linux-kernel - Re: [PATCH 1/2] ext2: fix race between setxattr and write back

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Date:   Mon, 14 Aug 2023 23:42:36 +0800
From:   "yebin (H)" <yebin10@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <linux-ext4@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] ext2: fix race between setxattr and write back



On 2023/8/14 20:46, Jan Kara wrote:
> On Fri 11-08-23 11:38:56, Ye Bin wrote:
>> There's a issue as follows:
>> Block Allocation Reservation Windows Map (ext2_try_to_allocate_with_rsv):
>> reservation window 0x000000006f105382 start: 0, end: 0
>> reservation window 0x000000008fd1a555 start: 1044, end: 1059
>> Window map complete.
>> kernel BUG at fs/ext2/balloc.c:1158!
>> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
>> RIP: 0010:ext2_try_to_allocate_with_rsv.isra.0+0x15c4/0x1800
>> Call Trace:
>>   <TASK>
>>   ext2_new_blocks+0x935/0x1690
>>   ext2_new_block+0x73/0xa0
>>   ext2_xattr_set2+0x74f/0x1730
>>   ext2_xattr_set+0x12b6/0x2260
>>   ext2_xattr_user_set+0x9c/0x110
>>   __vfs_setxattr+0x139/0x1d0
>>   __vfs_setxattr_noperm+0xfc/0x370
>>   __vfs_setxattr_locked+0x205/0x2c0
>>   vfs_setxattr+0x19d/0x3b0
>>   do_setxattr+0xff/0x220
>>   setxattr+0x123/0x150
>>   path_setxattr+0x193/0x1e0
>>   __x64_sys_setxattr+0xc8/0x170
>>   do_syscall_64+0x35/0x80
>>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>
>> Above issue may happens as follows:
>>          setxattr                             write back
>> ext2_xattr_set
>>    ext2_xattr_set2
>>      ext2_new_block
>>        ext2_new_blocks
>>          ext2_try_to_allocate_with_rsv
>> 	  alloc_new_reservation
>> 	  --> group=0 [0, 1023] rsv [1016, 1023]
>> 	                                do_writepages
>> 					  mpage_writepages
>> 					    write_cache_pages
>> 					      __mpage_writepage
>> 					        ext2_get_block
>> 						  ext2_get_blocks
>> 						   ext2_alloc_branch
>> 						    ext2_new_blocks
>> 						     ext2_try_to_allocate_with_rsv
>> 						       alloc_new_reservation
>> 		                     -->group=1 [1024, 2047] rsv [1044, 1059]
>> 	  if ((my_rsv->rsv_start > group_last_block) ||
>> 	      (my_rsv->rsv_end < group_first_block)
>> 	      rsv_window_dump
>> 	      BUG();
>> Now ext2 mkwrite delay allocate new blocks. So there maybe allocate blocks when
>> do write back. However, there is no concurrent protection between
>> ext2_xattr_set() and do_writepages().
>> To solve about issue hold '&ei->truncate_mutex' lock when new block for xattr.
>>
>> Signed-off-by: Ye Bin <yebin10@...wei.com>
> Thanks for the patch! I agree the handling of reservation window and its
> use for block allocation needs to be protected by ei->i_truncate_mutex.
> However in this particular case of xattr allocation where we want to
> allocate just one block which is completely independent of file data, I'd
> rather choose to make ext2_new_blocks() ignore the reservation window (set
> my_rsv to NULL). There's already a logic at the beginning of
> ext2_new_blocks() deciding whether to use the reservation window or not and
> we could extend it - probably by adding flags argument to it a introducing
> a NORESERVE flag.
>
> Also as a preparatory patch, I'd just remove ext2_new_block() and opencode
> it in the xattr code since it has only that one user anyway.
>
> 								Honza
>
Thanks for your suggestion.  I will send new version according to your 
suggestion.
>
>> ---
>>   fs/ext2/balloc.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/ext2/balloc.c b/fs/ext2/balloc.c
>> index c8049c90323d..039f655febfd 100644
>> --- a/fs/ext2/balloc.c
>> +++ b/fs/ext2/balloc.c
>> @@ -1432,8 +1432,14 @@ ext2_fsblk_t ext2_new_blocks(struct inode *inode, ext2_fsblk_t goal,
>>   ext2_fsblk_t ext2_new_block(struct inode *inode, unsigned long goal, int *errp)
>>   {
>>   	unsigned long count = 1;
>> +	struct ext2_inode_info *ei = EXT2_I(inode);
>> +	ext2_fsblk_t ret;
>> +
>> +	mutex_lock(&ei->truncate_mutex);
>> +	ret = ext2_new_blocks(inode, goal, &count, errp);
>> +	mutex_unlock(&ei->truncate_mutex);
>>   
>> -	return ext2_new_blocks(inode, goal, &count, errp);
>> +	return ret;
>>   }
>>   
>>   #ifdef EXT2FS_DEBUG
>> -- 
>> 2.31.1
>>