Re: [PATCH 3.16 105/306] vfs,mm: fix a dead loop in truncate_inode_pages_range()

[Wei Fang]

Hi, Ben,

Commit d05c5f7ba164 "vfs,mm: fix return value of read() at s_maxbytes"
need be backported with this patch, otherwise we'll get an error when
reading at the end of the file.

Thanks,
Wei

On 2017/2/16 6:41, Ben Hutchings wrote:
> 3.16.40-rc1 review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Wei Fang <fangwei1@xxxxxxxxxx>
> 
> commit c2a9737f45e27d8263ff9643f994bda9bac0b944 upstream.
> 
> We triggered a deadloop in truncate_inode_pages_range() on 32 bits
> architecture with the test case bellow:
> 
> 	...
> 	fd = open();
> 	write(fd, buf, 4096);
> 	preadv64(fd, &iovec, 1, 0xffffffff000);
> 	ftruncate(fd, 0);
> 	...
> 
> Then ftruncate() will not return forever.
> 
> The filesystem used in this case is ubifs, but it can be triggered on
> many other filesystems.
> 
> When preadv64() is called with offset=0xffffffff000, a page with
> index=0xffffffff will be added to the radix tree of ->mapping.  Then
> this page can be found in ->mapping with pagevec_lookup().  After that,
> truncate_inode_pages_range(), which is called in ftruncate(), will fall
> into an infinite loop:
> 
>  - find a page with index=0xffffffff, since index>=end, this page won't
>    be truncated
> 
>  - index++, and index become 0
> 
>  - the page with index=0xffffffff will be found again
> 
> The data type of index is unsigned long, so index won't overflow to 0 on
> 64 bits architecture in this case, and the dead loop won't happen.
> 
> Since truncate_inode_pages_range() is executed with holding lock of
> inode->i_rwsem, any operation related with this lock will be blocked,
> and a hung task will happen, e.g.:
> 
>   INFO: task truncate_test:3364 blocked for more than 120 seconds.
>   ...
>      call_rwsem_down_write_failed+0x17/0x30
>      generic_file_write_iter+0x32/0x1c0
>      ubifs_write_iter+0xcc/0x170
>      __vfs_write+0xc4/0x120
>      vfs_write+0xb2/0x1b0
>      SyS_write+0x46/0xa0
> 
> The page with index=0xffffffff added to ->mapping is useless.  Fix this
> by checking the read position before allocating pages.
> 
> Link: http://lkml.kernel.org/r/1475151010-40166-1-git-send-email-fangwei1@xxxxxxxxxx
> Signed-off-by: Wei Fang <fangwei1@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> [bwh: Backported to 3.16: adjust context]
> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> ---
>  mm/filemap.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1464,6 +1464,10 @@ static ssize_t do_generic_file_read(stru
>  	unsigned int prev_offset;
>  	int error = 0;
>  
> +	if (unlikely(*ppos >= inode->i_sb->s_maxbytes))
> +		return -EINVAL;
> +	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
> +
>  	index = *ppos >> PAGE_CACHE_SHIFT;
>  	prev_index = ra->prev_pos >> PAGE_CACHE_SHIFT;
>  	prev_offset = ra->prev_pos & (PAGE_CACHE_SIZE-1);
>