Re: [PATCH V2] fat: Prevent the race of read/write the FAT32 entry

[OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>]

Edward Adam Davis <eadavis@qq.com> writes:

> syzbot reports data-race in fat32_ent_get/fat32_ent_put. 
>
> 	CPU0(Task A)			CPU1(Task B)
> 	====				====
> 	vfs_write
> 	new_sync_write
> 	generic_file_write_iter
> 	fat_write_begin
> 	block_write_begin		vfs_statfs
> 	fat_get_block			statfs_by_dentry
> 	fat_add_cluster			fat_statfs
> 	fat_ent_write			fat_count_free_clusters
> 	fat32_ent_put			fat32_ent_get
>
> Task A's write operation on CPU0 and Task B's read operation on CPU1 occur
> simultaneously, generating an race condition.
>
> Add READ/WRITE_ONCE to solve the race condition that occurs when accessing
> FAT32 entry.

I mentioned about READ/WRITE_ONCE though, it was about possibility. Do
we really need to add those?  Can you clarify more?

> diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
> index 1db348f8f887..a9eecd090d91 100644
> --- a/fs/fat/fatent.c
> +++ b/fs/fat/fatent.c
> @@ -146,8 +146,8 @@ static int fat16_ent_get(struct fat_entry *fatent)
>  
>  static int fat32_ent_get(struct fat_entry *fatent)
>  {
> -	int next = le32_to_cpu(*fatent->u.ent32_p) & 0x0fffffff;
> -	WARN_ON((unsigned long)fatent->u.ent32_p & (4 - 1));
> +	int next = le32_to_cpu(READ_ONCE(*fatent->u.ent32_p)) & 0x0fffffff;
> +	WARN_ON((unsigned long)READ_ONCE(fatent->u.ent32_p) & (4 - 1));

Adding READ_ONCE() for pointer is broken.

>  	if (next >= BAD_FAT32)
>  		next = FAT_ENT_EOF;
>  	return next;
> @@ -187,8 +187,8 @@ static void fat16_ent_put(struct fat_entry *fatent, int new)
>  static void fat32_ent_put(struct fat_entry *fatent, int new)
>  {
>  	WARN_ON(new & 0xf0000000);
> -	new |= le32_to_cpu(*fatent->u.ent32_p) & ~0x0fffffff;
> -	*fatent->u.ent32_p = cpu_to_le32(new);
> +	new |= le32_to_cpu(READ_ONCE(*fatent->u.ent32_p)) & ~0x0fffffff;
> +	WRITE_ONCE(*fatent->u.ent32_p, cpu_to_le32(new));
>  	mark_buffer_dirty_inode(fatent->bhs[0], fatent->fat_inode);
>  }

Also READ_ONCE() is really required here?
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>