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@xxxxxxxxx>
> ---
> 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:
