Re: [PATCH 14/19] fs: Permit FIBMAP for users with CAP_SYS_RAWIO in s_user_ns

From: Andreas Dilger <adilger@dilger.ca>
To: Seth Forshee <seth.forshee@canonical.com>,
	"Serge E. Hallyn" <serge@hallyn.com>
Cc: Theodore Ts'o <tytso@mit.edu>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@canonical.com>,
	Richard Weinberger <richard.weinberger@gmail.com>,
	Austin S Hemmelgarn <ahferroin7@gmail.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	linux-bcache@vger.kernel.org, dm-devel@redhat.com,
	linux-raid@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	linux-mtd@lists.infradead.org,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	fuse-devel@lists.sourceforge.net,
	linux-security-module@vger.kernel.org, selinux@tycho.nsa.gov
Subject: Re: [PATCH 14/19] fs: Permit FIBMAP for users with CAP_SYS_RAWIO in s_user_ns
Date: Fri, 4 Dec 2015 17:00:58 -0700	[thread overview]
Message-ID: <DB9A6172-D1AF-4704-9D0D-936365A44AD0@dilger.ca> (raw)
In-Reply-To: <20151204231152.GE18359@thunk.org>

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> On Dec 4, 2015, at 4:11 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> 
> On Fri, Dec 04, 2015 at 02:45:32PM -0600, Seth Forshee wrote:
>> On Fri, Dec 04, 2015 at 02:07:36PM -0600, Serge E. Hallyn wrote:
>>> Heh, I was looking over http://www.gossamer-threads.com/lists/linux/kernel/103611
>>> a little while ago :)  The same question was asked 16 years ago.  Apparently
>>> the answer then was that it was easier than fixing the code.
>> 
>> So it seems then that either it still isn't safe and so unprivileged
>> users shouldn't be allowed to do it at all, or else it's safe and we
>> should drop the requirement completely. I can't say which is right,
>> unfortunately.
> 
> It may not have been safe 16 years agoo, but giving invalid arguments
> to FIBMAP is safe for ext4 and ext2.  This is the sort of thing that
> tools like trinity should and does test for, so I think it should be
> fine to remove the root check for FIBMAP.

You can use FIEMAP on regular files and directories without special permission:

$ filefrag -v /etc
Filesystem type is: ef53
File size of /etc is 12288 (3 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..       0:    8396832..   8396832:      1:
   1:        1..       2:    8397051..   8397052:      2:    8396833: last,eof
/etc: 2 extents found

FIEMAP also has the benefit that you don't need to call it millions of times
for large files, like is needed for FIBMAP.

Cheers, Andreas

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