Re: [PATCH bpf-next v5] bpf: Fix a race condition between btf_put() and map_free()

[Martin KaFai Lau <martin.lau@linux.dev>]

On 12/7/23 8:16 PM, 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, 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@huaweicloud.com/

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>