Re: [PATCH] fix UAF in bfq_io_set_weight_legacy()

[Jens Axboe]

On 9/9/21 7:37 AM, Li Jinlin wrote:
> From: Li Jinlin <lijinlin3@xxxxxxxxxx>
> 
> KASAN reports a use-after-free report when doing fuzz test:
> 
> [693354.104835] ==================================================================
> [693354.105094] BUG: KASAN: use-after-free in bfq_io_set_weight_legacy+0xd3/0x160
> [693354.105336] Read of size 4 at addr ffff888be0a35664 by task sh/1453338
> 
> [693354.105607] CPU: 41 PID: 1453338 Comm: sh Kdump: loaded Not tainted 4.18.0-147
> [693354.105610] Hardware name: Huawei 2288H V5/BC11SPSCB0, BIOS 0.81 07/02/2018
> [693354.105612] Call Trace:
> [693354.105621]  dump_stack+0xf1/0x19b
> [693354.105626]  ? show_regs_print_info+0x5/0x5
> [693354.105634]  ? printk+0x9c/0xc3
> [693354.105638]  ? cpumask_weight+0x1f/0x1f
> [693354.105648]  print_address_description+0x70/0x360
> [693354.105654]  kasan_report+0x1b2/0x330
> [693354.105659]  ? bfq_io_set_weight_legacy+0xd3/0x160
> [693354.105665]  ? bfq_io_set_weight_legacy+0xd3/0x160
> [693354.105670]  bfq_io_set_weight_legacy+0xd3/0x160
> [693354.105675]  ? bfq_cpd_init+0x20/0x20
> [693354.105683]  cgroup_file_write+0x3aa/0x510
> [693354.105693]  ? ___slab_alloc+0x507/0x540
> [693354.105698]  ? cgroup_file_poll+0x60/0x60
> [693354.105702]  ? 0xffffffff89600000
> [693354.105708]  ? usercopy_abort+0x90/0x90
> [693354.105716]  ? mutex_lock+0xef/0x180
> [693354.105726]  kernfs_fop_write+0x1ab/0x280
> [693354.105732]  ? cgroup_file_poll+0x60/0x60
> [693354.105738]  vfs_write+0xe7/0x230
> [693354.105744]  ksys_write+0xb0/0x140
> [693354.105749]  ? __ia32_sys_read+0x50/0x50
> [693354.105760]  do_syscall_64+0x112/0x370
> [693354.105766]  ? syscall_return_slowpath+0x260/0x260
> [693354.105772]  ? do_page_fault+0x9b/0x270
> [693354.105779]  ? prepare_exit_to_usermode+0xf9/0x1a0
> [693354.105784]  ? enter_from_user_mode+0x30/0x30
> [693354.105793]  entry_SYSCALL_64_after_hwframe+0x65/0xca
> 
> [693354.105875] Allocated by task 1453337:
> [693354.106001]  kasan_kmalloc+0xa0/0xd0
> [693354.106006]  kmem_cache_alloc_node_trace+0x108/0x220
> [693354.106010]  bfq_pd_alloc+0x96/0x120
> [693354.106015]  blkcg_activate_policy+0x1b7/0x2b0
> [693354.106020]  bfq_create_group_hierarchy+0x1e/0x80
> [693354.106026]  bfq_init_queue+0x678/0x8c0
> [693354.106031]  blk_mq_init_sched+0x1f8/0x460
> [693354.106037]  elevator_switch_mq+0xe1/0x240
> [693354.106041]  elevator_switch+0x25/0x40
> [693354.106045]  elv_iosched_store+0x1a1/0x230
> [693354.106049]  queue_attr_store+0x78/0xb0
> [693354.106053]  kernfs_fop_write+0x1ab/0x280
> [693354.106056]  vfs_write+0xe7/0x230
> [693354.106060]  ksys_write+0xb0/0x140
> [693354.106064]  do_syscall_64+0x112/0x370
> [693354.106069]  entry_SYSCALL_64_after_hwframe+0x65/0xca
> 
> [693354.106114] Freed by task 1453336:
> [693354.106225]  __kasan_slab_free+0x130/0x180
> [693354.106229]  kfree+0x90/0x1b0
> [693354.106233]  blkcg_deactivate_policy+0x12c/0x220
> [693354.106238]  bfq_exit_queue+0xf5/0x110
> [693354.106241]  blk_mq_exit_sched+0x104/0x130
> [693354.106245]  __elevator_exit+0x45/0x60
> [693354.106249]  elevator_switch_mq+0xd6/0x240
> [693354.106253]  elevator_switch+0x25/0x40
> [693354.106257]  elv_iosched_store+0x1a1/0x230
> [693354.106261]  queue_attr_store+0x78/0xb0
> [693354.106264]  kernfs_fop_write+0x1ab/0x280
> [693354.106268]  vfs_write+0xe7/0x230
> [693354.106271]  ksys_write+0xb0/0x140
> [693354.106275]  do_syscall_64+0x112/0x370
> [693354.106280]  entry_SYSCALL_64_after_hwframe+0x65/0xca
> 
> [693354.106329] The buggy address belongs to the object at ffff888be0a35580
>                  which belongs to the cache kmalloc-1k of size 1024
> [693354.106736] The buggy address is located 228 bytes inside of
>                  1024-byte region [ffff888be0a35580, ffff888be0a35980)
> [693354.107114] The buggy address belongs to the page:
> [693354.107273] page:ffffea002f828c00 count:1 mapcount:0 mapping:ffff888107c17080 index:0x0 compound_mapcount: 0
> [693354.107606] flags: 0x17ffffc0008100(slab|head)
> [693354.107760] raw: 0017ffffc0008100 ffffea002fcbc808 ffffea0030bd3a08 ffff888107c17080
> [693354.108020] raw: 0000000000000000 00000000001c001c 00000001ffffffff 0000000000000000
> [693354.108278] page dumped because: kasan: bad access detected
> 
> [693354.108511] Memory state around the buggy address:
> [693354.108671]  ffff888be0a35500: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> [693354.116396]  ffff888be0a35580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> [693354.124473] >ffff888be0a35600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> [693354.132421]                                                        ^
> [693354.140284]  ffff888be0a35680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> [693354.147912]  ffff888be0a35700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> [693354.155281] ==================================================================
> 
> Freeing bfqg is protected by queue lock in blkcg_deactivate_policy(), 
> but getting/using bfqg is protected by blkcg lock in
> bfq_io_set_weight_legacy(). If bfq_io_set_weight_legacy() get bfqg
> before freeing bfqg and use bfqg in the after, the use-after-free
> will occur.
> 
> CPU0                             CPU1
> blkcg_deactivate_policy
>   spin_lock_irq(&q->queue_lock)
>                                  bfq_io_set_weight_legacy  
>                                    spin_lock_irq(&blkcg->lock)
>                                    blkg_to_bfqg(blkg)
>                                      pd_to_bfqg(blkg->pd[pol->plid])
>                                      ^^^^^^blkg->pd[pol->plid] != NULL
>                                            bfqg != NULL
>   pol->pd_free_fn(blkg->pd[pol->plid])
>     pd_to_bfqg(blkg->pd[pol->plid])
>     bfqg_put(bfqg)
>       kfree(bfqg)
>   blkg->pd[pol->plid] = NULL
>   spin_unlock_irq(q->queue_lock);
>                                    bfq_group_set_weight(bfqg, val, 0)
>                                      bfqg->entity.new_weight
>                                      ^^^^^^trigger uaf here 
>                                    spin_unlock_irq(&blkcg->lock);
> 
> To fix this use-after-free, instead of holding blkcg->lock while
> walking ->blkg_list and getting/using bfqg, RCU walk ->blkg_list and
> hold the blkg's queue lock while getting/using bfqg.
> 
> Fixes: e21b7a0b9887 ("block, bfq: add full hierarchical scheduling and cgroups support")
> Signed-off-by: Li Jinlin <lijinlin3@xxxxxxxxxx>
> ---
>  block/bfq-cgroup.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index e2f14508f2d6..7209060caa90 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -1025,21 +1025,25 @@ static int bfq_io_set_weight_legacy(struct cgroup_subsys_state *css,
>  	struct blkcg *blkcg = css_to_blkcg(css);
>  	struct bfq_group_data *bfqgd = blkcg_to_bfqgd(blkcg);
>  	struct blkcg_gq *blkg;
> +	struct bfq_group *bfqg;
>  	int ret = -ERANGE;
>  
>  	if (val < BFQ_MIN_WEIGHT || val > BFQ_MAX_WEIGHT)
>  		return ret;
>  
>  	ret = 0;
> -	spin_lock_irq(&blkcg->lock);
>  	bfqgd->weight = (unsigned short)val;
> -	hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) {
> -		struct bfq_group *bfqg = blkg_to_bfqg(blkg);
> +
> +	rcu_read_lock();
> +	hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
> +		spin_lock_irq(&blkg->q->queue_lock);
> +		bfqg = blkg_to_bfqg(blkg);
>  
>  		if (bfqg)
>  			bfq_group_set_weight(bfqg, val, 0);
> +		spin_unlock_irq(&blkg->q->queue_lock);
>  	}
> -	spin_unlock_irq(&blkcg->lock);
> +	rcu_read_unlock();
>  
>  	return ret;
>  }

Ping Paolo.


-- 
Jens Axboe