| Message ID | 20231205224812.813224-1-yonghong.song@linux.dev (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | [bpf-next,v3] bpf: Fix a race condition between btf_put() and map_free() | expand |
Hi, On 12/6/2023 6:48 AM, Yonghong Song wrote: > 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, let us increase the reference count for btf > so it won't be prematurely freed by btf_put() in bpf_map_free(). > The same strategy has been used by kptrs, and this patch did > similar things for graph node (list_head and rb_root). > Rerun './test_progs -j' with the above mdelay() hack for a couple > of times and didn't observe the error. > > Fixes: 958cf2e273f0 ("bpf: Introduce bpf_obj_new") > Signed-off-by: Yonghong Song <yonghong.song@linux.dev> > --- > kernel/bpf/btf.c | 1 + > kernel/bpf/syscall.c | 8 ++++++-- > 2 files changed, 7 insertions(+), 2 deletions(-) > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c > index 63cf4128fc05..f999e0c1bb32 100644 > --- a/kernel/bpf/btf.c > +++ b/kernel/bpf/btf.c > @@ -3696,6 +3696,7 @@ static int btf_parse_graph_root(const struct btf *btf, > if (offset % node_type_align) > return -EINVAL; > > + btf_get((struct btf *)btf); I think there will be memory leak for btf. Because btf_parse() calls btf_parse_graph_root() indirectly as well, here btf_get() pins the BTF, but the free of the passed btf_field is done in btf_free(), so the map BTF can not be freed. It seems we need to pin the passed BTF optionally and it depends on whether btf_record is freed by the passed BTF or not. And for the extra type casting (namely (struct btf *)), I think we should change the function signature of its callers to fix it, because btf_get() does modify the passed btf. > 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; > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c > index ebaccf77d56e..00efc82a8e36 100644 > --- a/kernel/bpf/syscall.c > +++ b/kernel/bpf/syscall.c > @@ -521,8 +521,10 @@ 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: > + 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 +572,10 @@ struct btf_record *btf_record_dup(const struct btf_record *rec) > } > break; > case BPF_LIST_HEAD: > - case BPF_LIST_NODE: > case BPF_RB_ROOT: > + btf_get(fields[i].graph_root.btf); > + break; > + case BPF_LIST_NODE: > case BPF_RB_NODE: > case BPF_SPIN_LOCK: > case BPF_TIMER:
On 12/5/23 7:56 PM, Hou Tao wrote: > Hi, > > On 12/6/2023 6:48 AM, Yonghong Song wrote: >> 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, let us increase the reference count for btf >> so it won't be prematurely freed by btf_put() in bpf_map_free(). >> The same strategy has been used by kptrs, and this patch did >> similar things for graph node (list_head and rb_root). >> Rerun './test_progs -j' with the above mdelay() hack for a couple >> of times and didn't observe the error. >> >> Fixes: 958cf2e273f0 ("bpf: Introduce bpf_obj_new") >> Signed-off-by: Yonghong Song <yonghong.song@linux.dev> >> --- >> kernel/bpf/btf.c | 1 + >> kernel/bpf/syscall.c | 8 ++++++-- >> 2 files changed, 7 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c >> index 63cf4128fc05..f999e0c1bb32 100644 >> --- a/kernel/bpf/btf.c >> +++ b/kernel/bpf/btf.c >> @@ -3696,6 +3696,7 @@ static int btf_parse_graph_root(const struct btf *btf, >> if (offset % node_type_align) >> return -EINVAL; >> >> + btf_get((struct btf *)btf); > I think there will be memory leak for btf. Because btf_parse() calls > btf_parse_graph_root() indirectly as well, here btf_get() pins the BTF, > but the free of the passed btf_field is done in btf_free(), so the map > BTF can not be freed. It seems we need to pin the passed BTF optionally > and it depends on whether btf_record is freed by the passed BTF or not. This is indeed a problem. I started to debug btf reference count and will share the result once I got a handle on it. > > And for the extra type casting (namely (struct btf *)), I think we > should change the function signature of its callers to fix it, because > btf_get() does modify the passed btf. The 'btf' declaration looks like const struct btf *btf which implies the pointee of 'btf' is a constant. But actually btf_get() does change pointee's. The btf_get() func signature: void btf_get(struct btf *btf) { refcount_inc(&btf->refcnt); } If we changed the btf_get argument to be 'const', then we will need to change refcount_inc parameter to be constant as well. But that will be incorrect since refcount_inc indeeds tries to modify the pointee. static inline void refcount_inc(refcount_t *r) { __refcount_inc(r, NULL); } But I do see a problem, we should not have 'const struct btf *btf' in the first place. I will try to fix it as well. >> 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; >> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c >> index ebaccf77d56e..00efc82a8e36 100644 >> --- a/kernel/bpf/syscall.c >> +++ b/kernel/bpf/syscall.c >> @@ -521,8 +521,10 @@ 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: >> + 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 +572,10 @@ struct btf_record *btf_record_dup(const struct btf_record *rec) >> } >> break; >> case BPF_LIST_HEAD: >> - case BPF_LIST_NODE: >> case BPF_RB_ROOT: >> + btf_get(fields[i].graph_root.btf); >> + break; >> + case BPF_LIST_NODE: >> case BPF_RB_NODE: >> case BPF_SPIN_LOCK: >> case BPF_TIMER:
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index 63cf4128fc05..f999e0c1bb32 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -3696,6 +3696,7 @@ static int btf_parse_graph_root(const struct btf *btf, if (offset % node_type_align) return -EINVAL; + btf_get((struct btf *)btf); 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; diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c index ebaccf77d56e..00efc82a8e36 100644 --- a/kernel/bpf/syscall.c +++ b/kernel/bpf/syscall.c @@ -521,8 +521,10 @@ 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: + 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 +572,10 @@ struct btf_record *btf_record_dup(const struct btf_record *rec) } break; case BPF_LIST_HEAD: - case BPF_LIST_NODE: case BPF_RB_ROOT: + btf_get(fields[i].graph_root.btf); + break; + case BPF_LIST_NODE: case BPF_RB_NODE: case BPF_SPIN_LOCK: case BPF_TIMER:
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, let us increase the reference count for btf so it won't be prematurely freed by btf_put() in bpf_map_free(). The same strategy has been used by kptrs, and this patch did similar things for graph node (list_head and rb_root). Rerun './test_progs -j' with the above mdelay() hack for a couple of times and didn't observe the error. Fixes: 958cf2e273f0 ("bpf: Introduce bpf_obj_new") Signed-off-by: Yonghong Song <yonghong.song@linux.dev> --- kernel/bpf/btf.c | 1 + kernel/bpf/syscall.c | 8 ++++++-- 2 files changed, 7 insertions(+), 2 deletions(-)