linux-ext4 - Re: BUG? ext3: Allocate blocks over quota limit with mmap

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Message-ID: <4C56534A.5030806@rs.jp.nec.com>
Date:	Mon, 02 Aug 2010 14:10:34 +0900
From:	Akira Fujita <a-fujita@...jp.nec.com>
To:	akpm@...ux-foundation.org, adilger@...ger.ca,
	Jan Kara <jack@...e.cz>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: BUG? ext3: Allocate blocks over quota limit with mmap

Hi ext3 maintainers,

Could you look into this?
If this is not a problem, it is good though.

Regards,
Akira Fujita


(2010/07/29 11:08), Akira Fujita wrote:
> Hi,
> 
> I found a problem that user can allocate blocks over quota limitation
> on ext3 (and ext2) with mmap.
> You can reproduce this with the following steps:
> 
> 1. Enable user quota on ext3
>   [akira@...086 mnt]$ uname -r
>   2.6.35-rc6
> 
>   [root@...086 mnt]# cat /proc/mounts  | grep  /dev/sda9
>   /dev/sda9 /mnt/mp1 ext3 rw,relatime,errors=continue,barrier=0,data=ordered,usrquota 0 0
> 
>   [root@...086 mnt]# quotaon -p /mnt/mp1
>   group quota on /mnt/mp1 (/dev/sda9) is off
>   user quota on /mnt/mp1 (/dev/sda9) is on
> 
>   [root@...086 mnt]# repquota -v /mnt/mp1
>   *** Report for user quotas on device /dev/sda9
>   Block grace time: 7days; Inode grace time: 7days
>                           Block limits                File limits
>   User            used    soft    hard  grace    used  soft  hard  grace
>   ----------------------------------------------------------------------
>   root      --    1229       0       0              4     0     0
>   akira     --       0     100    1000              0     0     0
> 
> 
> 2. Create sparse file on ext3
>   [akira@...086 mnt]$ df -T /mnt/mp1
>   Filesystem    Type   1K-blocks      Used Available Use% Mounted on
>   /dev/sda9     ext3       23300      1236     20861   6% /mnt/mp1
> 
>   [akira@...086 mnt]$ dd if=/dev/zero of=/mnt/mp1/file bs=4096 seek=1MB count=1
> 
> [akira@...086 mnt]$ ls -ls /mnt/mp1
>   total 26
>    7 -rw------- 1 root  root        7168 Jul 28 15:53 aquota.user
>    7 -rw-rw-r-- 1 akira akira 4096004096 Jul 28 15:53 file
>   12 drwx------ 2 root  root       12288 Jul 28 14:49 lost+found
> 
>   [root@...086 mnt]# repquota -v /mnt/mp1
>   *** Report for user quotas on device /dev/sda9
>   Block grace time: 7days; Inode grace time: 7days
>                           Block limits                File limits
>   User            used    soft    hard  grace    used  soft  hard  grace
>   ----------------------------------------------------------------------
>   root      --    1228       0       0              3     0     0
>   akira     --       8     100    1000              2     0     0
> 
> 3. Write data to "file" with mmap and msync.
>    (In this time, write size is 50MB. It's larger than partition size )
> 	e.g.
>          long long contents = 0x0002;
> 	fd = (file, O_APPEND | O_RDWR, 0666);
> 	p = mmap(NULL, psize, PROT_WRITE, MAP_SHARED, fd, offset);
> 	memset(p, contents++, psize);
> 	offset += psize
> 	munmap(p, psize);
> 	close(fd);
> 
> 4. Then run out disk space, user uses all of the blocks.
>   [akira@...086 mnt]$ df -T /mnt/mp1
>   Filesystem    Type   1K-blocks      Used Available Use% Mounted on
>   /dev/sda9     ext3       23300     23300         0 100% /mnt/mp1
>                                      ~~~~~
>   [root@...086 mnt]# repquota -v /mnt/mp1
>   *** Report for user quotas on device /dev/sda9
>   Block grace time: 7days; Inode grace time: 7days
>                           Block limits                File limits
>   User            used    soft    hard  grace    used  soft  hard  grace
>   ----------------------------------------------------------------------
>   root      --    1228       0       0              3     0     0
>   akira     +-   22065     100    1000  6days       2     0     0
>                  ~~~~~
> 
> memset() after mmap() triggers the pagefault and then __do_fault
> marks whole pages correspond to offset we specified as dirty.
> After 5 seconds (or call sync), the kjournald tries to write out all of dirtied pages
> with getting blocks to disk.
> kjournald has CAP_SYS_RESOURCE capability, therefore it can ignore
> quota limitation (also can use blocks for root user).
> As a result, user can have blocks over quota limitation,
> though quota is enabled.
> Note: ext4 has own page_mkwrite, so this problem does not happen on it.
> 
> I guess behavior of kjournald is correct (write out all dirty pages of file),
> so we need some consideration for pagefault behavior for ext3 and ext2.
> 
> Is this a bug?
> 
> Regards,
> Akira Fujita
> 
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