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

[yebin (H)]

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@xxxxxxxxxx>
> 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