When running `./test_progs -j` in my local vm with latest kernel,
I once hit a kasan error like below:
[ 1887.184724] BUG: KASAN: slab-use-after-free in bpf_rb_root_free+0x1f8/0x2b0
[ 1887.185599] Read of size 4 at addr ffff888106806910 by task kworker/u12:2/2830
[ 1887.186498]
[ 1887.186712] CPU: 3 PID: 2830 Comm: kworker/u12:2 Tainted: G OEL 6.7.0-rc3-00699-g90679706d486-dirty #494
[ 1887.188034] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 1887.189618] Workqueue: events_unbound bpf_map_free_deferred
[ 1887.190341] Call Trace:
[ 1887.190666] <TASK>
[ 1887.190949] dump_stack_lvl+0xac/0xe0
[ 1887.191423] ? nf_tcp_handle_invalid+0x1b0/0x1b0
[ 1887.192019] ? panic+0x3c0/0x3c0
[ 1887.192449] print_report+0x14f/0x720
[ 1887.192930] ? preempt_count_sub+0x1c/0xd0
[ 1887.193459] ? __virt_addr_valid+0xac/0x120
[ 1887.194004] ? bpf_rb_root_free+0x1f8/0x2b0
[ 1887.194572] kasan_report+0xc3/0x100
[ 1887.195085] ? bpf_rb_root_free+0x1f8/0x2b0
[ 1887.195668] bpf_rb_root_free+0x1f8/0x2b0
[ 1887.196183] ? __bpf_obj_drop_impl+0xb0/0xb0
[ 1887.196736] ? preempt_count_sub+0x1c/0xd0
[ 1887.197270] ? preempt_count_sub+0x1c/0xd0
[ 1887.197802] ? _raw_spin_unlock+0x1f/0x40
[ 1887.198319] bpf_obj_free_fields+0x1d4/0x260
[ 1887.198883] array_map_free+0x1a3/0x260
[ 1887.199380] bpf_map_free_deferred+0x7b/0xe0
[ 1887.199943] process_scheduled_works+0x3a2/0x6c0
[ 1887.200549] worker_thread+0x633/0x890
[ 1887.201047] ? __kthread_parkme+0xd7/0xf0
[ 1887.201574] ? kthread+0x102/0x1d0
[ 1887.202020] kthread+0x1ab/0x1d0
[ 1887.202447] ? pr_cont_work+0x270/0x270
[ 1887.202954] ? kthread_blkcg+0x50/0x50
[ 1887.203444] ret_from_fork+0x34/0x50
[ 1887.203914] ? kthread_blkcg+0x50/0x50
[ 1887.204397] ret_from_fork_asm+0x11/0x20
[ 1887.204913] </TASK>
[ 1887.204913] </TASK>
[ 1887.205209]
[ 1887.205416] Allocated by task 2197:
[ 1887.205881] kasan_set_track+0x3f/0x60
[ 1887.206366] __kasan_kmalloc+0x6e/0x80
[ 1887.206856] __kmalloc+0xac/0x1a0
[ 1887.207293] btf_parse_fields+0xa15/0x1480
[ 1887.207836] btf_parse_struct_metas+0x566/0x670
[ 1887.208387] btf_new_fd+0x294/0x4d0
[ 1887.208851] __sys_bpf+0x4ba/0x600
[ 1887.209292] __x64_sys_bpf+0x41/0x50
[ 1887.209762] do_syscall_64+0x4c/0xf0
[ 1887.210222] entry_SYSCALL_64_after_hwframe+0x63/0x6b
[ 1887.210868]
[ 1887.211074] Freed by task 36:
[ 1887.211460] kasan_set_track+0x3f/0x60
[ 1887.211951] kasan_save_free_info+0x28/0x40
[ 1887.212485] ____kasan_slab_free+0x101/0x180
[ 1887.213027] __kmem_cache_free+0xe4/0x210
[ 1887.213514] btf_free+0x5b/0x130
[ 1887.213918] rcu_core+0x638/0xcc0
[ 1887.214347] __do_softirq+0x114/0x37e
The error happens at bpf_rb_root_free+0x1f8/0x2b0:
00000000000034c0 <bpf_rb_root_free>:
; {
34c0: f3 0f 1e fa endbr64
34c4: e8 00 00 00 00 callq 0x34c9 <bpf_rb_root_free+0x9>
34c9: 55 pushq %rbp
34ca: 48 89 e5 movq %rsp, %rbp
...
; if (rec && rec->refcount_off >= 0 &&
36aa: 4d 85 ed testq %r13, %r13
36ad: 74 a9 je 0x3658 <bpf_rb_root_free+0x198>
36af: 49 8d 7d 10 leaq 0x10(%r13), %rdi
36b3: e8 00 00 00 00 callq 0x36b8 <bpf_rb_root_free+0x1f8>
<==== kasan function
36b8: 45 8b 7d 10 movl 0x10(%r13), %r15d
<==== use-after-free load
36bc: 45 85 ff testl %r15d, %r15d
36bf: 78 8c js 0x364d <bpf_rb_root_free+0x18d>
So the problem is at rec->refcount_off in the above.
I did some source code analysis and find the reason.
CPU A CPU B
bpf_map_put:
...
btf_put with rcu callback
...
bpf_map_free_deferred
with system_unbound_wq
... ... ...
... btf_free_rcu: ...
... ... bpf_map_free_deferred:
... ...
... ---------> btf_struct_metas_free()
... | race condition ...
... ---------> map->ops->map_free()
...
... btf->struct_meta_tab = NULL
In the above, map_free() corresponds to array_map_free() and eventually
calling bpf_rb_root_free() which calls:
...
__bpf_obj_drop_impl(obj, field->graph_root.value_rec, false);
...
Here, 'value_rec' is assigned in btf_check_and_fixup_fields() with following code:
meta = btf_find_struct_meta(btf, btf_id);
if (!meta)
return -EFAULT;
rec->fields[i].graph_root.value_rec = meta->record;
So basically, 'value_rec' is a pointer to the record in struct_metas_tab.
And it is possible that that particular record has been freed by
btf_struct_metas_free() and hence we have a kasan error here.
Actually it is very hard to reproduce the failure with current bpf/bpf-next
code, I only got the above error once. To increase reproducibility, I added
a delay in bpf_map_free_deferred() to delay map->ops->map_free(), which
significantly increased reproducibility.
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 5e43ddd1b83f..aae5b5213e93 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -695,6 +695,7 @@ static void bpf_map_free_deferred(struct work_struct *work)
struct bpf_map *map = container_of(work, struct bpf_map, work);
struct btf_record *rec = map->record;
+ mdelay(100);
security_bpf_map_free(map);
bpf_map_release_memcg(map);
/* implementation dependent freeing */
To fix the problem, we need to have a reference on btf in order to
safeguard accessing field->graph_root.value_rec in map->ops->map_free().
The function btf_parse_graph_root() is the place to get a btf reference.
The following are rough call stacks reaching bpf_parse_graph_root():
btf_parse
...
btf_parse_fields
...
btf_parse_graph_root
map_check_btf
btf_parse_fields
...
btf_parse_graph_root
Looking at the above call stack, the btf_parse_graph_root() is indirectly
called from btf_parse() or map_check_btf().
We cannot take a reference in btf_parse() case since at that moment,
btf is still in the middle to self-validation and initial reference
(refcount_set(&btf->refcnt, 1)) has not been triggered yet.
But even if refcount_set() is moved earlier, taking reference
will have other issues at btf_put() stage.
btf_put <== to start destruction process unless refcount == 0
...
btf_record_free <== in which if graph_root, a btf reference will be hold
<== reference is taken during btf_parse()
In order to free the reference taken during btf_parse(), we need to
have refcount 0 in btf_put(), but this is impossible since we need to
free the reference taken during btf_parse() in order to get refcount 0
for btf_put(). So the end result is btf_put() will not be able to
free btf-related memories and we have memory leak here.
Luckily, there is no need to take a btf reference during btf_parse()
since eventually graph_root.value_rec will refer to the rec in btf
struct_meta_tab.
But we do need to take a reference for map_check_btf() case as
the value_rec refers to a record in struct_meta_tab and we want
to make sure btf is not freed until value_rec usage at map_free
is done.
So the fix is to get a btf reference for map_check_btf() case
whenever a graph_root is found. A boolean field 'has_btf_ref' is added to
struct btf_field_graph_root so later the btf reference can be properly
released.
Rerun './test_progs -j' with the above mdelay() hack for a couple
of times and didn't observe the error.
Running Hou's test ([1]) is also successful.
[1] https://lore.kernel.org/bpf/20231206110625.3188975-1-houtao@xxxxxxxxxxxxxxx/
Cc: Hou Tao <houtao@xxxxxxxxxxxxxxx>
Fixes: 958cf2e273f0 ("bpf: Introduce bpf_obj_new")
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
include/linux/bpf.h | 1 +
include/linux/btf.h | 2 +-
kernel/bpf/btf.c | 27 ++++++++++++++++++---------
kernel/bpf/syscall.c | 12 +++++++++---
4 files changed, 29 insertions(+), 13 deletions(-)
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index c1a06263a4f3..cdeb0c95ec74 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -216,6 +216,7 @@ struct btf_field_graph_root {
u32 value_btf_id;
u32 node_offset;
struct btf_record *value_rec;
+ bool has_btf_ref;
};
struct btf_field {
diff --git a/include/linux/btf.h b/include/linux/btf.h
index 59d404e22814..f7d4f6594308 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -217,7 +217,7 @@ bool btf_member_is_reg_int(const struct btf *btf, const struct btf_type *s,
const struct btf_member *m,
u32 expected_offset, u32 expected_size);
struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type *t,
- u32 field_mask, u32 value_size);
+ u32 field_mask, u32 value_size, bool from_map_check);
int btf_check_and_fixup_fields(const struct btf *btf, struct btf_record *rec);
bool btf_type_is_void(const struct btf_type *t);
s32 btf_find_by_name_kind(const struct btf *btf, const char *name, u8 kind);
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 63cf4128fc05..f23d6a7b8b93 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3665,7 +3665,8 @@ static int btf_parse_graph_root(const struct btf *btf,
struct btf_field *field,
struct btf_field_info *info,
const char *node_type_name,
- size_t node_type_align)
+ size_t node_type_align,
+ bool from_map_check)
{
const struct btf_type *t, *n = NULL;
const struct btf_member *member;
@@ -3696,6 +3697,9 @@ static int btf_parse_graph_root(const struct btf *btf,
if (offset % node_type_align)
return -EINVAL;
+ if (from_map_check)
+ btf_get((struct btf *)btf);
+ field->graph_root.has_btf_ref = from_map_check;
field->graph_root.btf = (struct btf *)btf;
field->graph_root.value_btf_id = info->graph_root.value_btf_id;
field->graph_root.node_offset = offset;
@@ -3706,17 +3710,19 @@ static int btf_parse_graph_root(const struct btf *btf,
}
static int btf_parse_list_head(const struct btf *btf, struct btf_field *field,
- struct btf_field_info *info)
+ struct btf_field_info *info, bool from_map_check)
{
return btf_parse_graph_root(btf, field, info, "bpf_list_node",
- __alignof__(struct bpf_list_node));
+ __alignof__(struct bpf_list_node),
+ from_map_check);
}
static int btf_parse_rb_root(const struct btf *btf, struct btf_field *field,
- struct btf_field_info *info)
+ struct btf_field_info *info, bool from_map_check)
{
return btf_parse_graph_root(btf, field, info, "bpf_rb_node",
- __alignof__(struct bpf_rb_node));
+ __alignof__(struct bpf_rb_node),
+ from_map_check);
}
static int btf_field_cmp(const void *_a, const void *_b, const void *priv)
@@ -3732,7 +3738,7 @@ static int btf_field_cmp(const void *_a, const void *_b, const void *priv)
}
struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type *t,
- u32 field_mask, u32 value_size)
+ u32 field_mask, u32 value_size, bool from_map_check)
{
struct btf_field_info info_arr[BTF_FIELDS_MAX];
u32 next_off = 0, field_type_size;
@@ -3798,12 +3804,14 @@ struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type
goto end;
break;
case BPF_LIST_HEAD:
- ret = btf_parse_list_head(btf, &rec->fields[i], &info_arr[i]);
+ ret = btf_parse_list_head(btf, &rec->fields[i], &info_arr[i],
+ from_map_check);
if (ret < 0)
goto end;
break;
case BPF_RB_ROOT:
- ret = btf_parse_rb_root(btf, &rec->fields[i], &info_arr[i]);
+ ret = btf_parse_rb_root(btf, &rec->fields[i], &info_arr[i],
+ from_map_check);
if (ret < 0)
goto end;
break;
@@ -5390,7 +5398,8 @@ btf_parse_struct_metas(struct bpf_verifier_log *log, struct btf *btf)
type = &tab->types[tab->cnt];
type->btf_id = i;
record = btf_parse_fields(btf, t, BPF_SPIN_LOCK | BPF_LIST_HEAD | BPF_LIST_NODE |
- BPF_RB_ROOT | BPF_RB_NODE | BPF_REFCOUNT, t->size);
+ BPF_RB_ROOT | BPF_RB_NODE | BPF_REFCOUNT, t->size,
+ false);
/* The record cannot be unset, treat it as an error if so */
if (IS_ERR_OR_NULL(record)) {
ret = PTR_ERR_OR_ZERO(record) ?: -EFAULT;
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index aff045eed375..bdc81221baf7 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -521,8 +521,11 @@ void btf_record_free(struct btf_record *rec)
btf_put(rec->fields[i].kptr.btf);
break;
case BPF_LIST_HEAD:
- case BPF_LIST_NODE:
case BPF_RB_ROOT:
+ if (rec->fields[i].graph_root.has_btf_ref)
+ btf_put(rec->fields[i].graph_root.btf);
+ break;
+ case BPF_LIST_NODE:
case BPF_RB_NODE:
case BPF_SPIN_LOCK:
case BPF_TIMER:
@@ -570,8 +573,11 @@ struct btf_record *btf_record_dup(const struct btf_record *rec)
}
break;
case BPF_LIST_HEAD:
- case BPF_LIST_NODE:
case BPF_RB_ROOT:
+ if (fields[i].graph_root.has_btf_ref)
+ btf_get(fields[i].graph_root.btf);
+ break;
+ case BPF_LIST_NODE:
case BPF_RB_NODE:
case BPF_SPIN_LOCK:
case BPF_TIMER:
@@ -1034,7 +1040,7 @@ static int map_check_btf(struct bpf_map *map, struct bpf_token *token,
map->record = btf_parse_fields(btf, value_type,
BPF_SPIN_LOCK | BPF_TIMER | BPF_KPTR | BPF_LIST_HEAD |
BPF_RB_ROOT | BPF_REFCOUNT,
- map->value_size);
+ map->value_size, true);
if (!IS_ERR_OR_NULL(map->record)) {
int i;
