 128 4 */
atomic_t usercnt; /* 132 4 */
struct work_struct work; /* 136 32 */
char name[16]; /* 168 16 */

/* size: 192, cachelines: 3, members: 21 */
/* sum members: 146, holes: 1, sum holes: 38 */
/* padding: 8 */
/* forced alignments: 2, forced holes: 1, sum forced holes: 38 */
} __attribute__((__aligned__(64)));

AFTER (same 3 cache lines, no extra padding now):
struct bpf_map {
const struct bpf_map_ops * ops __attribute__((__aligned__(64))); /* 0 8 */
struct bpf_map * inner_map_meta; /* 8 8 */
void * security; /* 16 8 */
enum bpf_map_type map_type; /* 24 4 */
u32 key_size; /* 28 4 */
u32 value_size; /* 32 4 */
u32 max_entries; /* 36 4 */
u32 map_flags; /* 40 4 */
int spin_lock_off; /* 44 4 */
u32 id; /* 48 4 */
int numa_node; /* 52 4 */
u32 btf_key_type_id; /* 56 4 */
u32 btf_value_type_id; /* 60 4 */
/* --- cacheline 1 boundary (64 bytes) --- */
struct btf * btf; /* 64 8 */
struct bpf_map_memory memory; /* 72 16 */
bool unpriv_array; /* 88 1 */
bool frozen; /* 89 1 */

/* XXX 38 bytes hole, try to pack */

/* --- cacheline 2 boundary (128 bytes) --- */
atomic64_t refcnt __attribute__((__aligned__(64))); /* 128 8 */
atomic64_t usercnt; /* 136 8 */
struct work_struct work; /* 144 32 */
char name[16]; /* 176 16 */

/* size: 192, cachelines: 3, members: 21 */
/* sum members: 154, holes: 1, sum holes: 38 */
/* forced alignments: 2, forced holes: 1, sum forced holes: 38 */
} __attribute__((__aligned__(64)));

This patch, while modifying all users of bpf_map_inc, also cleans up its
interface to match bpf_map_put with separate operations for bpf_map_inc and
bpf_map_inc_with_uref (to match bpf_map_put and bpf_map_put_with_uref,
respectively). Also, given there are no users of bpf_map_inc_not_zero
specifying uref=true, remove uref flag and default to uref=false internally.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191117172806.2195367-2-andriin@fb.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
.../net/ethernet/netronome/nfp/bpf/offload.c | 4 +-
include/linux/bpf.h | 10 ++--
kernel/bpf/inode.c | 2 +-
kernel/bpf/map_in_map.c | 2 +-
kernel/bpf/syscall.c | 51 ++++++++-----------
kernel/bpf/verifier.c | 6 +--
kernel/bpf/xskmap.c | 6 +--
net/core/bpf_sk_storage.c | 2 +-
8 files changed, 34 insertions(+), 49 deletions(-)

diff --git a/drivers/net/ethernet/netronome/nfp/bpf/offload.c b/drivers/net/ethernet/netronome/nfp/bpf/offload.c
index 88fab6a82acff..06927ba5a3ae0 100644
--- a/drivers/net/ethernet/netronome/nfp/bpf/offload.c
+++ b/drivers/net/ethernet/netronome/nfp/bpf/offload.c
@@ -46,9 +46,7 @@ nfp_map_ptr_record(struct nfp_app_bpf *bpf, struct nfp_prog *nfp_prog,
/* Grab a single ref to the map for our record. The prog destroy ndo
* happens after free_used_maps().
*/
- map = bpf_map_inc(map, false);
- if (IS_ERR(map))
- return PTR_ERR(map);
+ bpf_map_inc(map);

record = kmalloc(sizeof(*record), GFP_KERNEL);
if (!record) {
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 3bf3835d0e866..78d5233b4f8ec 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -99,8 +99,8 @@ struct bpf_map {
/* The 3rd and 4th cacheline with misc members to avoid false sharing
* particularly with refcounting.
*/
- atomic_t refcnt ____cacheline_aligned;
- atomic_t usercnt;
+ atomic64_t refcnt ____cacheline_aligned;
+ atomic64_t usercnt;
struct work_struct work;
char name[BPF_OBJ_NAME_LEN];
};
@@ -649,9 +649,9 @@ void bpf_map_free_id(struct bpf_map *map, bool do_idr_lock);

struct bpf_map *bpf_map_get_with_uref(u32 ufd);
struct bpf_map *__bpf_map_get(struct fd f);
-struct bpf_map * __must_check bpf_map_inc(struct bpf_map *map, bool uref);
-struct bpf_map * __must_check bpf_map_inc_not_zero(struct bpf_map *map,
- bool uref);
+void bpf_map_inc(struct bpf_map *map);
+void bpf_map_inc_with_uref(struct bpf_map *map);
+struct bpf_map * __must_check bpf_map_inc_not_zero(struct bpf_map *map);
void bpf_map_put_with_uref(struct bpf_map *map);
void bpf_map_put(struct bpf_map *map);
int bpf_map_charge_memlock(struct bpf_map *map, u32 pages);
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index a70f7209cda3f..2f17f24258dc8 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -34,7 +34,7 @@ static void *bpf_any_get(void *raw, enum bpf_type type)
raw = bpf_prog_inc(raw);
break;
case BPF_TYPE_MAP:
- raw = bpf_map_inc(raw, true);
+ bpf_map_inc_with_uref(raw);
break;
default:
WARN_ON_ONCE(1);
diff --git a/kernel/bpf/map_in_map.c b/kernel/bpf/map_in_map.c
index fab4fb134547d..4cbe987be35b4 100644
--- a/kernel/bpf/map_in_map.c
+++ b/kernel/bpf/map_in_map.c
@@ -98,7 +98,7 @@ void *bpf_map_fd_get_ptr(struct bpf_map *map,
return inner_map;

if (bpf_map_meta_equal(map->inner_map_meta, inner_map))
- inner_map = bpf_map_inc(inner_map, false);
+ bpf_map_inc(inner_map);
else
inner_map = ERR_PTR(-EINVAL);

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index ace1cfaa24b6b..c8668ac70c982 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -313,7 +313,7 @@ static void bpf_map_free_deferred(struct work_struct *work)

static void bpf_map_put_uref(struct bpf_map *map)
{
- if (atomic_dec_and_test(&map->usercnt)) {
+ if (atomic64_dec_and_test(&map->usercnt)) {
if (map->ops->map_release_uref)
map->ops->map_release_uref(map);
}
@@ -324,7 +324,7 @@ static void bpf_map_put_uref(struct bpf_map *map)
*/
static void __bpf_map_put(struct bpf_map *map, bool do_idr_lock)
{
- if (atomic_dec_and_test(&map->refcnt)) {
+ if (atomic64_dec_and_test(&map->refcnt)) {
/* bpf_map_free_id() must be called first */
bpf_map_free_id(map, do_idr_lock);
btf_put(map->btf);
@@ -577,8 +577,8 @@ static int map_create(union bpf_attr *attr)
if (err)
goto free_map;

- atomic_set(&map->refcnt, 1);
- atomic_set(&map->usercnt, 1);
+ atomic64_set(&map->refcnt, 1);
+ atomic64_set(&map->usercnt, 1);

if (attr->btf_key_type_id || attr->btf_value_type_id) {
struct btf *btf;
@@ -655,21 +655,19 @@ struct bpf_map *__bpf_map_get(struct fd f)
return f.file->private_data;
}

-/* prog's and map's refcnt limit */
-#define BPF_MAX_REFCNT 32768
-
-struct bpf_map *bpf_map_inc(struct bpf_map *map, bool uref)
+void bpf_map_inc(struct bpf_map *map)
{
- if (atomic_inc_return(&map->refcnt) > BPF_MAX_REFCNT) {
- atomic_dec(&map->refcnt);
- return ERR_PTR(-EBUSY);
- }
- if (uref)
- atomic_inc(&map->usercnt);
- return map;
+ atomic64_inc(&map->refcnt);
}
EXPORT_SYMBOL_GPL(bpf_map_inc);

+void bpf_map_inc_with_uref(struct bpf_map *map)
+{
+ atomic64_inc(&map->refcnt);
+ atomic64_inc(&map->usercnt);
+}
+EXPORT_SYMBOL_GPL(bpf_map_inc_with_uref);
+
struct bpf_map *bpf_map_get_with_uref(u32 ufd)
{
struct fd f = fdget(ufd);
@@ -679,38 +677,30 @@ struct bpf_map *bpf_map_get_with_uref(u32 ufd)
if (IS_ERR(map))
return map;

- map = bpf_map_inc(map, true);
+ bpf_map_inc_with_uref(map);
fdput(f);

return map;
}

/* map_idr_lock should have been held */
-static struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map,
- bool uref)
+static struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map, bool uref)
{
int refold;

- refold = atomic_fetch_add_unless(&map->refcnt, 1, 0);
-
- if (refold >= BPF_MAX_REFCNT) {
- __bpf_map_put(map, false);
- return ERR_PTR(-EBUSY);
- }
-
+ refold = atomic64_fetch_add_unless(&map->refcnt, 1, 0);
if (!refold)
return ERR_PTR(-ENOENT);
-
if (uref)
- atomic_inc(&map->usercnt);
+ atomic64_inc(&map->usercnt);

return map;
}

-struct bpf_map *bpf_map_inc_not_zero(struct bpf_map *map, bool uref)
+struct bpf_map *bpf_map_inc_not_zero(struct bpf_map *map)
{
spin_lock_bh(&map_idr_lock);
- map = __bpf_map_inc_not_zero(map, uref);
+ map = __bpf_map_inc_not_zero(map, false);
spin_unlock_bh(&map_idr_lock);

return map;
@@ -1456,6 +1446,9 @@ static struct bpf_prog *____bpf_prog_get(struct fd f)
return f.file->private_data;
}

+/* prog's refcnt limit */
+#define BPF_MAX_REFCNT 32768
+
struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i)
{
if (atomic_add_return(i, &prog->aux->refcnt) > BPF_MAX_REFCNT) {
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ffc3e53f53009..87181cd5bafd7 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8008,11 +8008,7 @@ static int replace_map_fd_with_map_ptr(struct bpf_verifier_env *env)
* will be used by the valid program until it's unloaded
* and all maps are released in free_used_maps()
*/
- map = bpf_map_inc(map, false);
- if (IS_ERR(map)) {
- fdput(f);
- return PTR_ERR(map);
- }
+ bpf_map_inc(map);

aux->map_index = env->used_map_cnt;
env->used_maps[env->used_map_cnt++] = map;
diff --git a/kernel/bpf/xskmap.c b/kernel/bpf/xskmap.c
index 82a1ffe15dfaa..08dccf733991c 100644
--- a/kernel/bpf/xskmap.c
+++ b/kernel/bpf/xskmap.c
@@ -18,10 +18,8 @@ struct xsk_map {

int xsk_map_inc(struct xsk_map *map)
{
- struct bpf_map *m = &map->map;
-
- m = bpf_map_inc(m, false);
- return PTR_ERR_OR_ZERO(m);
+ bpf_map_inc(&map->map);
+ return 0;
}

void xsk_map_put(struct xsk_map *map)
diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
index da5639a5bd3b9..458be6b3eda97 100644
--- a/net/core/bpf_sk_storage.c
+++ b/net/core/bpf_sk_storage.c
@@ -798,7 +798,7 @@ int bpf_sk_storage_clone(const struct sock *sk, struct sock *newsk)
* Try to grab map refcnt to make sure that it's still
* alive and prevent concurrent removal.
*/
- map = bpf_map_inc_not_zero(&smap->map, false);
+ map = bpf_map_inc_not_zero(&smap->map);
if (IS_ERR(map))
continue;

--
2.20.1