linux-ext4 - Re: [RFC][PATCH 7/9]ext4: Add the EXT4_IOC_FIEMAP

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Date:	Tue, 04 Nov 2008 14:42:50 -0700
From:	Andreas Dilger <adilger@....com>
To:	Akira Fujita <a-fujita@...jp.nec.com>
Cc:	linux-ext4@...r.kernel.org, Theodore Tso <tytso@....edu>,
	Mingming Cao <cmm@...ibm.com>, hch@...radead.org
Subject: Re: [RFC][PATCH 7/9]ext4: Add the EXT4_IOC_FIEMAP_INO ioctl

On Oct 31, 2008  18:46 +0900, Akira Fujita wrote:
>> Why does a regular user need to do the ioctl on a file that it may not
>> have read permission to access?  I can see this is useful for root
>> doing a defrag of the whole filesystem instead of opening and closing
>> all of the files, but for regular users we need to validate via the
>> full path to ensure they can even access the file before defragmenting it.
>
> The FIEMAP_INO ioctl just passes a inode number belongs to
> the target block group from user space to kernel space
> and then the owner check is done in the kernel space.
>
> If the regular user (defrag -f excecutant) is owner of a file,
> defrag handles this file as the candidate of victim file which would
> be moved to the other block group to make free space.
>
> So I think the full path check is unneeded because the owner check
> is done in the kernel space (I'm not sure it's good enough).
> If it's not good in the security point of view,
> I will make defrag -f mode be done only by root user.

If the defrag operation is limited to the owner of the file (or root
via CAP_DAC_OVERRIDE) then this is probably OK also.  The data never
gets to userspace so there is relatively little risk to this operation.

>>>> This was mentioned last time these patches were posted, but there was
>>>> no reply from you.  Christoph suggested a more generic VFS open-by-inum,
>>>> which isn't impossible to do but would cause a lot of controversy I
>>>> think, while the EXT4_IOC_WRAPPER is at least contained within ext4,
>>>> but is more generically useful than EXT4_IOC_FIEMAP_INO.
>
> How do the other ext4 developers think about
> implementing EXT4_IOC_WRAPPER?
> Will it be used only for defrag so far?

I expect the initial users of this ioctl will be FIEMAP and DEFRAG, but
it might also be useful for other ioctls in the future.

I haven't really asked other ext4 developers about it yet, and nobody
else has commented the last time I posted the patch.

I don't have an objection to Christoph's open-by-FH API, if there is
acceptance of this from other kernel developers (Al Viro in particular),
but that exposes a lot more security issues than just the ioctl wrapper.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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