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

["yukuai (C)" <yukuai3@huawei.com>]

在 2022/02/26 17:43, Greg KH 写道:
> 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?

Hi,

Yes, usually user will use blktrace to access these files.
> 
>>
>> ==================================================================
>> 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.

Ok, I'll do this in the v2 patch.
> 
>>   	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.

In the case tracing the whole disk, bt->dir is NULL, if dentries are not
saved, they should be looked up from 'q->debugfs_dir'. Perhaps the
following:

if (bt->dir) {
	debugfs_remove(bt->dir);
} else {
	/* lookup from q->debugfs_dir and remove */
}

Thanks,
Kuai
> 
> So this can just be a 1 line change :)
> 
> thanks,
> 
> greg k-h
> .
>