Re: [PATCH] workqueue: fix UAF in pwq_unbound_release_workfn()

[Lai Jiangshan <jiangshanlai@gmail.com>]

On Thu, Jul 8, 2021 at 5:28 PM Yang Yingliang <yangyingliang@huawei.com> wrote:
>
> I got a UAF report when doing fuzz test:
>
> [  152.880091][ T8030] ==================================================================
> [  152.881240][ T8030] BUG: KASAN: use-after-free in pwq_unbound_release_workfn+0x50/0x190
> [  152.882442][ T8030] Read of size 4 at addr ffff88810d31bd00 by task kworker/3:2/8030
> [  152.883578][ T8030]
> [  152.883932][ T8030] CPU: 3 PID: 8030 Comm: kworker/3:2 Not tainted 5.13.0+ #249
> [  152.885014][ T8030] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
> [  152.886442][ T8030] Workqueue: events pwq_unbound_release_workfn
> [  152.887358][ T8030] Call Trace:
> [  152.887837][ T8030]  dump_stack_lvl+0x75/0x9b
> [  152.888525][ T8030]  ? pwq_unbound_release_workfn+0x50/0x190
> [  152.889371][ T8030]  print_address_description.constprop.10+0x48/0x70
> [  152.890326][ T8030]  ? pwq_unbound_release_workfn+0x50/0x190
> [  152.891163][ T8030]  ? pwq_unbound_release_workfn+0x50/0x190
> [  152.891999][ T8030]  kasan_report.cold.15+0x82/0xdb
> [  152.892740][ T8030]  ? pwq_unbound_release_workfn+0x50/0x190
> [  152.893594][ T8030]  __asan_load4+0x69/0x90
> [  152.894243][ T8030]  pwq_unbound_release_workfn+0x50/0x190
> [  152.895057][ T8030]  process_one_work+0x47b/0x890
> [  152.895778][ T8030]  worker_thread+0x5c/0x790
> [  152.896439][ T8030]  ? process_one_work+0x890/0x890
> [  152.897163][ T8030]  kthread+0x223/0x250
> [  152.897747][ T8030]  ? set_kthread_struct+0xb0/0xb0
> [  152.898471][ T8030]  ret_from_fork+0x1f/0x30
> [  152.899114][ T8030]
> [  152.899446][ T8030] Allocated by task 8884:
> [  152.900084][ T8030]  kasan_save_stack+0x21/0x50
> [  152.900769][ T8030]  __kasan_kmalloc+0x88/0xb0
> [  152.901416][ T8030]  __kmalloc+0x29c/0x460
> [  152.902014][ T8030]  alloc_workqueue+0x111/0x8e0
> [  152.902690][ T8030]  __btrfs_alloc_workqueue+0x11e/0x2a0
> [  152.903459][ T8030]  btrfs_alloc_workqueue+0x6d/0x1d0
> [  152.904198][ T8030]  scrub_workers_get+0x1e8/0x490
> [  152.904929][ T8030]  btrfs_scrub_dev+0x1b9/0x9c0
> [  152.905599][ T8030]  btrfs_ioctl+0x122c/0x4e50
> [  152.906247][ T8030]  __x64_sys_ioctl+0x137/0x190
> [  152.906916][ T8030]  do_syscall_64+0x34/0xb0
> [  152.907535][ T8030]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> [  152.908365][ T8030]
> [  152.908688][ T8030] Freed by task 8884:
> [  152.909243][ T8030]  kasan_save_stack+0x21/0x50
> [  152.909893][ T8030]  kasan_set_track+0x20/0x30
> [  152.910541][ T8030]  kasan_set_free_info+0x24/0x40
> [  152.911265][ T8030]  __kasan_slab_free+0xf7/0x140
> [  152.911964][ T8030]  kfree+0x9e/0x3d0
> [  152.912501][ T8030]  alloc_workqueue+0x7d7/0x8e0
> [  152.913182][ T8030]  __btrfs_alloc_workqueue+0x11e/0x2a0
> [  152.913949][ T8030]  btrfs_alloc_workqueue+0x6d/0x1d0
> [  152.914703][ T8030]  scrub_workers_get+0x1e8/0x490
> [  152.915402][ T8030]  btrfs_scrub_dev+0x1b9/0x9c0
> [  152.916077][ T8030]  btrfs_ioctl+0x122c/0x4e50
> [  152.916729][ T8030]  __x64_sys_ioctl+0x137/0x190
> [  152.917414][ T8030]  do_syscall_64+0x34/0xb0
> [  152.918034][ T8030]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> [  152.918872][ T8030]
> [  152.919203][ T8030] The buggy address belongs to the object at ffff88810d31bc00
> [  152.919203][ T8030]  which belongs to the cache kmalloc-512 of size 512
> [  152.921155][ T8030] The buggy address is located 256 bytes inside of
> [  152.921155][ T8030]  512-byte region [ffff88810d31bc00, ffff88810d31be00)
> [  152.922993][ T8030] The buggy address belongs to the page:
> [  152.923800][ T8030] page:ffffea000434c600 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x10d318
> [  152.925249][ T8030] head:ffffea000434c600 order:2 compound_mapcount:0 compound_pincount:0
> [  152.926399][ T8030] flags: 0x57ff00000010200(slab|head|node=1|zone=2|lastcpupid=0x7ff)
> [  152.927515][ T8030] raw: 057ff00000010200 dead000000000100 dead000000000122 ffff888009c42c80
> [  152.928716][ T8030] raw: 0000000000000000 0000000080100010 00000001ffffffff 0000000000000000
> [  152.929890][ T8030] page dumped because: kasan: bad access detected
> [  152.930759][ T8030]
> [  152.931076][ T8030] Memory state around the buggy address:
> [  152.931851][ T8030]  ffff88810d31bc00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> [  152.932967][ T8030]  ffff88810d31bc80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> [  152.934068][ T8030] >ffff88810d31bd00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> [  152.935189][ T8030]                    ^
> [  152.935763][ T8030]  ffff88810d31bd80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> [  152.936847][ T8030]  ffff88810d31be00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> [  152.937940][ T8030] ==================================================================
>
> If apply_wqattrs_prepare() fails in alloc_workqueue(), it will call put_pwq()
> which invoke a work queue to call pwq_unbound_release_workfn() and use the 'wq'.
> The 'wq' allocated in alloc_workqueue() will be freed in error path when
> apply_wqattrs_prepare() fails. So it will lead a UAF.
>
> CPU0                                          CPU1
> alloc_workqueue()
> alloc_and_link_pwqs()
> apply_wqattrs_prepare() fails
> apply_wqattrs_cleanup()
> schedule_work(&pwq->unbound_release_work)
> kfree(wq)
>                                               worker_thread()
>                                               pwq_unbound_release_workfn() <- trigger uaf here
>
> In the error path of alloc_workqueue(), the variables will be freed, we
> don't need the worker to free them, so just free the 'ctx' and its members
> in the error path in apply_wqattrs_prepare().
>
> Fixes: 2d5f0764b526 ("workqueue: split apply_workqueue_attrs() into 3 stages")
> Reported-by: Hulk Robot <hulkci@huawei.com>
> Co-developed-by: Xu Qiang <xuqiang36@huawei.com>
> Signed-off-by: Xu Qiang <xuqiang36@huawei.com>
> Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>

Thanks for the report and the fix.

> ---
>  kernel/workqueue.c | 27 ++++++++++++++++++++++++++-
>  1 file changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 50142fc08902..1e71524fab31 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -3898,6 +3898,31 @@ struct apply_wqattrs_ctx {
>         struct pool_workqueue   *pwq_tbl[];
>  };
>
> +static void free_pwq(struct pool_workqueue *pwq)
> +{
> +       if (!pwq || --pwq->refcnt)
> +               return;
> +
> +       put_unbound_pool(pwq->pool);
> +       kmem_cache_free(pwq_cache, pwq);
> +}
> +
> +static void free_wqattrs_ctx(struct apply_wqattrs_ctx *ctx)
> +{
> +       int node;
> +
> +       if (!ctx)
> +               return;
> +
> +       for_each_node(node)
> +               free_pwq(ctx->pwq_tbl[node]);
> +       free_pwq(ctx->dfl_pwq);
> +
> +       free_workqueue_attrs(ctx->attrs);
> +
> +       kfree(ctx);
> +}
> +
>  /* free the resources after success or abort */
>  static void apply_wqattrs_cleanup(struct apply_wqattrs_ctx *ctx)
>  {
> @@ -3981,7 +4006,7 @@ apply_wqattrs_prepare(struct workqueue_struct *wq,
>  out_free:
>         free_workqueue_attrs(tmp_attrs);
>         free_workqueue_attrs(new_attrs);
> -       apply_wqattrs_cleanup(ctx);
> +       free_wqattrs_ctx(ctx);
>         return NULL;
>  }
>

For workqueue_apply_unbound_cpumask(), there will be the same problem
because the new pwqs are not yet linked(link_pwq()) to the wq.

Please also add this to your patch.

@@ -5313,9 +5309,12 @@ static int workqueue_apply_unbound_cpumask(void)
        }

        list_for_each_entry_safe(ctx, n, &ctxs, list) {
-               if (!ret)
+               if (!ret) {
                        apply_wqattrs_commit(ctx);
-               apply_wqattrs_cleanup(ctx);
+                       apply_wqattrs_cleanup(ctx);
+               } else {
+                       free_wqattrs_ctx(ctx);
+               }
        }

        return ret;