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

[Jan Kara <jack@suse.cz>]

On Mon 02-08-10 14:10:34, Akira Fujita wrote:
> Hi ext3 maintainers,
> 
> Could you look into this?
> If this is not a problem, it is good though.
  It's a bug and I'm aware of problems of this sort for some time already.
But I never realized this particular effect which is really nasty. Thanks
for letting me now. I'll give more priority to rebasing my patches fixing
this and pushing them upstream.

									Honza

> (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@bsd086 mnt]$ uname -r
> >   2.6.35-rc6
> > 
> >   [root@bsd086 mnt]# cat /proc/mounts  | grep  /dev/sda9
> >   /dev/sda9 /mnt/mp1 ext3 rw,relatime,errors=continue,barrier=0,data=ordered,usrquota 0 0
> > 
> >   [root@bsd086 mnt]# quotaon -p /mnt/mp1
> >   group quota on /mnt/mp1 (/dev/sda9) is off
> >   user quota on /mnt/mp1 (/dev/sda9) is on
> > 
> >   [root@bsd086 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@bsd086 mnt]$ df -T /mnt/mp1
> >   Filesystem    Type   1K-blocks      Used Available Use% Mounted on
> >   /dev/sda9     ext3       23300      1236     20861   6% /mnt/mp1
> > 
> >   [akira@bsd086 mnt]$ dd if=/dev/zero of=/mnt/mp1/file bs=4096 seek=1MB count=1
> > 
> > [akira@bsd086 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@bsd086 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@bsd086 mnt]$ df -T /mnt/mp1
> >   Filesystem    Type   1K-blocks      Used Available Use% Mounted on
> >   /dev/sda9     ext3       23300     23300         0 100% /mnt/mp1
> >                                      ~~~~~
> >   [root@bsd086 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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> > 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR