Re: [PATCH] blktrace: Revert "blktrace: remove debugfs file dentries from struct blk_trace"

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Feb 26, 2022 at 05:53:43PM +0800, Yu Kuai wrote:
> This reverts commit c0ea57608b691d6cde8aff23e11f9858a86b5918.
> 
> When tracing the whole disk, 'dropped' and 'msg' will be created
> under 'q->debugfs_dir' and 'bt->dir' is NULL, thus blk_trace_free()
> won't remove those files. What's worse, the following UAF can be
> triggered because of stale 'dropped' and 'msg':

Only root has access to these files, right?

> 
> ==================================================================
> BUG: KASAN: use-after-free in blk_dropped_read+0x89/0x100
> Read of size 4 at addr ffff88816912f3d8 by task blktrace/1188
> 
> CPU: 27 PID: 1188 Comm: blktrace Not tainted 5.17.0-rc4-next-20220217+ #469
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-4
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x34/0x44
>  print_address_description.constprop.0.cold+0xab/0x381
>  ? blk_dropped_read+0x89/0x100
>  ? blk_dropped_read+0x89/0x100
>  kasan_report.cold+0x83/0xdf
>  ? blk_dropped_read+0x89/0x100
>  kasan_check_range+0x140/0x1b0
>  blk_dropped_read+0x89/0x100
>  ? blk_create_buf_file_callback+0x20/0x20
>  ? kmem_cache_free+0xa1/0x500
>  ? do_sys_openat2+0x258/0x460
>  full_proxy_read+0x8f/0xc0
>  vfs_read+0xc6/0x260
>  ksys_read+0xb9/0x150
>  ? vfs_write+0x3d0/0x3d0
>  ? fpregs_assert_state_consistent+0x55/0x60
>  ? exit_to_user_mode_prepare+0x39/0x1e0
>  do_syscall_64+0x35/0x80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7fbc080d92fd
> Code: ce 20 00 00 75 10 b8 00 00 00 00 0f 05 48 3d 01 f0 ff ff 73 31 c3 48 83 1
> RSP: 002b:00007fbb95ff9cb0 EFLAGS: 00000293 ORIG_RAX: 0000000000000000
> RAX: ffffffffffffffda RBX: 00007fbb95ff9dc0 RCX: 00007fbc080d92fd
> RDX: 0000000000000100 RSI: 00007fbb95ff9cc0 RDI: 0000000000000045
> RBP: 0000000000000045 R08: 0000000000406299 R09: 00000000fffffffd
> R10: 000000000153afa0 R11: 0000000000000293 R12: 00007fbb780008c0
> R13: 00007fbb78000938 R14: 0000000000608b30 R15: 00007fbb780029c8
>  </TASK>
> 
> Allocated by task 1050:
>  kasan_save_stack+0x1e/0x40
>  __kasan_kmalloc+0x81/0xa0
>  do_blk_trace_setup+0xcb/0x410
>  __blk_trace_setup+0xac/0x130
>  blk_trace_ioctl+0xe9/0x1c0
>  blkdev_ioctl+0xf1/0x390
>  __x64_sys_ioctl+0xa5/0xe0
>  do_syscall_64+0x35/0x80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Freed by task 1050:
>  kasan_save_stack+0x1e/0x40
>  kasan_set_track+0x21/0x30
>  kasan_set_free_info+0x20/0x30
>  __kasan_slab_free+0x103/0x180
>  kfree+0x9a/0x4c0
>  __blk_trace_remove+0x53/0x70
>  blk_trace_ioctl+0x199/0x1c0
>  blkdev_common_ioctl+0x5e9/0xb30
>  blkdev_ioctl+0x1a5/0x390
>  __x64_sys_ioctl+0xa5/0xe0
>  do_syscall_64+0x35/0x80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> The buggy address belongs to the object at ffff88816912f380
>  which belongs to the cache kmalloc-96 of size 96
> The buggy address is located 88 bytes inside of
>  96-byte region [ffff88816912f380, ffff88816912f3e0)
> The buggy address belongs to the page:
> page:000000009a1b4e7c refcount:1 mapcount:0 mapping:0000000000000000 index:0x0f
> flags: 0x17ffffc0000200(slab|node=0|zone=2|lastcpupid=0x1fffff)
> raw: 0017ffffc0000200 ffffea00044f1100 dead000000000002 ffff88810004c780
> raw: 0000000000000000 0000000000200020 00000001ffffffff 0000000000000000
> page dumped because: kasan: bad access detected
> 
> Memory state around the buggy address:
>  ffff88816912f280: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
>  ffff88816912f300: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
> >ffff88816912f380: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
>                                                     ^
>  ffff88816912f400: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
>  ffff88816912f480: fa fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
> ==================================================================
> 
> Fixes: c0ea57608b69 ("blktrace: remove debugfs file dentries from struct blk_trace")
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  include/linux/blktrace_api.h | 2 ++
>  kernel/trace/blktrace.c      | 8 ++++++--
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/blktrace_api.h b/include/linux/blktrace_api.h
> index 22501a293fa5..f288d229727c 100644
> --- a/include/linux/blktrace_api.h
> +++ b/include/linux/blktrace_api.h
> @@ -23,6 +23,8 @@ struct blk_trace {
>  	u32 pid;
>  	u32 dev;
>  	struct dentry *dir;
> +	struct dentry *dropped_file;
> +	struct dentry *msg_file;

No need to save these dentries.  Just look them up when you want to
remove the files.

>  	struct list_head running_list;
>  	atomic_t dropped;
>  };
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 19514edc44f7..13152a17fdb3 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -312,6 +312,8 @@ static void __blk_add_trace(struct blk_trace *bt, sector_t sector, int bytes,
>  
>  static void blk_trace_free(struct blk_trace *bt)
>  {
> +	debugfs_remove(bt->msg_file);
> +	debugfs_remove(bt->dropped_file);
>  	relay_close(bt->rchan);
>  	debugfs_remove(bt->dir);

Why not just move this line up above relay_close()?

Then you the whole directory is properly removed, along with the files
in it.

So this can just be a 1 line change :)

thanks,

greg k-h