Re: [PATCH] eCryptfs: Quota check incorrectly ignored

['Tyler Hicks']

On 2012-02-10 13:31:49, 'Tyler Hicks' wrote:
> On 2012-02-10 22:44:05, Li Wang wrote:
> > > -----Original Message-----
> > > From: Jan Kara [mailto:jack@xxxxxxx]
> > > Sent: Friday, February 10, 2012 6:32 PM
> > > To: Li Wang
> > > Cc: Tyler Hicks; ecryptfs@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > Jan
> > > Kara; Yong Peng
> > > Subject: Re: [PATCH] eCryptfs: Quota check incorrectly ignored
> > > 
> > > On Thu 09-02-12 19:39:32, Li Wang wrote:
> > > > eCryptfs recently modified the write path to perform encryption
> > > > and write down in ecryptfs_writepage(). This function invokes
> > vfs_write()
> > > > to write down the encrypted data to lower page cache. vfs_write() will
> > > > first make sure this write will not exceed the quota limit for the owner
> > > > of the file being written into, if quota is supported by
> > > > the underlying file system, and it is turned on. Normally, it
> > accomplishs
> > > > this job by calling check_idq()/check_bdq() (fs/quota/dquot.c). When
> > system
> > > > dirty ratio is not high, ecryptfs_writepage() is normally invoked by the
> > > > write back kernel thread, who has the capability CAP_SYS_RESOURCE,
> > > > this priority will let check_idq()/check_bdq() directly bypass the quota
> > > > check. This sometimes makes data safely written into the disk in spite
> > of
> > > > exceeding the quota limit.
> > > >
> > > > This patch temporarily removes the CAP_SYS_RESOURCE capability from the
> > > kernel
> > > > thread before invoking vfs_write_lower(), to let it undergo quota check
> > by
> > > > the lower file system, if necessary. After that, reassign the
> > capability.
> > >   Hmm, but then the error will just be thrown away by the flusher thread
> > > and the application never learns about it? That doesn't sound like a good
> > > solution.
> > > 
> > > 								Honza
> > 
> > Yes. we are aware of that, but we have not found better solution, since it
> > seems 
> > VFS does not supply a file system independent interface to check quota early
> > and only
> > (fix us if we are wrong), The file systems just perform the quota check in
> > their own way, 
> > the routine is wrapped deeply inside the file system specific code path.
> > Maybe we could
> > copy some codes from _dquot_alloc_space() & dquot_alloc_inode
> > (fs/quota/dquot.c) to 
> > perform quota check on lower inode ourselves, but that is ugly, and we are
> > not sure if it works
> > for any file system or not..., On the other side, due to the existence of
> > write buffer, and io schedule,
> > the successful write call does not gurantee the data will be written into
> > disk, in terms of that,
> > we do not change much of the semantic of write call. 
> 
> Maybe I should just revert 57db4e8d73ef2b5e94a3f412108dff2576670a8a
> (and friends) and go back to the write-through cache model for eCryptfs.
> The write-back model has some nice performance improvements, but we keep
> running into little issues like this. For write-through to work
> reliably, we're going to need some support from the VFS that isn't there
> yet.

Sorry, that should be "For write-back to work reliably, we're going to
need some support from the VFS that isn't there yet."

Tyler

> 
> Handling ENOSPC correctly is another area that is lacking after the
> switch to write-back. Currently, an application will continue to write
> out data without having any indication that the disk is full. Thieu
> proposed a fix, but it was never merged:
> 
> http://article.gmane.org/gmane.linux.kernel/1209265
> 
> So, what do folks think about going back to a write-through cache until
> we have the ability to alert the application of these error conditions?
> 
> Tyler
> 
> >  
> > > 
> > > 
> > > >
> > > > Signed-off-by: Li Wang <liwang@xxxxxxxxxxx>
> > > > Singed-off-by: Yong Peng <pengyong@xxxxxxxxxxxxxx>
> > > > ---
> > > >
> > > > To repeat this bug,
> > > > 	mount -o usrquota /dev/sda3 /tmp
> > > > 	cd /tmp
> > > > 	edquota -u foo // set the disk quota limit for user foo be m1 bytes
> > > > 	quotaon -a
> > > > 	mount -t ecryptfs cipher plain
> > > >
> > > > 	login the system as user foo
> > > > 	cd /tmp/plain
> > > > 	execute the following simple program
> > > >
> > > > 	int main()
> > > > 	{
> > > > 		char buf[m2]; // m2>m1
> > > > 		FILE *f = fopen("dummy", "w");
> > > > 		fwrite(buf, 1, m2, f);
> > > > 		sleep(60); // let the kernel thread do the write back job
> > > > 		fclose(f);
> > > > 		return 0;
> > > > 	}
> > > >
> > > > 	This program can write as much of data as it wants, provided sleep
> > > enough long time before closing the file.
> > > >
> > > >  fs/ecryptfs/crypto.c |   27 +++++++++++++++++++++++++++
> > > >  1 files changed, 27 insertions(+), 0 deletions(-)
> > > >
> > > > diff --git a/fs/ecryptfs/crypto.c b/fs/ecryptfs/crypto.c
> > > > index 63ab245..2c1da29 100644
> > > > --- a/fs/ecryptfs/crypto.c
> > > > +++ b/fs/ecryptfs/crypto.c
> > > > @@ -457,6 +457,7 @@ int ecryptfs_encrypt_page(struct page *page)
> > > >  	struct page *enc_extent_page = NULL;
> > > >  	loff_t extent_offset;
> > > >  	int rc = 0;
> > > > +	struct cred *cred;
> > > >
> > > >  	ecryptfs_inode = page->mapping->host;
> > > >  	crypt_stat =
> > > > @@ -487,8 +488,34 @@ int ecryptfs_encrypt_page(struct page *page)
> > > >  				   * (PAGE_CACHE_SIZE
> > > >  				      / crypt_stat->extent_size))
> > > >  				  + extent_offset), crypt_stat);
> > > > +		if (current->flags & PF_KTHREAD) {
> > > > +			/*
> > > > +                         * Temporarily remove the
> > > CAP_SYS_RESOURCE capability
> > > > +                         * from the write back kernel thread to let it
> > > undergo
> > > > +                         * quota check by the lower file system
> > > > +                         */
> > > > +			cred = prepare_creds();
> > > > +			if (unlikely(!cred)) {
> > > > +				rc = -ENOMEM;
> > > > +				goto out;
> > > > +			}
> > > > +			cap_lower(cred->cap_effective, CAP_SYS_RESOURCE);
> > > > +			commit_creds(cred);
> > > > +		}
> > > >  		rc = ecryptfs_write_lower(ecryptfs_inode, enc_extent_virt,
> > > >  					  offset, crypt_stat->extent_size);
> > > > +		if (current->flags & PF_KTHREAD) {
> > > > +			/*
> > > > +                         * Reassign the CAP_SYS_RESOURCE
> > > capability
> > > > +                         */
> > > > +			cred = prepare_creds();
> > > > +			if (unlikely(!cred)) {
> > > > +				rc = -ENOMEM;
> > > > +				goto out;
> > > > +			}
> > > > +			cap_raise(cred->cap_effective, CAP_SYS_RESOURCE);
> > > > +			commit_creds(cred);
> > > > +		}
> > > >  		if (rc < 0) {
> > > >  			ecryptfs_printk(KERN_ERR, "Error attempting "
> > > >  					"to write lower page; rc = [%d]"
> > > > --
> > > > 1.7.6.5
> > > --
> > > Jan Kara <jack@xxxxxxx>
> > > SUSE Labs, CR
> >

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